Modbus and Phone Modems
Kenneth Jenningsi
(6/29/00)
Today's modems are radically different from their predecessors.
Gone are the plug and play days of the past. Also gone is the getting
of coffee and waiting for the data to be sent. These days, it is
possible to send a one-megabyte data file in six minutes or less.
Obviously, these changes are affecting our modem choices for
transmitting Modbus data. The new, high-speed modems may be the
right solution for demanding SCADA (supervisory control and data
acquisition) applications.
This paper gives some insight into the selection and implementation
of a telephone modem installation using Modbus protocol.ii
Introduction
Since 1978, Modbus has been successfully implemented in varieties of applications.
No other protocol can boast a wide industrial acceptance as Modbus. The prosperity of
Modbus is due to its ease of use and its ability to communicate over a wide variety of
media: wire, fiber optics, radios, microwaves, satellites and telephones.
The telephone modem is the primary instrument for communicating Modbus messages
over long distances. Phone modems are capable of sending and receiving data virtually
anywhere in the world. Many people choose them because they are inexpensive and
readily available.
Modems have undergone radical changes in the past several years. These changes
enable them to send large amounts of data quickly over regular telephone wires. The
new “V.90” standardiii permits modem download transmission rates up to 57,600 bits per
second (BPS) over normal telephone lines.
The newer technology does have a penalty. The penalty involves the loss of a simple
hookup. Newer modems have numerous (and complex) options. They include the
following:

Buffering (for communicating at different speeds)

Fallback and fall-forward modes (for adjusting speeds on noisy lines)

Error correction and data compression (for maximum transmission rates)

Negotiation (for determining the best communication connection)

