High-Speed Serial: V.35

V.35 is an old specification that has been obsolete since 1988. Most “V.35” interfaces are actually V.36, V.37, or V.38 interfaces. V.35 is fairly sparse—it does not even specify the form factor of the interface. ISO 2593 contains the specifications for the blocky, long-pinned Winchester connector. However, the industry refers to such interfaces and cables as V.35, so I will too in order to maintain consistency.

To save space, V.35 uses common-ground transmission on signaling leads, but the data and clock signals use differential signaling. Connectors have 34 possible pins, which are lettered A through NN. (G, I, O, and Q are not used.) When used for data-communications applications, connectors typically only have 17 or 18 pins. To ensure that the cables are inserted correctly, the screw attachment mechanism allows connectors to be attached only one way. Figure E-4 shows male and female V.35 connectors.

Male and female V.35 connectors

Figure E-4. Male and female V.35 connectors

Each pin is given a purpose and is assigned to a particular serial circuit. Figure E-5 shows the circuit assignments by abbreviation. More detail on the circuits is in Table E-2.

Circuit identifiers

Figure E-5. Circuit identifiers

Table E-2. V.35 pin assignments

Pin

V.24 circuit

Circuit name

Circuit abbrev.

Circuit type

Circuit direction

A

101

Protective Ground Shield

PGNDSHD

Ground

N/A

B

102

Signal Ground

GND or SGND

Ground for common-ground circuits

N/A

C

105

Request to Send

RTS

Common-ground control

To DCE

D

106

Clear to Send

CTS

Common-ground control

From DCE

E

107

Data Set Ready

DSR

Common-ground control

From DCE

F

109

Received Line Signal DetectorCarrier Detect

RLSDDCD

Common-ground control

From DCE

H

108.2

Data Terminal Ready

DTR

Common-ground control

To DCE

L

141

Local Loopback

LL

Common-ground control

To DCE

N

140

Remote Loopback

RL

Common-ground control

To DCE

P

103

Send Data A

TD ATX+, SD+

Differential Data

To DCE

R

104

Receive Data A

RD ARX+, RD+

Differential data

From DCE

S

103

Send Data B

TD BTX-, SD-

Differential data

To DCE

T

104

Receive Data B

RD ARX-, RD-

Differential data

From DCE

U

113

Serial ClockTransmit External A

SCTE A

Differential timing

To DCE

V

115

Serial ClockReceive AReceive Timing

SCR ART+ RxClk+

Differential timing

From DCE

W

113

Serial Clock Transmit External B

SCTE B

Differential timing

To DCE

X

115

Serial ClockReceive AReceive Timing

SCR BRT-RxClk-

Differential timing

From DCE

Y

114

Serial Clock Transmit ASend Timing

SCT AST+TxClk+

Differential timing

From DCE

AA

144

Serial Clock Transmit B

SCT BST-TxClk-

Differential timing

From DCE

NN

142

Test Mode

TM

Common-ground control

From DCE

Even though Pin NN, the Test Mode signal, can occasionally be used, it is not frequently pinned out. Pin J, the Received Line Signal Detector (RLSD) is not used and not pinned on data communications equipment. Pins M, Z, BB, CC, DD, EE, FF, and MM are reserved for future standards and are not pinned. Pins HH, KK, JJ, and LL are reserved for country-specific features, but are not pinned out.

Common V.35 Cabling Problems

In T1 circuits, V.35 runs at 1.5 Mbps. Do not be tempted to save money by making your own cable according to a manufacturer pinout. V.35 needs to run fast within tight electrical tolerances. Hand-built cable will not work. Any money saved by scrimping on cable will haunt you when the cable turns out to have a slightly flaky lead or improper grounding.

Warning

Custom-built V.35 cables will not work. Do not try this at home!

Long V.35 connections may cause problems if the ground level at the two ends of the connection is too different. If a voltage differential exists between a third-floor router and a basement CSU/DSU, control signals may not be interpreted correctly.

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