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, is the issue of speed and distance. The network architect always wants to get the highest speed over the longest distance - and of course in the most economical way.

Secondly, there is the issue of cabling. In many premises environments cabling to form the data communications link connecting the modems is not always readily available. Additional pairs of cable are 'tough to get.' This presents a particular problem when full duplex transmission is needed. Being able to realize it by dedicating a single pair of cables to each particular direction may be 'out of the question.' The network architect may have to look to achieve full duplex transmission using just a single cable pair to implement the link.

Finally, there is the matter of isolation. The need for isolation arises when ground currents are present. In many instances the data equipment communicating must be grounded at different points. Different grounds imply different reference levels for voltages. Ground currents are generated in an attempt to achieve equilibrium between different ground points. From the perspective of communications such currents make themselves known as an additional interference mode. Isolation provides a barrier between different ground points. This barrier does not allow ground currents to be generated. Isolation allows equipment being grounded at different points to be connected together without the need to worry about the interference due to ground currents.

The Model 214A can handle all of these issues. Its employment in a typical 'store' application is shown in the illustration above. Here we have a PC communicating with a host computer. The two computers are interchanging files of accounting data. The PC located at a point of sale is sending the host rapid updates on 'sales being made.' The host is sending back continuous updates on 'available inventory.' The only way the two computers can keep up with each other is by having full duplex transmission. However, note that the communication is asynchronous, as it is being driven by individual sales that occur randomly in time. Here speed and distance are definitely issues. For the computers to keep up with the processing they have to be able to receive the data as fast as possible over the distance, which is perhaps 1000 feet. However, cabling is a problem. There is only one spare cable available to configure the link. The Model 214A works well in this situation. It can provide a data transmission speed of up to 38.4 KBPS at a distance of up to 3 miles, asynchronously and full duplex on just a single cable pair.

Returning to the application in the illustration. The two computers as well as the modems are most likely grounded at different points. Ground currents may develop which can interfere with the communications. The Model 214A though has transformer isolation, which can ameliorate this deleterious effect.