Handshaking (for preventing overrun)
Understanding the various options, determining when to use them and making sure that
they are compatible is the first step in establishing a successful communication’s system.
1
Types of Modems
There are three types of phone modems: dial-up, leased line and cellular. A dial-up
modem requires the host computer to dial the telephone number of the remote site
before communicating. This lengthens the time it takes to communicate to the remote
unit. A leased line modem, on the other-hand, provides instantaneous communication.
Cellular phone modems provide mobile communications for today’s person on the go.
This offers tremendous advantages for maintenance personnel that are responsible for
numerous sites over large distances. Additionally, these data ported portable phones
provide fax, e-mail and file transfer functions. Cellular phone modems are usable with
either analog or digital data phones.
Wired phone modems are available in both analog and digital formats. Analog modems
are by far the most common. Digital phone modems, more properly called data service
units (DSU’s), are gaining in popularity (especially with utility users using Adtran’s
models IIIAR and DSU5600iv, Eazy’s MT 132A-R2v and Motorola’s DDSMR1).
Options
Some modems have redundancy capabilities. They may either have multiple leased
line accessing or dial-up fallback modes.vi If using a fallback modem, make sure it can
automatically dial the remote site’s telephone number since most host‘s software does
not provide dialing capabilities.
Other modems can combine voice and data on the same phone line. Select Modbus
ASCII mode whenever using this method, since data gaps are likely during the voice
transmission period.
Jargon
When it comes to communicating, modems use their own jargon. A virtual alphabet soup
combination of letters and numbers is used in communicating to the modem. Fortunately,
most modems converse using a standard "Hayes" protocol for their initialization.
Initializing a modem means configuring it for a particular mode of operation. Each
modem possesses a set of commands for establishing these modes of operation (see
table D). Small variations in commands do exist between different manufacturers and
models; so pay careful attention to the modem's manual.
The most commonly used commands are those that begin with a single letter (A-Z).
These commands instruct the modem to perform a specific operation such as dialing a
telephone number, resetting the modem, turning off the modem's speaker or displaying
log-on information. Commands that begin with the ampersand (&) usually involve the
modem's communications parameters. Commands that begin with the percent sign (%)
and slash (/ or \) involve Microcom Networking Protocol (MNP) operations; avoid them
when using Modbus communications.
Cellular phone modems have unique commands for controlling additional functions and
have commands that deal with wireless networking. MNP10 protocol is permissible
when using Modbus ASCII with cellular phones.
The modem must be in the command mode to receive and process these command
instructions. If the modem is not in the command mode, it simply passes the
2
information on to the receiving modem as if it were data. To enter the command mode,
issue three plus signs (+++) in rapid succession followed by silence. This causes the
modem to stop communicating and to start listening for commands.
The first command, issued before all others, is the AT command. Use it to gain the
modem's attention. It tells the modem that a command or group of commands follows.
A command line may contain up to 40 characters. Terminate all commands with a
carriage return (enter). This causes the modem to process the given command.
After initializing the modem, save the parameters to permanent memory using the &W0
command; a power loss will clear the configuration. The modem is now ready for
dialing telephone numbers (ATDT) or returning on-line (ATO).
Guidelines
The primary factor in selecting a communication mode depends upon the options that
the devices offer and those needed.
If speed is an issue, select Modbus RTU instead of Modbus ASCII. It is 25% faster than
the ASCII mode when using a phone modem (see table A). Use the same data rate
throughout the system because speed buffering causes data gaps and produces errors.
If the devices need to communicate at different speeds, select the ASCII mode. The
ASCII mode uses a carriage return with line feed to signify the end of message.
Modbus ASCII mode permits timing gaps between characters and allows messages to
get through with no problems. It is also much easier to troubleshoot than the RTU
mode. While it may be slower than the RTU mode, it's not appreciably slower.
Give special consideration when choosing between software and hardware communication
port setups. Software configurations offer the most selections (baud rates, data bits, parity
bits and stop bits). However, a total loss of power or CPU failure may change the software
configuration. Hardware configurations may offer fewer selections. Unlike software
selectable configurations, hardware setups do not change during power loss. This affords
the opportunity of reloading the program and attempting to restart the unit.
Sometimes, selecting one option enables or disables another option. Selecting the DIP
(hardware configuration) mode rather than the memory (software configuration) mode
on Modicon's 984-xx5 series PLC enables the Modbus Plus bridge mode. This enables
a single modem to communicate to numerous PLC's.
If one requires multiple bridging networks at different locations, choose the bridge
multiplexer with its silent master (X-mode) option. The host receives multiple replies
whenever more than one PLC is in a bridging mode on a network: PLC's report back
with bad slave ID’s along with the good reply from the intended PLC. This causes
garbled replies. The X-mode operation solves this problem by inhibiting the invalid
address reply that normally returns to the host.
Use the bridge multiplexer when a device does not support a particular communication
mode. For example, if all PLC's use the RTU mode and a non-PLC device uses the
ASCII mode, then use a bridge multiplexer to convert the message from RTU to ASCII.
Another very important consideration in selecting a modem and choosing amongst the
various communication parameters is the data transmission length. Most modems use
ten bit data words (start+data+parity+stop); Modicon's PLC default is eleven bits. See if
3
your slave unit requires eleven bits or can use ten bits. Newer model 984’s permit
configurable bridging; older units do not.
External modems are easier to troubleshoot since they have status lights. These lights
indicate transmission and reception of data, handshaking status, carrier detection,
transmission speed, power and diagnostic conditions. External modems are also easier
to install. If the modem stops functioning or locks up, it is easier to reset. Internal
modems have no diagnostic lights and are more difficult to install but are very
convenient for portable computers.
Since high-speed modems use error correction and compression methods for high data
transfer rates, one may think about employing these modes. In most cases, disable or
turn off these modes because they slow down the system. Additional error checking,
performed by the modem, is the source of the delay and defeats the effectiveness of
compression. When using compression, small messages reduce throughput. Effective
high-speed compression requires the master to send large messages (200 registers not
the present 16) and faster port speeds (57K baud). Table A summarizes the effects of
using V.42bis data compression.
Handshaking is a new factor to consider. Some Modbus devices, such as Modicon’s
Compact E984-275 port number 2, do not support hardware (RTS/CTS) handshaking.
On dialup modems this is not a problem but it is a problem with radio and lease line
modems. Be careful about selecting which communication port to use.
Virtually any telephone modem with asynchronous RS232 capabilities is useable with
Modbus. Use the same modem at both ends for it is much easier to deal with a single
manufacturer, if problems arise, than two. Pay careful attention to initializing the
modems when mixing and matching
different units (see Table B). Table C
110XCA20401
provides additional information about the
RJ45 to 25 Pin Male Adapter
modems used in this report.
1
