Model 271 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 under 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 matter of isolation. The need for isolation arises when ground currents are present. In many instances the data equipment communicating must be grounded at different points. Different grounds imply different reference levels for voltages. Ground currents are generated in an attempt to achieve equilibrium between different ground points. From the perspective of communications such currents make themselves known as an additional interference mode. Isolation provides a barrier between different ground points. This barrier does not allow ground currents to be generated. Isolation allows equipment being grounded at different points to be connected together without the need to worry about the interference due to ground currents.

Thirdly, there is the issue of the data interface. The RS-232 interface still presents the most ubiquitous interface in the world. The ability to take/direct data from/to this interface without any conversion eases link implementation.

Fourthly, there is the matter of powering the modems. A modem needs electrical power to work. Many times the network installer can look around for an electrical outlet and find that either one is not available or is quite far from the device and currently in use.

Carrying out premises data communications using fiber optic cables in the harsh interference environment presents several ready advantages. First, there is the bandwidth potential. Applications that require very higher 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 271 fiber optic line driver presents a convenient way of dealing with many of the issues described above and bringing effective data communications to the manufacturing environment. To begin with it realizes a data communications link over fiber optic cable. Thus, it obtains all of the benefits of fiber optic transmission with respect to interference and isolation. The data interface to this modem is the ubiquitous RS-232. The modem can achieve a higher than usual data transmission rate compared to RS-232. In particular, it can achieve 56 KBPS asynchronously and operate either half or full duplex. Finally, the Model 271 does not require any external power. It can power itself from the transmit data line. Of course, if the user wants to employ wall power there is the Model 271A which has this feature.

The illustration above shows the Model 271 in a typical application. Here we have the entire operations of a corporation, both office and manufacturing, housed in the same building. The interference environment throughout the building is harsh. There is a need for a point-to-point data communications link from a PC, located in the factory portion of the building, to a mini-computer located in the office portion of the building. There is no available electrical outlet near the mini-computer. The link must be full duplex, asynchronously. Required data transmission rates are not high, at most 50 KBPS. This is an ideal situation for the Model 271. By realizing a fiber optic link it can provide the protection against interference. It can protect against ground loops if these are present. It can satisfy the moderate data transmission rate needs in a full duplex, asynchronous manner. Furthermore, it can operate at the mini-computer side without the need for an available electrical outlet.
 
 
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