ITU-TSS V.35 interface

V.35 is an interface (V.35 is the ITU - formerly CCITT standard termed "Data Transmission at 48 Kbps Using 60-108 KHz Group-Band Circuits") commonly used on higher speed circuits of 56kbps and above. Recognizable by its blocky, 34-pin connector; V.35 combines the bandwidth of several telephone circuits to provide the high-speed interface between a DTE or DCE and a CSU/DSU (Channel Service Unit/Data Service Unit). To achieve such high speeds and great distances, V.35 combines both balanced and unbalanced voltage signals on the same interface. Transmission is usually a synchronous protocol (note clocking pins i.e. receive and transmit clock). Although V.35 is commonly used to support speeds ranging anywhere from 48 to 64 Kbps, much higher rates are possible [ ISDN (64 or 128Kbps), Factional T1@ 128 Kbps to 1.544Mbps T1, ATM and Frame Relay]. V.35 cable distances theoretically can range up to 4000 feet (1200 m) at speeds up to 100 Kbps. Actual distances will depend on your equipment and the quality of the cable . Lan routers often come equipped with a V.35 electrical interface but many today are using a HSSI interface at speeds higher than T1 and above (typical T3 45Mbps).

Pin	Name	Dir	Description
A			Chassis Ground
B			Signal Ground
C	RTS		Request To Send
D	CTS		Clear To Send
E	DSR		Data Set Ready
F	DCD		Data Carrier Detect
H	DTR		Data Terminal Ready
J	LL		Local Loopback
K			Local Test
L			unused
M			unused
N			unused
P	TxD-		Send Data A
R	RxD-		Receive Data A
S	TxD+		Send Data B
T	RxD+		Receive Data B
U			Terminal Timing A
V			Receive Timing A
W			Terminal Timing B
X			Receive Timing B
Y			Send Timing A
Z			unused
AA			Send Timing B
BB			unused
CC			unused
DD			unused
EE			unused
FF			unused
HH			unused
JJ			unused
KK			unused
LL			unused
MM			unused
NN			unused

V.35 cable is balanced multipair with characteristic impedance 80-120 Om at the fundamental frequency.
Voltage is 0.55v+/-20% with 100 Ohm load.

There seem to be two versions of the V.35 pinning. One is called V.35 Winchester, the other is called V.35 Straight. One is mirrored to the other. We don"t know how it is mirrored. This information came from a 3Com source. Their Access Builder 4000 V.35 Card interface is V.35 Straight they say. The Dutch PTT supplies V.35 interface according to V.35 Winchester. Connecting these two requires some sort of crossed cable.

Note: Direction is DTE (Computer) relative DCE (Modem)