black
2
red
3
yellow
4
brown
5
Wiring
Prefabricated modem cables are available
for most PLC's. These cables make for
easy installation when connecting a PLC
to a modem. The slot mount 984 PLC
requires an AS-W954-006 cable (refer to
the system's manual, GM-0984-SYS page
63, or Modfax document #1954).
To
Micro
PLC
+5V
DSR
TX
RX
GND
RTS
CTS
SHD
1
2
3
4
5
6
7
8
blue
orng
6
green
7
8
white
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
RJ45
Different PLC’s require different cablesvii
due to physical constraints, such as tight
DB25
spaces and protective covers.
The
Figure 1
Quantum PLC, for example, requires a
new thin plastic molded connector
because of its shallow front door cover. The Micro PLC requires an adapter to convert
from a RJ45 connector to a DB25 connector (see Figure 1).
Figure 2 shows the wiring for a Bay Network Alliance lease line modem.viii
4
NC
RX
TX
GND
DTR
DSR
RTS
CTS
NC
1
2
3
5
4
6
7
8
9
1
3
2
7
4
5
6
20
8
SHLD
RD
TD
GND
RTS
CTS
DSR
DTR
CD
Telco
SHLD
TD
RD
GND
RTS
CTS
DSR
DTR
CD
1
2
3
7
4
5
6
20
8
1
3
2
5
4
5
7
8
9
NC
TX
RX
GND
DTR
DSR
RTS
CTS
NC
(Wiring Is For Bay Alliance Modem Only!)
Figure 2
Newer and smaller modems usually have a DB9 connector instead of a DB25
connector, so check your modem first before wiring.
After obtaining or making your RS232 cables, you are ready to connect your modems.
Connecting the modems is even easier. Simply plug a telephone cable, with its RJ11
connector on it, into the modem and then plug the other end of the cable into the
telephone line.
Make sure that the telephone line is an analog one and not a digital one. Digital lines,
like those found in offices (e.g., PBX: Public Branch Exchange), will not work and may
even damage the modem due to high voltages.
Initializing the Modem
After connecting the modem to the computer, it is ready for initialization. Initializing the
modem involves sending it a series of letters and numbers, such as those from Table D
or the modem's manual. These combinations of letters and numbers instruct the
modem to perform a particular function. Refer to Table B for initialization suggestions.
Use a terminal emulator package, such as ProComm Plus, to communicate to the modem.
Select the desired communication parameters and issue an AT command. This instructs
the modem that command information is being sent. If the modem is working properly and
the cabling is correct, both transmit and receive data lights should blink and the modem
should respond with an OK. If not, the modem may not be in the command mode. Issue
three plus signs (+++) to make the modem enter the command state and try the command
again. If the transmit data indicator light does not blink, you should check the wiring.
It is advisable to reset the modem to the factory default mode before beginning. Use
the &F (or &F0,1,2) command to do this. Afterwards, issue single or multiple
commands. End all commands with a carriage return (enter). Write the configuration
into the modem's memory or use it on a temporary basis until no longer needed.
If the RTU mode is being used, turn off the modem’s error correction, compression and
speed buffering. If the modem does not default to this, issue an &Q0. If the modem
has a problem with handshaking, try issuing an &K0. If that fails, try jumping the
request to send and clear to send handshake lines together on the Modbus device.
A slave modem has an additional requirement. It needs to auto-answer the incoming
calls. Use the S0 command to select the auto-answering mode. Enter the number of
rings to wait before answering after the S0 command (e.g., S0=3 for wait three rings).
Remember to write the configuration to memory, using the &W0 command, before
powering down the modem.
5
Testing
First, connect the slave and master modem with a telephone cable (without connecting to
the telephone line). Then, connect each modem to a computer running a communications
program. Select the same communications settings as those used with Modbus. Turn on
the modems and issue an AT. If the master modem needs special initialization, do so.
Issue an ATD to the master modem and then an ATA to the slave modem. The ATD
and ATA cause the modems to connect to each other without having to dial out on the
telephone line. Try chatting back and forth after the modems connect. If this works, the
modems’ configurations are likely correct.
Troubleshooting
If the modems fail to connect or cannot pass data back and forth, the configuration is
not correct. Use the modem's indicator lights to diagnose the problem. The modem's
ready (MR), terminal ready (TR), request-to-send (RS), clear-to-send (CS), carrier
detect (CD) and on-hook (OH) indicator lights should all be lit. The slave modem's auto
answer (AA) indicator should light, too. The transmit (TX) and (RX) indicator lights
should blink on and off when sending data.
If either the terminal ready light or the request to send light fails to light, the problem is
likely incorrect wiring or an improperly functioning terminal. Try a different modem
configuration; one that ignores the RTS input signal. If that fails, then you must jumper
out the missing signal.
If the indicator lights are functioning properly and the data is not being received
correctly, then a mismatch between the modems' communication parameters must
exist. Check the baud rate, parity type, number of data bits and stop bits for
correctness. Also, check the modem's ability to use that mode. The Aceex modem, for
example, strips off the eleventh bit when using &Q5 or &Q6 but not when using &Q0.
Other modems may claim to transmit eleven data bits but unexpectedly change to ten
bits upon receiving a modem command function.
Some modems, like the Practical Peripheral, can provide connection information that
aids in diagnosing communication problems. Issuing an I6 command causes the
Practical Peripheral modem to display a data connection screen. This screen tells the
amount of data sent, number dialed, connection time, communication mode, protocol
used, compression method, flow control and parity type used and the speed of the
modem and terminal. It also displays the reason for terminating the last call. This
information is very handy for determining connection failures.
Occasionally, manufactures must change components due to parts obsolescence. This
may cause a working system to stop functioning when adding these newer components.
Either find older components or wait until manufacturing revisions become available.
Contact the manufacturer to see if this is a possibility and find out if there is an available
update.
Another troubleshooting technique involves monitoring the communication’s link with
either a hardware data analyzer or a software snooper program such as Win-Tech’s
Listen32ix. By examining the data stream being sent back and forth, communication
faults can be found. Missing characters, extra characters (due to noise or tail-squelch)
and timing issues become readily apparent when using the analyzer.
6
Modsoft
Using Modsoft with a telephone link is the same as using Modsoft with a direct wire
connection. Before communicating over the telephone, one must first establish a
modem connection using a communications program, such as ProComm Plus.
Enter the modem's initialization string, port number, baud rate, parity type and number
of stop bits into the communication’s program. Dial the telephone number of the remote
PLC. After the modems connect, exit to DOS (ALT+X) without hanging up the line and
start Modsoft.
Next, go to the communications setup. Select Modbus I using the same parameters as
with ProComm Plus and use Modsoft as usual. When finished, remember to hang up
the line or turn off the computer. To hang up the line, go back to ProComm Plus and
hang up (ALT+H).
ProWORX
Both ProWORX Plus and ProWORX NxT have the capability of connecting to the PLC
via a modem dial-up connection. ProWORX needs no additional dialing programs. Just
select the communications port, the dial-up modem option type (an optional password
connection is also available), enter the modem’s initialization string and then click OK.
Leave the other entries at their default value. Figure 3 shows a typical communications
setup for ProWORX NxT.x
Figure 3
Connection to the PLC is 10 bits, so set the communications to either ASCII with even
parity or RTU with no parity. If not, ProWORX will prompt with a message when the OK
7
button is pressed. Also, make certain that the Modbus port on the PLC matches the
