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.
The first issue involves the need to satisfy data transmission and speed
requirements. They must meet the under application's needs. They also
need to be met relative to the interference environment within which the
communications is taking place. Certain environments, such as office
building settings, present relatively benign environments where
background noise is the only problem. Others, such as manufacturing
facilities, present harsh environments. Here one may have to deal with
Electromagnetic Interference (EMI) from high-powered production tools,
Radio Frequency Interference (RFI), power surges and other deleterious
effects. In many cases RS-422 communications by employing differential
signaling can effectively deal with the deleterious effects of such
interference. However, there are many manufacturing environments where
the RS-422 approach just does not provide enough protection.
Secondly, 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.
Carrying out premises data communications in the manufacturing
environment by using fiber optic cables presents several ready
advantages. First, there is tremendous bandwidth potential. It can deal
with traffic from relatively low data transmission rates to T1 (1.544
MBPS) to and well beyond. Applications that require very high data
transmission rates can be easily accommodated. Secondly, there is the
protection that fiber optic transmission provides against the variety of
deleterious effects which plague transmission over copper cable. These
include the resistance that fiber optic transmission has to
Electromagnetic Interference (EMI), lightning induced current surges and
ground loops. Finally, there is the protection that fiber optic
transmission has with respect to 'tapping.' It is much more secure with
no effective radiation of the communication occurring out of the cable.
The Model 272A fiber optic line driver presents a convenient way of
dealing with many of the issues described above and bringing effective
data communications to the manufacturing environment. To begin with it
realizes a data communications link over fiber optic cable. Thus, it
obtains all of the benefits of fiber optic transmission with respect to
interference and isolation. The data interface to this modem is by
RS-422. Therefore, the modem presents a convenient way of evolving a
network from the differential signaling protection provided by RS-422 to
the enhanced protection provided by fiber optic transmission. The modem
can achieve data rates of 2 MBPS over 2 km which is certainly suitable
for most settings.
The employment of the Model 272A in a typical application is provided by
the illustration above. Here we have the manufacturing environment with
all of the harsh interference and isolation needs that this implies.
There is need for a full duplex, point-to-point link, between a PC -
operating as a type of work station - and an Intelligent Machine
Controller. The Intelligent Machine Controller is itself directing
automated tools within the environment. The Intelligent Machine
Controller has the RS-422 interface and could use this type of
communications to deal with the harsh interference environment. However,
effecting the fiber optic link shown provides an even better solution.
The use of dual fiber cable allows a link in both directions and as a
result brings about full duplex communications.