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 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 which best addresses the issues raised above is one
for signaling over fiber optic cable but having an RS-485 interface.
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