ProWORX parameters.
When connecting to the PLC, the software will prompt for the phone number. Enter the
phone number and then click on the dial button. ProWORX dials the number and
makes the connection to the remote PLC. When finished, go back and click on the
hang-up button.
Operating in Windows
Obtaining data from the PLC via a Modbus direct cable hookup is very simple; however, it
is both tricky and frustrating to do so over phone lines when using Windows.
The problem with Windows is its unfriendly nature of communication port sharing; it
usually does not. When one Windows program (such as Windows terminal program) is
running and another program tries to communicate using the same port, Windows balks
and refuses to let the second program have port access. This is similar to file sharing
violations; great for protecting files but not so great in this application.
So how do we make the connection? One way is to use ProComm Plus for DOS in a
Window’s DOS session. Running ProComm Plus, within a DOS session, effectively
cloaks the port from Windows.
Another way is to use Window’s older Terminal program or Windows 95 newer
HyperTerminal program. These programs require modifying the system.ini file to allow
for communication port sharing between DOS and Windows applications. When using
communications port #2, add the statement “Com2AutoAssign=0” (without quotation
marks) to the [386ENH] section of the system.ini file.xi
When using a configurable modem (e.g. IBM’s Thinkpad 760E internal Mwave modem
or any other Digital Signal Processing modem), Windows “AutoAssign” will not work.
Use a DOS based dialing program for these types of modems.
Windows NT doesn’t support the “Auto Assign” feature either. The technique to make
the connection is to use an external modem and add &D0 to the modem’s initialization
command (or remove the wire between pin 20 of the computer and the modem). Next,
use HyperTerminal to dial the remote PLC. After making the connection, hang-up the
connection and launch the desired program. When finished, re-launch HyperTerminal
and then send +++ to the modem and then ATH0 to hang-up. This method works
because HyperTerminal attempts to hang-up the modem by signaling the modem to
hang-up via pin 20. Since pin 20 is ignored (&D0 or disconnected), the modem never
hang-ups until either power is removed or an ATH0 (hang-up) command is sent. This
technique will not work on an internal modem.
Modlink
Modlink is a Windows based DDE communications program that allows a computer to
communicate to a Modbus slave device. It is an easy to use SCADA Modbus master
communication’s program. Constructing an Excel spreadsheet with live data from the
PLC is easy using Modlink and takes only a matter of minutes.
Try connecting to the Los Angeles Technology Center using 9600 baud with 7 data bits,
even parity and one stop bit. Make sure to disable the modem’s data compression and
error correction feature before dialing.
8
If using a Practical Peripherals’ 14400FXSA modem and ProComm Plus, initialize the
modem with ATL1&Q0&K0^M using the setup utility (ALT+S). Next, select modem
options, general options, initialization options and enter the initialization string. After
making your connection, exit to DOS from ProComm without hanging up the phone line
using ALT+X (this returns you back to Windows).
Launch Excel and then Modlink. Create a new database by clicking on file and new.
Edit the serial port to match the modem’s communications parameters and adjust the
poll time to 1000 milliseconds. Add a new PLC into the PLC list with an address of 8
with the communications port matching the modem’s port number. Next, add a symbol
with a name of timer using the previously defined PLC name and reference a single
address of 1600 using an output register type of integer. This symbol, register 41600, is
the free running timer of the motion demo PLC.
Copy the created symbol from Modlink and then paste it into Excel using the Edit, Paste
Special and Paste Link commands. Select a background color for the cell using the
right mouse button and format cells option. Verify that the data is being updated in
Excel. If using an external modem, check the modem’s transmit and receive indicator
lights. Save the Modlink database and Excel spreadsheet for your next session. When
finished, run ProComm Plus again and hang up the line or shutdown the computer to
disconnect from the phone line.
Communicating via the Internet
Modbus messages are not limited to just RS232 serial communications. Modbus
messages may even travel within TCP-IP Ethernet message packets. This makes it
possible to communicate to a PLC via the Internet.
The world’s first Internet PLC serverxii simulates this functionality. Connecting to the
PLC server requires a modem, an Internet provider, and a Web browser. After
connecting with your Web browser, you can read the PLC’s data and configuration.
Portable Communications
Portable communication devices using cellular technology are capable of transmitting
Modbus messages. These devices employ both analog and digital techniques for
sending messages between master and slave devices. Today, the analog cellular
phone is the most popular of these type devices for data communications.
Cellular phone imperfections can cause considerable challenges to transmitting Modbus
data over mobile telephone networks. Problems with establishing and keeping quality
transmissions often occur. Connection failures (try redialing again later or possibly try
changing location) and excessive retry rates (try lowering transmission speeds or
remaining stationary) can happen.
Transmission power reductions, hand-offs
(transferring the call to a different cellular site for balancing customer load) and sending
cellular control information on the voice channel can cause a disconnection.
To ensure high transmission reliability, use Modbus ASCII protocol with cellular
protocols on both sides of the cellular connection (this requires both modems to be
cellular compatible). If cellular protocols do not work, disable both error correction and
data compression. Also, try using cellular modem pools when available (contact your
cellular provider for availability).
9
Analog cellular telephone modems often include ETC cellular protocol that combats
harsh line conditions during cellular transmissions. ETC protocol improves the initial
connection success rate, automatically calculates optimal transmission speeds,
provides error detection with data retransmission and conditions the signal line. In
addition to ETC, other numerous functionsxiii and commandsxiv are available for use with
cellular phones.xv
Digital cellular phones, utilizing the new digital Personal Communication Services xvi
(PCS) network, are far less susceptible to noise. These PCS phones are now
becoming available for use in large metropolitan areas.
These digital phones have multi-line liquid crystal displays (LCD’s) and are also multifunctional. Functions include paging, faxing, call waiting, conference calling, caller id,
text messaging, data communications and many more (many functions are standard,
some are optional).
Companies manufacturing PCS phones with modem capabilities include Ericsson,
Motorola and Nokia. Network service providers (AT&Txvii, Pacific Bellxviii and others) are
just now beginning to provide data communication capabilities. Before you buy, be sure
to check out their capabilities in your area before purchasing.
To use your modem with a cellular phone, turn off the phone and connect your
approved cellular phone modem cablexix to the phone. Next, connect the other end
of the cable into the modem connector of your running computer. Finally, turn on the
phone and have the computer dial the number.
The Technology Center
Square D’s Los Angeles Technology Center has a Modbus telephone modem set to
auto-answer incoming calls. It is available for access twenty-four hours a day. The
telephone number is (562) 692-3195xx. Please select 7 data bits, even parity and one
stop bit. Select any baud rate from 600 to 28,800 BPS. You may dial in and use
ProWORX, Modsoft, Modlink, Modsnoop, SPC or any other program.
The slave modem communicates to the PLC at 19,200 baud via the modem’s speed
buffering feature. The PLC’s bridging feature then transfers the communications to the
Modbus Plus network at one million bits per second.
The Modbus Plus network possesses many PLC (and non-PLC) devices. You may
access the following PLC nodesxxi on the network:

