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 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.