Model 272A 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 improves 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 the application data transmission and speed requirements. They also need to meet the interference environment within which the communications is taking place. Certain environments, such as office building settings, present relatively benign environments where background noise is the only problem. Others, such as manufacturing facilities, 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. In many cases RS-422 communications by employing differential signaling can effectively deal with the deleterious effects of such interference. However, there are many manufacturing environments where the RS-422 copper wire cable approach just does not provide enough protection.

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.

Carrying out premises data communications in the manufacturing environment by using fiber optic cables presents several ready advantages. First, there is tremendous bandwidth potential. It can deal with traffic from relatively low to high data transmission rates. Applications that require 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 272A 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 by RS-422. Therefore, the modem presents a convenient way of evolving a network from the differential signaling protection provided by RS-422 to the enhanced protection provided by fiber optic transmission. The modem can achieve data rates of 2 MBPS over 2 km which is certainly suitable for most settings.

The employment of the Model 272A in a typical application is provided by the illustration above. Here we have the manufacturing environment with all of the harsh interference and isolation needs that this implies. There is need for a full duplex, point-to-point link, between a PC - operating as a type of work station - and an Intelligent Machine Controller. The Intelligent Machine Controller is itself directing automated tools within the environment. The Intelligent Machine Controller has the RS-422 interface and could use this type of communications to deal with the harsh interference environment. However, effecting the fiber optic link shown provides an even better solution. The use of dual fiber cable allows a link in both directions and as a result brings about full duplex communications.


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