Unit 2 (Quantum): Concept (pharmaceutical demo)

Unit 6 (984-145): Beckwood IEC demo (#215)

Unit 7 (Quantum): Hot-standby demo

Unit 8 (984-145): Beckwood motion demo (connected to the modem)

Unit 9 (Quantum): MCC controller

Unit 10 (Quantum): Beckwood “Quantour” demo

Unit 14 (Quantum): Concept (reactor demo)

Unit 70 (Micro 612): Beckwood simulator (#205) (via BM85)
10

Unit 71 (Micro 512): Beckwood simulator (#206) (via BM85)

Unit 72 (984-141): Beckwood ATV16 demo (#221) (via BM85)

Unit 73 (984-145): Beckwood Compact demo (#208) (via BP85)

Unit 74 (Quantum): Interbus-S, thermocouple demo (via BP85)

Unit 75 (Pwrlink): Powerlink panelboard (via MEB)

Unit 76 & Unit 77: Powerlogic (via MEB) (future)
Summary
Today's high-speed phone modems provide a cost-effective solution for transmitting
Modbus data. Today’s modems cost less, communicate faster, are more reliable and
have many new advanced features. However, for most Modbus applications, many of
these advanced capabilities slow down rather than speed up data transmission.
These advanced capabilities make installations more difficult. Correctly configuring a
modem allows it to communicate to virtually any other modem using Modbus. Properly
configuring today's modem is challenging and will likely remain so.
Finally, newer high-speed communications evolving today may require new interfacing
techniques. Prepare for changes and modifications to your present system as this
happens. Since Modbus is an adaptable protocol, it will endure these changes and will
provide you service for many more years.
If you have any information regarding a modem that you would care to pass along,
please call (562) 463-3568. We appreciate your input.
11
TABLE A
MODBUS PERFORMANCE CHARTxxii
CONNECT MODBUS
PC
MODE
MODE
BAUD
MB+
PLUS
1M
DIRECT
7,N,2
2,400
V22bis
7,N,2
2,400
V22bis
7,N,2
2,400
DIRECT
7,N,2
9,600
V32
7,N,2
9,600
V32 (PCS)
7,N,2
9,600
V32/V42bis
7,N,2
9,600
V32/V42bis
7,N,2
9,600
V32/V42bis
7,N,2 19,200
DIRECT
7,N,2 19,200
DIRECT
8,N,1
2,400
V22bis
8,N,1
2,400
DIRECT
8,N,1
9,600
V32
8,N,1
9,600
DIRECT
8,N,1 19,200
MEDIA
PLC UPLOAD UPLOAD UPLOAD
BAUD BAUD
ALL
STATE 4XXXX'S
1M
12
6
4
1M
2,400
2,400
135
64
45
2,400
2,400
194
92
65
2,400 19,200
155
74
52
9,600
9,600
37
17
12
9,600
9,600
55
25
18
9,600
9,600
150
47
34
9,600
9,600
118
53
37
9,600 19,200
101
46
31
9,600 19,200
82
36
25
19,200 19,200
21
10
7
2,400
2,400
80
38
27
2,400
2,400
124
59
41
9,600
9,600
25
12
8
9,600
9,600
41
19
13
19,200 19,200
14
7
4
TABLE B
MODEM INITIALIZATION COMMANDS
MASTER
MODEM
MASTER
INITIALIZATION
COMMAND
AT&F
AT&F
AT&F1&Q0
AT&F1&Q0\N1
AT&F1&Q0
AT&F0
AT&F1&Q0\N1
AT&F&Q0S37=9&K0
AT&F\N1
SLAVE
INITIALIZATION
COMMAND
Zoom (V32) AT&F&K0S0=1
Zoom (V32) AT&F&K0&Q0S0=1
Compudyne AT&FS0=1
Compudyne AT&FS0=1
Zoom (V32) AT&F&K0S0=1
Zoom (V32) AT&F&K0S0=1
Zoom (V32) AT&F&K0&Q0S0=1
MI-G144E
AT&F\N1&K0\Q0S0=1
Zoom (V32) AT&F&K0S0=1
MBUS
MODE
ASCII
RTU
BOTH
BOTH
ASCII
ASCII
RTU
ASCII
ASCII
2,400
2,400
2,400
2,400
9,600
9,600
9,600
9,600
9,600
V22b
V22b
V22b
V22b
V32
V32
V32
V32
V32
Viva
Accura
Accura***
PPI
Mhz XJ2144
Nokia 2190
Robotics****
Sportsterxxiii
AT&F&Q6&K0
AT&F&Q6&K0
AT&F&K0
AT&F1&Q6&K0
AT&F&Q&K
AT&F
AT&F&B1&H0&M0&K0
S32=0S27=32S0=1
Zoom (V32)
Zoom (V34)
PPI (V34)
Zoom (V34)
Zoom (V34)
Zoom (V34)
Robotics****
Sportster
AT&F&K0S0=1
AT&F&K0S0=1
AT&F&K0S0=1
AT&F&K0S0=1
AT&F&K0S0=1
AT&F&K0S0=1
AT&F&B1&H0&M0&K0
S32=0S27=32S0=1
ASCII
ASCII
ASCII
ASCII
ASCII
ASCII
RTU
9,600
19,200
9,600
19,200
9,600
9,600
9,600
V32b
V34
V42b
V32b
V32b
V32b
V32b
Thinkpad &
Globalxxiv
AT&F&K0&Q6
Supra 28.8& AT&F&C1&D0&K0&Q6
Accura 28.8 N=0S37=9S0=2&W1
ASCII
9600
V32b
Thinkpad &
Global
AT&F&K0&Q6
Boca 33.6
ASCII
9600
V32b
Generic*
Generic*
PPI**
Aceex**
PPI
Aceex
Aceex
Accura
Paradyne
SLAVE
MODEM
BAUD
RATE
COM
MODE
3710-A2-203
AT&F&K0&Q0&D0
S0=1&W1***
*1200/2400 baud Hayes compatibles ** Terminal dialing baud and Modsoft baud = 19,200
*** &Q6 not used due to noise **** Switch 4 typically down (must be set to echo offline)
12
TABLE C
MODEM COMPARISON CHART
MANUFACTURER
PRACTICAL
ZOOM
ZOOM
ACEEX
BOCA RESEARCH
HAYES
ERICSSON (PCS)
MOTOROLA (PCS)
INTEL
PSION DACOM
NOKIA (PCS)
MODEL
14400FXSA
VX V.32
V34 28.8
1496I
M1440E
ACCURA288
DPY 101 711
SYN5182
146917
GOLD CARD
DTP-8
TYPE
FEATURES
MODBUS COSTxxv
EXT
V32b,42,42b/FAX/LCD MASTER
$150
EXT
V32,42,42b
SLAVE
$50
EXT
V34,32,42,42b/FAX
SLAVE
$100
INT
V32b,42,42b/FAX MASTER
$50
EXT
V32b,42b/FAX
SLAVE
$45
EXT
V34,32b/FAX MASTER
$100
PCMCIA
V34,32,32b,42/FAX MASTER
$499
PCMCIA
V34,32,32b,42/FAX MASTER
$428
PCMCIA
V32b,42,42b/FAX MASTER
$150
PCMCIA
$249
V90/K56/V.34/FAX MASTER
PCMCIA V110,34,32,32b,42/FAX MASTER
$349
TABLE D
MODEM COMMANDS
COMMAND
+++
AT
A
DP/DT
E(x)
H(x)
L(x)
M(x)
O
Q(x)
S(r)=x
S(r)?
V(x)
W(x)
X(x)
Z(x)
&C(x)
&D(x)
&F(x)
&K(x)
&Q(x)
&T(x)
&V
&W(x)
&Y(x)
DESCRIPTION
Go to command mode
Attention (Issue before all commands)
Answer
DP=Pulse dial, DT=Tone dial
Echo command characters (E0=no echo, E1=echo)
Hook control (H0=hang up, H1=on line)
Speaker volume (L0=lowest, L1=low, L2=medium, L3=highest)
