Within the premises or local area environment the short haul modem is a
convenient device for configuring a reliable communications link. When
the distance between communicating data equipment gets beyond 100 feet
signals need 'to be boosted' or they will not be received and decoded
reliably. Using a pair of short haul modems in the link, one for
transmitting and one for receiving in each direction, boosts the signals
and gives the reliability.
However, a number of items always seem to come up when using a pair of
short haul modems to deal with this rather straightforward problem.
First, there is the matter of powering the modems. A modem needs
electrical power to work. Many times the network installer can look
around for an electrical outlet and find that either one is not
available or is quite far from the device and currently in use.
Secondly, there is issue of distance and speed in a 'no frills'
approach. In many situations the network installer needs a short haul
modem but wants just a basic 'work horse.' - that is, a unit that can
give a reasonable speed of 19.2 KBPS over a reasonable distance of about
1 mile. The installer doesn't want or need or desire to pay for any
other 'fancy features.'
Thirdly, there is the issue of providing protection against power
surges. These often couple into data devices through modem connections
and can cause extensive damage.
Finally, there is the need for the modem to supply full duplex
communications over 2 pairs of twisted pair cable.
All of these issues can be conveniently handled by the Model 201 short
haul modem. Its employment in a typical application is provided by the
Here we have two communicating data devices, a badge swipe reader and a
host computer. The badge swipe reader may be located at the entrance of
some office building. As employees enter/exit they swipe their badges
through the reader. The badge swipe reader takes their employee code and
sends it to the host computer. This computer records the time an
employee has spent at work and forwards this to the payroll department.
Employees are, of course, lined up waiting to swipe their badges. At the
same time the host computer is sending back to the badge reader
acknowledgements of previous swipes and clock correct updates.
Consequently, the communication is full duplex and by its nature is
asynchronous. The data rate needed to support this type of application
is moderate, no more than 19.2 KBPS. Distances are also moderate - being
As you see the communication is accomplished by having two twisted pair
cables. Each cable is dedicated to a specific direction of the
communication, either badge reader-to-host computer or vice versa. While
you can not see it in the illustration there is no available, that is
unused, electric outlet either near the badge swipe reader or the host
The Model 201 shown in the illustration fits the needs of this
application quite well. It is a full duplex, short haul modem where full
duplex is achieved by using two separate cable pairs. It transmits data
in an asynchronous mode. The Model 201 can transmit at data rates up to
19.2 KBPS for links of about 1 mile on 26 AWG cable. It can provide
reliable transmission for up to about 10 miles if a lower data rate of
1.2 KBPS is OK and the cable is the lower attenuation 19 AWG.
The Model 201 not only reliably transmits data, but also has built-in
The modem also operates by powering itself from the interface to which
it is attached. It does not need an electrical outlet.