Digital Equipment ChannelWorks, Руководство по установке сети

Страница: 68 / 90

A-2 ChannelWorks Network Installer’s Guide
Serial Modem Cables
A modem cable is a type of serial cable that is used to connect a modem to the
ChannelWorks Bridge. If you are connecting a Hayes
®
or Hayes-compatible modem,
you must obtain a cable with a male connector for the modem end (normally 25 pins)
and a female connector for the ChannelWorks Bridge end (9 pins).
In a serial modem cable, the pins in the connectors are wired straight through. This
means that the pins function identically on the two connectors at either end of the cable.
You can obtain a serial modem cable from most computer dealers or computer supplies
stores.
Null Modem Cables
Null modem cables are another type of serial cable used to connect a PC to the
ChannelWorks Bridge. Null modem cable wires are attached to the pins in the connec-
tors differently from how they are attached in a modem cable. The cable should have a
9-pin female connector for the ChannelWorks Bridge end, and a 9-pin female connec-
tor for the PC end.
You can obtain ready-made null modem connectors from most computer dealers or
computer supplies stores.
The serial null modem cable is designed for devices that send and receive data on the
same pins. Like the ChannelWorks Bridge, PCs 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 wire, these wires must be crossed.
If you are making your own null modem cable, you must connect the wires as follows
(see Figure A-1):
• Connect the wire from pin 2 on the bridge end of the cable to pin 3 on the PC end.
• Connect the wire from pin 3 on the bridge end of the cable to pin 2 on the PC end.
• Connect the wire from pin 7 on the bridge end of the cable to pin 8 on the PC end.
• Connect the wire from pin 8 on the bridge end of the cable to pin 7 on the PC end.