Telos Zephyr Xstream User Manual
Page 169
USER’S MANUAL
Section 9: The V.35/X.21 Interface Option 157
9.3 Hook-Ups to Non-ISDN Synchronous Networks
The V.35/X.21 Connection
If present, the V.35/X.21 interface is supplied in slot B slot, looking from the rear. It permits
connection to transmission paths other than ISDN. It may also be used with external ISDN
Terminal Adapters in the (rare) case where the available ISDN service is not compatible with the
Zephyr’s internal TA, or for emergencies.
?
CURIOSITY NOTE!
You might wonder why the much more common RS-232 is not used. Answer: V.35 is
synchronous, meaning that the bit clock is transmitted between the two ends.
RS-232 is typically used for self-clocking (asynchronous) applications, and requires overhead
start and stop bits, slowing and chopping the bit flow. And, just as with audio, balanced
transmission is more reliable in a noisy environment, or in one which has ground potentials
at differing levels.
V.35 is a standard for connecting to synchronous digital data paths. Like RS‐232, it defines
signals and (not officially) connectors and pin‐outs so that equipment from various
manufacturers may talk with each other. The usual connector is a big boxy AMP type, which
was chosen by AT&T decades ago. Most terminal equipment sold for the US market supports
the V.35 standard (although an adapter cable may be required to connect to the usual
connector). The following Signals are provided at the V.35 end of the Telos cable:
V.35 CONNECTOR PIN-OUT
Pin
Description
Direction (Xstream ◄► DCE)
B Ground
C #
RTS (Request to Send)
►
F
CD (Carrier Detect)
◄
H #
DTR (Data Terminal Ready)
►
P *
TX Data
►
R
RX Data
◄
S *
/TX Data
►
T
/RX Data
◄
V RX
Clock
◄
X
/RX Clock
◄
Y TX
Clock
►
AA
/TX Clock
►