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GENERAL LL485 is a dual channel RS485 to fibre optic interface unit designed to extend either RS485 or RS422 data protocol over a fibre optic medium. OVERVIEW OF THE RS485 AND RS422 PROTOCOLS
RS422 is a differential interface where signals are transmitted on a twisted pair from one transmitter to one or more receivers.
The signals are transmitted on two wires (twisted pair) and are either 0 volt or 5 volt. Let's name the two conductors in the twisted pair A and B. The signals on the conductors are differential levels. A logic '1' signal is transmitted as a 5 volt level in conductor A and a 0 volt level in conductor B, and a logic "0" is transmitted as a 0 volt level in conductor A and a 5 volt level in conductor B. The receiver extracts the signal from the twisted pair by analysing the differential signal arriving. The advantage is that electrical disturbances (like lightning induced pulse or electrical motor noise) will appear on both lines and because only the difference signal is used, the effect of the disturbance has little or no influence. A ground (0 volt) conductor is not necessary.
RS485 (sometimes called RS422 multidrop) is similar to RS422 as far as the signals (voltage levels) are concerned. The major difference is that the transmitter can be controlled to go into a high impedance condition where it is equivalent to not being connected to (having no influence on) the twisted pair. This allows multiple transmitters to be connected to the same twisted pair. By having intelligence in the stations controlling these transmitters they can send signals from various places on the same twisted pair. With RS422 the data is permanently driven on the lines and if a second transmitter is connected, a conflict will exist between the output levels of the transmitters. (Assume one transmitter tries to drive the line to a 5 volt level and another transmitter tries to drive the line to a 0 volt level, the line level will be undefined and the transmitters could be damaged if this situation exists permanently.)
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