Advanced IC Engineering Inc.

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.

V.35 Signal Descriptions

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 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).

V.35 Differential Driver

The resistors Za and Zb are optional. I recommend 10 Ohms to bring the interface to 50 Ohms and to 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

The typical driver is the standard RS232 driver 1488, however this requires + and - 12 Volts.  Many of the new interfaces are using the V.24 / RS232 driver chips with the charge pump built in, so they only need +5V to operate.  These come in many configuration some include both drivers and recievers.  One of the typical is the DS14C232 from TI or Max 232. 

V.35 Unbalanced Receiver

The typical IC used for the reciever is the 1489, it only requires 5 volts to operate, but most new designs are using the charge pump IC which have both the receivers and drivers in the same chip. 

V.35 Interface
(some signals not shown)

This shows a typical V.35 interface.  A few signals like LT have not been included.   The box is a M.34 connector set up in the DTE configuration.

V.35 Breakout, Testing and Conversion

ADVICE manufactures unique products for working with the V.35 interface:

• V.35 Modify And Test Set - a V.35 breakout
• Universal Modify Test Set - a V.35/RS232/RS449/RS530 breakout and interface converter
• Exchanger - Interface converter (10 permutations)

The V.35 MATS and UMATS products allow you to make/break connections to diagnose and solve cabling connection problems. 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.

If you have comments or questions about the information presented, please contact us.