V.35 Technical Reference
Page 1 of 5
V.35 Interface
V.35 has been around for quite some time. It was originally designed for a 48 kbps synchronous modem
- that's right, officially its top rated speed is 48 kbps. However, V.35 has been used for many years in
applications running from 20 kbps up to and past 2 Mbps. In 1989, CCITT BLUE BOOK (ITU)
recommended the interface become obsolete and replaced it with the V.10/V.11 standard. However,
V.35 still remains popular, and has evolved to using the specifications from V.11 for the differential
signals, while the control signals remain unbalanced. The V.11/V.35 BLUE is fully interoperable with
the old V.35 RED interface, except V.35 RED may not handle the speed and distance of the newer spec.
In many years of testing, I have not found any system, DSU/CSU, Router, Frame Relay Bridge, etc. with
incompatible V.35 interfaces.
Most of the V.35 signals are for control and handshake purposes (like RTS, CTS, DSR, DTR) and these
are implemented in unbalanced fashion, similar to RS232/V.24. This approach is simple, inexpensive,
and is usually adequate for these relatively invariant signals.
V.35 gets its superior speed and noise immunity by using differential signaling on the data and clock
lines. Unlike RS232/V.24 which uses signals with reference to ground, V.35 receivers look for the
difference in potential between a pair of wires. The wires can be at any potential, the signal is carried by
voltage differences between the two wires. Now the secret; by twisting these two wires, it becomes
likely that noise picked up on one wire will also be picked up on the other. When both wires pick up the
same noise it has the affect of cancelling itself - as the same noise impulse on both wires is invisible to
the receiver. Remember the receivers are only looking at the difference in voltage level of each wire to
the other, not to ground. Many high speed interfaces use this same technique: RS530, V.11, RS449,
10/100/1000baseT, etc.
The differential signals for V.35 are commonly labeled as either "A" and "B". Wire A always connects
to A, and B connects to B. Crossing the wires just inverts the data or clock. I have never seen any piece
of equipment damaged from this, but they don't work this way, either.
V.35 Connector and Pinout
V.35 Connector (Female M34)
V.35 Pinout (Male M34)
file://C:\A-DATA\AA-EDU\AA-FLDR-LEC\FILE-RAW\V35\V_35%20Technical%20Refer
... 5/15/2004
V.35 Technical Reference
Page 2 of 5
V.35 Signal Descriptions
Name
FG
SG
SDA
SDB
RDA
RDB
RTS
CTS
DSR
DTR
Pin
A
B
P
S
R
T
C
D
E
H
RLSD
F
TCEA
TCEB
TCA
TCB
RCA
RCB
LL
RLB
TM
U
W
Y
AA
V
X
J
BB
K
L
Description
Frame/Chassis Ground
Signal Ground
Send Data A
Send Data B
Receive Data A
Receive Data B
Request To Send
Clear To Send
Data Set Ready
Data Terminal Ready
Received Line Signal
Detect
Transmit Clock Ext A
Transmit Clock Ext B
Transmit Clock A
Transmit Clock B
Receive Clock A
Receive Clock B
Local Loopback
Remote Loopback
Test Mode
Test Pattern
Type
Differential
Differential
Differential
Differential
Unbalanced
Unbalanced
Unbalanced
Unbalanced
Unbalanced
Differential
Differential
Differential
Differential
Differential
Differential
Unbalanced
Unbalanced
Unbalanced
Unbalanced
V.35 Cable Design
The design of your cable depends on what you are connecting together and the interfaces involved.
There are two standard interfaces types "Data Terminal Equipment" (DTE) and "Data Communication
Equipment" (DCE). Usually, but not always, the interface "facing away" from the network is the DCE,
and the interface "facing toward" the network is the DTE. The DCE normally supplies the clock.
DTE to DCE (differential pairs must be twisted)
DTE
DCE
P ------------------- P
S ------------------- S
file://C:\A-DATA\AA-EDU\AA-FLDR-LEC\FILE-RAW\V35\V_35%20Technical%20Refer
... 5/15/2004
V.35 Technical Reference
Page 3 of 5
R
T
C
D
E
H
Y
AA
V
X
-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
R
R
C
D
E
H
Y
AA
Y
X
DTE to DTE
This assumes that the devices have an external transmit clock, not all equipment does, in which case a
modem eliminator with clock will be needed. If only one device has a clock you might be able to get
away with using the one clock to drive tranmsit and receive in both devices, however if it is the old type
interface it probably won't work (the impedance will be too low).
DTE
P
S
R
T
C
D
E
H
Y&U
W & AA
V
X
-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
DTE
R
T
P
S
D
C
H
E
V
X
Y&U
W & AA
How to Build a V.35 Interface
It is important to remember that the CCITT (UIT) in 1989 recommended that V.35 RED is obsolete and
recommended that the V.11 (RS422) interface be used for the differential interface. This makes the V.35
BLUE interface easier to design and better in performance. It also removes the need for a -5 Volt power
supply. I have built both the new and old V.35 interfaces and have found them to be fully compatible.
V.35 Differential Driver
The resistors Za and Zb are optional. I recommend 10 Ohms to bring the interface to 50 Ohms and to
file://C:\A-DATA\AA-EDU\AA-FLDR-LEC\FILE-RAW\V35\V_35%20Technical%20Refer
... 5/15/2004
V.35 Technical Reference
Page 4 of 5
provide some protection from EMF. Here is a Tip: Note that the A or + signal is on the inverted output
pin of the driver, most designers get this switched in their first design. This happens because some data
books call the positive pin A and the negative pin B. The V.35 A or + must be the inverted pin. This is
also true of the receivers.
V.35 Differential Receiver
The resistor Zt is optional. I recommend 150 Ohms to reduce reflectance. However If you are trying to
build a non intrusive receiver I would leave this out. Note this interface when left floating will have an
unpredictable output. Some engineers place pull up (to pin A) and pull down (to pin B) resistors of 10K
to provide a known state when the cable is unplugged or connected equipment is turned off.
V.35 Unbalanced Driver
V.35 Unbalanced Receiver
V.35 Interface (some signals not shown)
file://C:\A-DATA\AA-EDU\AA-FLDR-LEC\FILE-RAW\V35\V_35%20Technical%20Refer
... 5/15/2004
V.35 Technical Reference
Page 5 of 5
This interface is designed as a DTE, to build a DCE just reverse the Drivers and Receivers.
V.35 Breakout and Conversion Tools
ADVICE now manufactures unique products for working with the V.35 interface:
o V.35 Modify And Test Set - a V.35 breakout
o Universal Modify Test Set - a V.35/RS232/RS449/RS530 breakout and interface converter
o Exchanger - Interface converter (10 permutations)
The V.35 MATS and UMATS products allow you to break connections and test different signal
configurations. The V.35 MATS has a built in converter for monitoring the interface on a RS232 data
scope. The UMATS and Exchanger allows conversion between many types of interface: V.35 to RS232
or RS449 or RS530.
Legal
file://C:\A-DATA\AA-EDU\AA-FLDR-LEC\FILE-RAW\V35\V_35%20Technical%20Refer
... 5/15/2004