Model 277 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.
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 realize 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 widely used in the world. But, it is not ubiquitous.
It is not universal. 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 3 of these interfaces and possibly
more may be present. A modem which 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 which 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 277 presents
an attractive modem candidate. It can provide signaling over fiber
optic cable - with the interference and ground loop protection this
implies - and operate over either a point-to-point connection or
in a ring topology.
The illustration above provides a ready application of the Model
277. Here we have 2 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 277 can provide this type of transmission. Multiple
Model 277s are being used here to extend the link length to the
needed distance. The Model 277 is being used as a tandem repeater
to extend the 'top' link in the left-to-right direction and the
'bottom' link in the right-to-left direction.
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