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 illustration above.

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 just 'in-building.'

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 computer.

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 surge protection.

The modem also operates by powering itself from the interface to which it is attached. It does not need an electrical outlet.