Serial Cables
89
D
Null Modem Cables
Null modem cables are another type of serial cable. However, these cable
wires are not attached to the pins in the connectors in the same way as a
modem cable. Terminals and printers use a null modem cable.
If you cannot purchase a ready-made null modem cable, you can make one by
connecting a null modem converter to a modem cable, as the following figure
shows. The end that connects to your patch panel must be male. The gender
of the connector at the other end of the cable depends on the peripheral device
you are connecting to your patch panel.
Figure D-2
An Assembled Null Modem Cable
The serial null modem cable is designed for devices that send and receive data
on the same pins. Terminals and printers are DTE devices — both expect to
send data on pin 2 and receive it on pin 3. Because both devices are trying to
send and receive on the same wire, these wires must be crossed.
If you are making your own null modem cable, you must connect the wire
from pin 2 on the system unit end of the cable to pin 3 on the device end, and
connect the wire from pin 3 on the system unit end of the cable to pin 2 on the
device end. See the following figure for wiring your cable.
A null modem cable also disables certain features of a peripheral device by
jumpering wire(s) from one pin to another pin on the same connector. The
following figure shows pins 5 and 6 jumpered, and then connected to pin 20.
Also, pins 4 and 8 are wired to each other. A cable wired like the one in this
figure will connect your patch panel and a Wyse-compatible terminal.
Null modem converter
Modem cable
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