Speaker control (M0=off, M1=on until carrier, M2=on, M3=on while connecting)
Exit command mode and go to on-line communication mode
Return response control (Q0=enable, Q1=disable)
Set S register to value x (S0=1 is for autoanswer)
Display S register value
Return response format (V0=numeric form, V1=word form)
Display connection messages (W0=none, W1=display negotiation and DTE speed,
W2=display DCE speed only)
Display extended connect codes (X0,1=no detection, X2=detect dialtone, X3=detect
busy, X4=detect dialtone and busy)
Restore user configuration profile (Saved by &W0,1,2)
Carrier detect (&C0=always on, &C1=on when carrier present)
Data terminal transition control (&D0=ignore, &D1=go to command state,
&D2=hang up & accept commands, &D3=reset)
Reset modem to factory [not user] defaults
&F0: Standard Hayes modem (no error detection or compression)
&F1: Error detection & compression (use &F for Zoom)
Flow control selection (&K0=none, &K3=hardware control, &K4=software control,
&K5=transparent software control) [N/A when &Q0 selected]
Communications mode for V.42 modems
&Q0: Direct mode (no buffering and no error correction/compression)
[Aceex modem includes MNP5 operation with &Q0]
&Q5: Negotiate speed and error correction mode
&Q6: Normal mode (speed buffering only) [N/A with Aceex modem]
Test mode (refer to your modem for procedures to follow)
Display modem's configuration
Write configuration to memory profile 0, 1 or 2
Profile reset selection (&Y0=profile 0, &Y1=profile 1)
13
Troubleshooting Appendixxxvi
The PLC will not respond when I communicate to it when using a modem.
Your modem is a 10-bit modem and you are trying to communicate using the 11-bit default. Modicon
PLC’s use 8 data bits, even parity and one stop bit. Most modems use no parity, 8 bits. Reconfigure
your PLC to 8,N, 1 and make sure to place it into the memory mode. (This is a problem for Micro
users with only one port; you need to buy a BM85 or obtain a two port Micro.) [9]
I am using 10 bits but I still cannot log-on with Modsoft.
A) Your modem needs the correct configuration (see Table B for initialization strings). Make sure you
disable both data compression and error correction. [10]
B) Turn off the automatic speed buffering mode when using Modbus RTU mode and make sure your
modem’s carrier is in V.32 mode (9600 baud) not V.32bis mode (14400 baud). Use the special “S”
command function for setting the carrier’s baud rate (S37=9 for a Practical Peripheral modem). [3]
C) You forgot to setup ProComm as 8,N,1 or (more likely) forgot to specify Modsoft as 8,N,1. Most
people just hit the return button without carefully looking at the Modsoft parameters. [2]
D) You are selecting different communication ports between Modsoft and ProComm. Make sure that
you select the same serial port for both Modsoft and ProComm. Modsoft remembers your last port
selection. Change it whenever you change connection modes (i.e., going from a direct connection to
a modem connection). [4]
E) The PLC is not in memory mode (default mode is 8/E/1) or the Modbus port configuration is
incorrect. [2]
F) You plugged the cable into the wrong PLC Modbus port (usually port 2). [2]
G) You forgot to set your files and buffers large enough for Modsoft [3]. Try increasing the buffers
and file size in the config.sys file. Make sure you can run Modsoft without the modem.
I still cannot connect. Is the problem at my host site or at my remote site?
Try connecting to the Los Angeles Technology Center using seven data bis, even parity and one stop
bit (7/E/1). If you can log-on and connect using Modsoft in ASCII mode, then the problem is most
likely at your slave site (PLC, modem or cable). Reconfigure your slave site for 7/E/1 and then try
connecting to it. If you are successful, change a single parameter and then try again.
Try sending the following ASCII string inside the quotation marks “:087E011960(CTL+M)(CTL+J)” to
address 8. If a response returns, then communications is occurring and solution C, D or G (from
above) will likely fix the problem.
If you can not connect to the Technology Center, then the problem is with your host site. See if you
can connect to a bulletin board.
I cannot log-on with Modsoft but FactoryLink, Panelmate or other MMI devices communicate
to the PLC OK.
A) Your PLC or computer does not have the DSR line connected properly; it must be high (+V). The
DSR connection may be loose. In this case, Modsoft connects but fails to display the networks
properly and then stops. [3]
B) If using a bridge multiplexer, the DSR on the bridge multiplexer must be high (+V) as well as the
DSR on the PLC. A PLC ‘not there’ error, which shows up immediately when using a bridge
multiplexer, indicates that the DSR line is missing. The usual solution is to jumper the DTR line to the
DSR line (at the PLC and at bridge multiplexer). This is a good way to prevent program changes, yet
allow data transfers.
I can connect to bulletin boards just fine with ProComm but not to the PLC using Modsoft.
