Model 72A Application Notes
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'
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.