Model 279 Application Notes

Fiber optic cable as a transmission medium has, for some time, been employed in the long haul or wide area network environment. Its advantages with respect to large bandwidth, immunity to electrical interference and security are well known.

In recent years there has also been a steady growth in the use of fiber optic cable in the premises environment. As fiber optic cable prices have decreased and cable connectorization has become easier to accomplish, this medium has become increasingly popular for implementing premises data communication networks - networks for either in-building or manufacturing plant use.

When fiber optic cable is employed in long haul or the wide area environment it is of the single-mode variety. In very elementary terms you can think of the cable as allowing only a single 'ray of light' to propagate from one end to another. Why is it single-mode? Because this type of fiber optic cable has very low attenuation and consequently can achieve the long distances between repeater required in this environment.

When fiber optic cable is employed in the premises environment, it is generally of the multi-mode variety. Again, in very elementary terms, you can think of the cable as allowing a multiplicity of 'rays of light' to propagate from one end to another. Multi-mode fiber optic cable has more attenuation than single-mode fiber optic cable - you often lose the 'rays' propagating near the walls of the cable. However, this is OK. In the premises environment you do not have to go as far as in the wide area network environment and your signals can tolerate the increased attenuation. Multi-mode fiber optic cable is also much less expensive than single-mode fiber optic cable. This works for the premises environment where cost is a much more sensitive issue.

There has been more and more activity in taking data being transported on a premises data communications network and sending out on a wide area network. One case may be taking data on a corporate LAN at one location and then sending it out onto a wide area network to a corporate LAN at another location. As the employment of fiber optic cable as a transmission medium has grown carrying out this task has come to mean converting the fiber optic signals from the multi-mode cable on the LAN to signals appropriate for the single-mode cable used in the wide area network. That is, a multi-mode to single-mode conversion has to be done. The Model 279 is well suited to this task.

The illustration above shows a pair of Model 279s being used in just this type of application. Here we see a process control computer at one location communicating with a local controller on a factory floor, some distance away, say in excess of 10 km. Considering that so much of the communication is being carried out in the factory environment where electrical interference is so much of the problem, it is desired to effect the communication be fiber optics as much as possible.

Both the process control computer and the local controller send and receive data through their RS-422 interface. The differential signaling of RS-422 provides some protection against electrical interference. However, it can not compete with fiber optic transmission. Thus, at both ends of the link we show a conversion from RS-422 to multi-mode fiber optic signals using the Model 272. The resulting communication can employ the low cost multi-mode fiber optic cable to out to the walls of the building where the computer and controller are located, the boundaries of the respective premises. At these points conversion of the multi-mode fiber optic signals to single-mode signals can be carried out using a Model 279 at either end. The Models 279 can then be linked by single-mode fiber optic cable which can reliably support transmission over the 'longer than premises' 10 km + distance.
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