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 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, usually present relatively benign environments where
background noise is the only problem. However, they are not always
benign. The presence of air conditioning equipment and fluorescent
lights may present harsh interference conditions. Others settings, such
as manufacturing facilities, always 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.
Secondly, there is the issue of topology. The data equipment
communicating may be able to be served by just a straight point-to-point
link. However, there are situations where the data equipment
communicating may be organized in a ring (daisy chain) configuration.
Ring configurations are often employed to realized multi-drop
topologies, commonly used in polling networks. Such networks have a
'master' host computer successively poll 'slave' devices. The master and
slaves are set up in a ring. The polling queries are sent from the
master around the ring. A query is passed from one slave to the next
until the intended destination slave is reached. Likewise a response
from a slave is sent around the ring until it reaches the master.
Thirdly, there is the issue of the data interface. The RS-232 interface
is the most widespread in the world, but it is not ubiquitous. There are
other interfaces, notably RS-422 and RS-485. RS-232 is found, most
commonly, when data is being sent on a simple point-to-point link and at
relatively low transmission speeds. RS-422 is encountered when higher
speeds are demanded over longer distances. It is also encountered when
you want the interference resistance provided by the differential
signaling associated with this standard. RS-485 is encountered when
dealing with polling networks which are set-up in a multi-dropped
topology. However, there may be communications networks where all three
of these interfaces and possibly more may be present. A modem that can
accommodate this situation would be attractive.
If truly harsh interference conditions are expected along with the
possibility of ground loops, then you should be considering modems that
operate with fiber optic cable. Carrying out data communications using
fiber optic cables in the premises environment presents several ready
advantages. First, there is tremendous bandwidth potential.
Consequently, 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.
In addition, if your data communications environment is such that you
may have to deal with either point-to-point or ring topologies and with
a variety of data interfaces, then the Model 8277 presents an attractive
modem candidate. It can provide signaling over fiber optic cable - with
the interference and ground loop protection which this implies - and
operate over either a point-to-point connection or in a ring topology.
What is more, the Model 8277 is also DIN Rail mounted. This makes it
ideal for the factory environment, where messy cabling is often a
problem - an environment where the Model 8277 would often find use.
DIN Rail is a cabling system that was developed in particular for
factory automation and is now available for general computer room use.
The system is very simple and straightforward. It uses a steel channel
called a DIN Rail. The rail has slotted holes for mounting. The rail is
normally mounted in a horizontal position. DIN Rail products are then
placed in the rail by snapping them into place after which the wiring is
completed. DIN Rail mounting helps organize the computer room and
prevents 'messy cabling' in the factory environment.
The illustration below provides a ready application of the Model 8277.
Here we have two computers set up in a simple point-to-point link. The
data interface is RS-485. The computers are located in a manufacturing
environment where harsh interference and ground loops prevail.
Furthermore, the computers are all fairly distant from one another -
several miles apart. Transmission over fiber optic cable is attractive
in the interference environment over these distances. As shown, the
Model 8277 can provide this type of transmission. Multiple Model 8277s
are being used here to extend the link length to the needed distance.
The Model 8277 is being used as a tandem repeater to extend the 'top'
link in the left-to-right direction and the 'bottom' link in the