Understanding Network Hardware
Connecting RS-232 Devices with Short-Haul Modems
While some might not consider a short-haul modem to be a true network device, it is a
device that your network might require to provide point-to-point connectivity between
a workstation or terminal and another device. Short-haul modems (sometimes called
line drivers) enable you to connect two distant RS-232C devices to one another.
Standard RS-232C cables are limited in distance to 15 to 30 meters (50 to 100 feet).
Short-haul modems allow the same connection to run as far as 5 miles using simple
telephone-grade twisted-pair cabling.
Short-haul modems can often be perfect solutions when a computer needs terminal
access to a remote device. For example, a user might need to access a terminal on a PBX
telephone system, which uses an RS-232C port. You have two options to provide this
Install regular modems on each end and use a telephone connection to connect
from the workstation to the PBX.
Use two short-haul modems and run a twisted-pair cable between the two
Depending on how frequently access is needed and how distant the device is, either
approach can be good. Generally, short-haul modems are preferred when the two
devices often or always need to be connected, and running a twisted-pair wire between
the locations is not prohibitively expensive or difficult. Short-haul modems are fairly
inexpensive, at about $100 each.
In most short-haul modem systems, two pairs of wire connect each short-haul
modem, although one-pair variants exist. With the two-pair variety, one pair is used to
transmit data and the other to receive data. Most short-haul modems are full duplex,
allowing transmission to take place in both directions simultaneously.
To hook up two devices using short-haul modems, you use a standard RS-232C
cable to connect each device to its short-haul modem. Then you wire the twisted-pair
wire to the short-haul modem, using the instructions that come with the modem.
Finally, most short-haul modems require external power, so you need to plug them into
a power outlet. Figure 6-5 shows an example of a short-haul modem connection.
If you frequently do RS-232C interfacing, you should invest in a device called a breakout
box. This is a small device that has two RS-232C connectors on each end. In the box, each of
the RS-232C pin signals is represented with a light-emitting diode (LED). Special patch posts and
switches in the breakout box enable you to reconfigure the RS-232C connection on the fly. Breakout
boxes can be invaluable for achieving RS-232C communications between two devices that aren't
communicating. They can show what is actually happening with the signals and enable you to try
different cable configurations dynamically. Once you use the breakout box to figure out how to make
the devices communicate, a permanent cable can then be made to those specifications.