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
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 the data interface. Many environments
employ polling networks which are realized as RS-485 communications.
Often these networks operate with a 'master' host computer polling
'slave' data devices in a, half-duplex, query-response mode.
The polling network protocol is realized in the host computer/data
devices. Data comes into/out of the master/slave through the
RS-485 interface. Speeds can be as high as 1 MBPS. Many RS-485
based polling networks are found in heavy industrial environments.
The type of modem that best addresses the issues raised above
is one for signaling over fiber optic cable but having an
Carrying out data communications using fiber optic cable 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.
The Model 276A is a short haul modem with an RS-485 data interface
that signals over fiber optic cable. It carries out half duplex
communications at rates up to 1 MBPS at distances of 2 km.
The illustration above shows a ready application of the Model
276A. Here we have a computer workstation on the left. This
workstation is serving as the master for two different RS-485
networks. To do this it needs to have two different RS-485 interfaces.
These interfaces are provided by the Model 290 Hub.
The first RS-485 network is the 'upper branch.' This is a polling
network. As seen, the workstation, that is located in a medical
facility, is polling an environmental control unit and a patient
monitoring console. Ground loops are a concern in medical facilities.
Ground currents may not only cause inaccurate measurements on
instruments like patient monitoring consoles but may also hurt
personnel. Transmission by fiber optic cable provides the isolation
to ameliorate the effects of ground loops. The Model 276A then
is an attractive candidate to effect RS-485, half duplex, communications
for this polling network over fiber optic cable.
The second RS-485 network is the 'lower branch.' This is a simple
point-to-point link connecting the workstation to an isolated
patient monitoring console. Only half duplex communication is
required. There is no polling network present here. However,
the patient monitoring console does have an RS-485 interface
as it is usually connected to computers through such a topology.
With ground loops continuing to be a concern realizing this
point-to-point link by a fiber optic cable connection appears
attractive. The Model 276A can effect such a link in a half