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. We mention two of interest when communication is in the heavy industrial environment. These are the issues of isolation and 'equipment generated' electromagnetic interference.

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. In an attempt to achieve equilibrium between different ground points ground currents are generated. 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 heavy industrial environment is quite harsh from the viewpoint of electromagnetic interference. The presence of high-powered electrical tools and other equipment generally means the generation of electromagnetic waves. These are often coupled onto cables that are used to realize the communication link. The coupled waves are usually referred to as 'common mode interference.' A convenient way of combating this type of interference is to use modems that employ differential signaling. Here the data being communicated is represented by the difference between voltage levels on different cables in the pair. Any common mode interference will get cancelled in taking the difference.

The Model 72A short haul modem is extremely attractive for configuring a data communications link in the heavy industrial environment. It has isolation to protect against ground currents and employs differential signaling to protect against common mode interference. A typical application is shown in the illustration above. Here we have a factory environment with a PC controlling a machine tool located in the factory's machine shop. The PC is located some distance from the machine shop, say at least, several hundred feet. In order to effect the control there must be full duplex communications. The PC issues instructions to the machine tool. The machine tool responds back with either acknowledgements and/or status updates. The nature of the communication is asynchronous. The Model 72A realizes such full duplex communications by using two cable pairs. A cable pair is dedicated to each direction. It can support data rates up to 19.2 KBPS over a distance of 1 mile, more than enough for factory communications.