You may have some incompatibility with your computer or the ProComm Plus configuration. Modsoft
assumes you are using the standard IRQ for each communication port (COM 1,3,5 with IRQ4 and
COM 2,4,6 with IRQ3). Use Windows’ device manager program to find the modem’s interrupt
number. A non-standard interrupt number causes Modsoft and LModsoft to fail when using Modbus.
[1]
14
When attaching, the modem’s (PLC and/or computer) indicator lights do not look right.
If missing a critical light (TX, RX, DSR, CD or CTS when RTS comes on), the cable’s wiring is
incorrect. Sometimes a modem initialization string can compensate. You need a breakout box or an
in-line indicator to troubleshoot this problem (Radio Shack has them for about $15). What looks
right? Hard to tell; it just comes with experience. [3]
I get random characters after ProComm (Windows) connects, but not with the DOS version.
Are you using an Orchid video board in Windows? Their older driver has a problem with COM4. It
uses COM4 to communicate to the video board. Obtain their new COM4 driver or move your modem
to COM 1 or COM3. [1]
I can connect to the PLC, with and without a modem, using the default but not 8/N/1.
This is not a modem problem but you may experience this problem when using a modem. See if you
can make a direct wire connection using 8/N/1. If you can, then your mouse driver is likely the
problem.
This problem was first seen in 1993 with Saeger notebooks. These notebooks have mouse drivers
that search for a mouse on COM 1/2. The problem, which appears random, occurs when you boot
the computer without a mouse. It loads the mouse TSR using COM1 when no mouse is present.
When using Modsoft with COM1, the mouse driver should cause conflicts (or so one would think).
However, the mouse driver uses 8/N/2 so it’s apparently OK with your defaults. When using Modsoft
with other parameters, it produces errors. The solution is to prevent the mouse driver from loading
(remark it out in the autoexec.bat file) or use COM2 for Modsoft. [3]
This is not a likely problem now since Windows does not use DOS drivers.
Sometimes I can connect to the PLC with a modem and sometimes not. I see the RX light on
the PLC modem blinking when I am not transmitting data. What causes this to happen?
A) A TSR or a hardware device conflict (scanner, digitizer, or audio card) may exist. If you are using
COM 3/4, it shares a common IRQ between those devices. This is OK providing you do not get TSR
conflicts and the two devices do not try to talk at the same time. Try removing the card(s) and move
your modem to COM 1 or 2. [2]
B) A noisy phone line may cause this problem. Dial the phone number and listen to how the line
sounds. The modem may not tolerate the line noise. Try a phone line in another part of the city or try
another modem and try C) below. [2]
C) Replace the slave PLC with a computer and try communicating both ways between the two
computers. If you can not communicate, a bad modem or cable is likely.
D) Try Modbus ASCII mode, as it is more forgiving than RTU.
I can no longer communicate to my PLC after upgrading from a 584 or chassis mount 984 to a
slot mount 984.
The older units process Modbus RTU messages differently. Supposedly, older PLC’s incorrectly use
a parsing technique to determine the end of the Modbus message. Parsing allows for processing
messages with large delays between characters. The slot mount 984 PLC adheres to the 3-character
time-out function for determining the end of message. [2]
I get a message “communication port in use” when trying to connect in Windows or the
modem’s transmit light fails to blink when Modsoft tries to connect.
Add Com1AutoAssign=0 or Com2AutoAssign=0 to the [386ENH] section of the system.ini file when
using Windows 9X [3]. This does not work for Windows NT or Windows 2000.
I can not get Modsoft to work in DOS with my PCMCIA modem but it works with an external
modem.
Modsoft does not work with a PCMCIA modem in DOS. Use Windows instead or use an external
modem. It is unclear, at this time, why this happens.
15
I receive a non-supported PLC when connecting to my PLC with Modsoft.
You are using Modsoft Light and trying to connect to a PLC that the software does not support. Try a
different PLC address that has a Compact or Micro PLC. Use the full version Modsoft, if you wish to
connect to a 984E or Quantum. [3]
My modem says it supports 11 bits but whenever I dial a phone number, it reverts to 10 bits.
New information indicates that placing a phone call with an eleven bit “Hayes” compatible modem
causes it to revert to ten bits. It appears that the only way to get it to work is to store the number in
the modem’s memory and let it automatically dial the phone number. [1]
Some of my modems communicate properly and some don’t. Why?
Modem vendors change firmware from time to time. Firmware changes may affect handshaking,
timing, or processing options. This problem is most likely to occur when changing out a modem.
