The extension cable supplies power down the cable. The extension cable is used in fixed applications where the base equipment is powered from a liberal power source. In all cases the hand held unit is powered from the cable. However in mobile applications the base equipment is battery powered and thus indirectly the handheld is also battery powered. The dsign of the handheld was optimized for the mobile application. This extended cable mode came about as an after thought as the client found some new applications for their gear.

I looked at the concepts of the actual RS482 repeater as linked to by Jacob. After studying how an actual RS485 repeater works I can see that the same concept can be used directly by my application. These work on the basis that for a protocol to function propery on RS485 it by nature has to be half duplex. As such, when nobody is transmitting the line goes quiet between packets or bursts. During these quiet times the RS485 repeater places both sides of itself in receiver mode. The first side of the repeater to detect transitions realizes that data is coming from that direction and so it enables the driver on the opposite side to let this data flow onwards. A re-triggerable circuit keeps the downwind driver enables as long as the input side continues to see transitions. After the line goes quiet again the repeater goes back to listening on both sides. There also needs to be a lockout in the repeater design such that once one of the transmitters is enabled the other one is prevented from becoming the transmitter till after the re-triggerable circuit times out.

It turns out that my problem with the FSK signalling works exactly the same way in concept. A packet consists of a continuous burst of frequency (either the 2550 or 3200 Hz depending on 1s and 0s) and then goes quiet after the end of the packet. There is a protocol defined dead time between the packets as well. Thus it would appear that I can build two repeaters, one at each end of the long cable. They will convert the FSK bus to long distance differential bus of RS485 characteristics down the cable (good for over 4000 feet at what amounts to a pretty slow comm link at a max frequency of 3200 Hz. (This is about the
equivalent of a 6400 baud async link).

So thanks all.....I'm off to buy some ICs so I can prototype up this concept.

Michael Karas