V.35 Technical Reference

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, examples are: RS530, RS449, 10/100/1000baseT.

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.

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V.35 Signal Descriptions

Name Pin Description Type

FG A Frame/Chassis Ground -

SG B Signal Ground -

SDA P Send Data A Differential

SDB S Send Data B Differential

RDA R Receive Data A Differential

RDB T Receive Data B Differential

RTS C Request To Send Unbalanced

CTS D Clear To Send Unbalanced

DSR E Data Set Ready Unbalanced

DTR H Data Terminal Ready Unbalanced

RLSD F Received Line Signal Detect Unbalanced

TCEA U Transmit Clock Ext A Differential

TCEB W Transmit Clock Ext B Differential

TCA Y Transmit Clock A Differential

TCB AA Transmit Clock B Differential

RCA V Receive Clock A Differential

RCB X Receive Clock B Differential

LL J Local Loopback Unbalanced

RLB BB Remote Loopback Unbalanced

TM K Test Mode Unbalanced

- L Test Pattern Unbalanced

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V.35 Cable Design

The design of your cable depends on what you are connecting together and the interfaces involved. There are two standard interface 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. All differential pairs must be twisted.

DTE to DCE

DTE DCE

P ------------------- P

S ------------------- S

R ------------------- R

T ------------------- R

C ------------------- C

D ------------------- D

E ------------------- E

H ------------------- H

Y ------------------- Y

AA ------------------- AA

V ------------------- Y

X ------------------- X

DTE to DTE

This cable design assumes that both devices provide their own 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 transmit 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 DTE

P ------------------- R

S ------------------- T

R ------------------- P

T ------------------- S

C ------------------- D

D ------------------- C

E ------------------- H

H ------------------- E

Y & U ------------------- V

W & AA ------------------- X

V ------------------- Y & U