U.S. Robotic modems exhibiting this problem may not work when using Modbus RTU mode. [1]
This problem may also occur when replacing an older PLC or using a new PLC. This problem first
began in July of 1999 when Intel stopped producing a communication chip and an alternative chip
was substituted. IBM, Zoom, US Robotics modems work fine but Aprotek modems do not work with
the newer PLC’s. [6]
My Accura modem doesn’t communicate to my Practical Peripheral modem. I have tried
everything to get it to work. However, I can get a Zoom modem to communicate to it.
This appears to be similar to the above problem. It appears that a chipset or firmware may not be
very tolerant to noisy lines. Try using data compression and error correction (remove the &Q6). This
should provide better immunity to noise. It will, though, slow down communications throughput [2].
Hyperterminal doesn’t work with Modsoft. It works with another computer but not this one.
If your modem is a built-in internal modem, it is likely a DSP modem. The IBM Thinkpad 760E uses a
MWAVE internal DSP modem. AutoAssign appears not to work with this type of modem. Therefore,
Windows based dialing programs won’t work with DOS based programs. To see if this is happening,
view Hyperterminal’s window along with Modsoft. If Hyperterminal displays data when trying to
connect to the PLC, then this is the problem. Use an external modem or use a DOS based dialing
program to solve the problem [1].
I appear to connect immediately when logging onto my PLC and never see any logic.
This happens when the modem echoes back the characters to the computer before receiving the
Modbus reply message. If using a U.S. Robotic’s modem with option switches, make certain that
switch number four is down, not up [4]. A defective cable (a short between the transmitted and
receive data lines) will cause this as well [2]. A modem that has been improperly initialized can also
do this [1]. To find out if this is occurring, remove the cable where it connects to the PLC and then log
onto the PLC. An echo-back problem exists if a log-on occurs instead of a receiving a timeout error
message.
16
Notes:
i
Kenneth Jennings runs the Automation Technology Center in Los Angeles, CA. (Phone: 562/463-3568). This
document contains hyperlinks to various Internet sites. Microsoft’s Word displays these hyperlinks as blue
underlined text.
ii
Hyperlink to http://www.modicon.com/intr7.html.
iii
The V.90 standard was ratified by the International Telecommunications Union (ITU) during 1998. The V.90
standard replaces the x2 and K56 protocols.
iv
Contact Greg Meyer at Com-Net Industries (818/767-4440) for more information on Adtran or Advanced Digital
Networks.
v
Contact the Eazy company for more information on their modems.
The Bay Networks’ Alliance modem (model DD3804001) is a 2/4 wire, 10 bit leased line modem with an
AutoDial fallback mode. For more information, contact Signal Enterprises, Inc. (916/791-8802).
vi
vii
Hyperlink to http://www.modicon.com/wincgi/printform.pl.
viii
Thanks to Brad Nohr (formerly Square D), Mike Nease and Lloyd Trick (from MSO) for this information.
ix
Hyperlink to ftp://www.win-tech.com/demos/listen32.zip and http://www.win-tech.com.
ProWORX Plus, version 1.55, works with IBM’s Thinkpad PC. ProWORX NxT, version 2.0, works with
Compaq’s PC and an internal modem (V56VL global). Other PC’s and modems will likely also work but have not
been tested.
x
xi
Consult Microsoft’s Knowledge base document number Q130402.
xii
Hyperlink to http://www.pep1.modicon.com.
xiii
Hyperlink to ftp://www.mhz.com/pub/info/techbull/3116.txt.
xiv
Hyperlink to ftp://www.mhz.com/pub/info/techbull/3112.txt.
xv
For information about Megahertz PCMCIA modems, refer to their technical document #3112 (Cellular for
Megahertz PC Card Users) and technical document #3116 (Cellular Modem Functions).
xvi
Personal Communication Services was created in late 1993 and became operational during the latter part of 1996.
xvii
Hyperlink to http://www.attws.com/nohost/cellular/ce_new4.html.
xviii
Hyperlink to http://www.pacbell.com/products/wireless/pcs/index.html.
xix Using an unapproved cable can damage your modem and/or phone.
xx
Square D’s Los Angeles office will move in September 2000. The phone number will change at that time.
xxi
PLC addresses are likely to change.
xxii
Upload time in seconds for C984-145, with PLC running, using Modsoft. Modsoft uploads 16 registers for each
transaction. The PLC has 41 networks, 1083 words of logic, 32 segments, 1056 coils, 512 inputs, 128 input
registers and 1600 holding registers. The master modem is a PPI 14400 FXSA and the slave modem is a Zoom V32.
The direct connection is via a Modbus RS232 programming cable.
xxiii
Thanks to Mr. Peter Thiessen of Thiessen Engineering.
17
xxiv
Thanks to James Banks of Kintronic.
xxv
Many older modems are no longer available. Older modem prices are a best estimate.
xxvi
Numbers in brackets [] indicate the number of occurrences of this problem.
18