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4.4
CONNECTION TO THE MODEM
The cable connecting the HyTerm to the modem is
approximately 10 feet (3 meters) long. It contains ten size
22 AWG conductors. It is permanently connected to the
HyTerm internally, and exits from the rear. The modem
end of the cable is terminated in a 25-pin subminature
connector, a Cannon or Cinch DB-25P, or AMP housing
205208-1 with 1-66506 pin inserts, or equivalent. This
mates with the socket found on most modems, a Cannon or
Cinch DB-25S, or AMP housing 205207-1 with socket
contacts 1-66504 or 1-66505. The plug is enclosed in a
hood (AMP 206472-1, or equivalent) to protect the plug
wiring. Pin assignments are shown in Figure 4-2. (Note that
this figure depicts the male connector on the end of the
cable; the modem plugs would be a mirror image of this
drawing.) See also Table 4-1.
Extension cables are available for use when the modem
must be located more than ten feet from the HyTerm.
Note, however, that the total length of all cables between
the HyTerm and the modem shou Id not exceed 50 feet (15
meters).
4.5
SIGNAL LEVELS
Signal voltages on the interface lines are nominally +12
and -12 volts. The EIA standard allows for voltages
anywhere between +25 and -25 volts, and the HyTerm
input receivers are capable of withstanding these levels. The
actual values measured on the HyTerm output lines will be
approximately 7 to 9 volts. Because of the possible wide
variation in actual voltage levels, the nominal values of +12
and -12 will be used in this discussion.
OFF
ON
OFF
+12V
Data signal states are referred to as "mark" and "space",
where a mark denotes a logic 1 and space denotes a logic O.
All other signals (control signals) are referred to as either
"on" or "off". Table 4-2 summarizes the relationship
between these terms and their voltage levels.
Table 4-2.
Interface Signal Terminology ,
Data
Control
Nominal
Circuits
Circuits
Voltage
MARK ("1")
OFF
-12
SPACE ("0")
ON
+12
Input receivers on the control lines provide "fail-safe"
operation: if an input control line should go to ground level
or be open-circuited (as when the connecting cable is
unplugged), it will be interpreted as being "off". Data
inputs are handled differently: if a data input is open-
circuited, it will be interpreted as still being in the same
state it was in previously. This provides greater noise
immunity on the data input lines. This is illustrated in
Figure 4-3, which also shows the actual voltages at which
the transitions between states occur.
Note also in Figure 4-3 that the level at which the input
signal changes from off to on (or from mark to space) is
different from the level at which it changes back. This
"hysteresis" provides additional noise immunity.
4.6
INTERFACE SIGNALS
The EIA signals wired to the interface connector are
I isted in Table 4-1.
MARK
SPACE
MARK
OV ----~~--------~~--------------------~----------~~------
-12V
FAIL-SAFE OPERATION
(CONTROL
SIGNALS)
NORMAL OPERATION
(RECEIVED
DATA)
Figure 4-3. Input Line Switching Levels
4-3
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