Author: Roy Eassa
Date: 11:36:34 01/16/01
Go up one level in this thread
A modem is considered Data Communications Equipment (DCE). Most everything else
you'll encounter, including a PC, is considered Data Termination Equipment
(DTE). The use the pins in a serial connection differently (each receives on
the pin the other sends on, etc.). The cable to connect them is a standard
serial cable, often referred to as "straight-through" because each pin connects
to the same numbered pin on the other connector.
To connect two DTE devices, like 2 computers, using a serial cable, you need a
special cable that acts as if it were connecting the 2 PCs via a pair of modems
and a phone line. That's why it's called a null modem cable (you have a null
modem because you are NOT using a modem, I guess). To get this effect from a
single cable, certain signals need to be "crossed over" from one pin to another.
That's why this cable is often called a crossover cable.
In the datacomm world, you often see both kinds of cables lying around in labs,
and people get really angry if the straight-through and crossover cables are not
properly labeled as such! And when there's a problem, the first suspect is
always that you have the wrong kind of cable.
Finally, there are small adapters ("breakout boxes") that can be used to view
and/or change the pin settings at a connector. These cost a little more but are
very useful and flexible.
(PS: The datacomm field pays its engineers very well. Now you know one reason
why!)
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