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 duplex mode.