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.
Fourthly, there is the need for the modem to supply full duplex
communications over 2 pairs of twisted pair cable.
Finally, there is the question of the data interface to the modem, the
connector where data comes in from a data device and goes out of the
modem to a data device. In most cases this is a 25 pin DB25 connector.
However, with the exploding use of PCs there has been an ever increasing
need to connect short haul modems to the PC Com Port. The Com Port has 9
pins, and a short haul modem would need a correspondingly 9 pin DB9
All of these issues can be conveniently handled by the Model 209 short
haul modem. Its employment in a typical application is provided by the
Here we have two communicating data devices, a PC on the left and a
Terminal server for and Ethernet LAN on the right. The communications
required is full duplex and asynchronous. It is at a maximum rate of
19.2 KBPS over less than 1 mile. The PC is sending/receiving data
to/from the LAN by way of its 9 pin Com Port.
The Model 209 is ideally suited to this application on the PC end. It
can handle the speed and distance requirements. Furthermore it connects
to Com Port of the PC.
Notice in the illustration the Model 201 handling the task on the
Terminal Server side. It is connected to a DB25 port. Its
transmission/reception is fully compatible with the Model 209 as is also
the Model 221. This is another special feature of the Model 209 - its
compatibility with these other modems.