I have a product I worked on that has a half duplex FSK port on it that talks via a short distance to a piece of military gear. Under normal circumstances this interface according to the client specification is capable if working out to 4000 feet or so using a 640 bps data rate using 2550/3200 Hz frequencies. To get that distance the signal is normally coupled via a transformer to the wire and driven with a low impedance driver to an amplitude of some 6 volts on the cable. The receive side would have an amplifer and comparator circuit to recover the waveform at signal levels as low as 100 mVolts or so.

When I worked on the recent equipment however, the decision was made to save battery power and not put a full distance drive capability for the interface into the hand held gear since it was reasoned that it would never be more than 6 feet away from the base equipment. So we built a simple circuit between our Cygnal C8051F126 processor and the interface using a transmitter with 0-5 volt swing through 1K resistor and a receiver with about 250 mV sensitivity. The transmitter is DC coupled to the signal wire and the receiver is AC coupled via a capacitor. No transformer was used to save space, cost and weight. It all works great at the 6 foot distance, even out to over 25 feet.

Now the client built a 50 foot test cable and is experiencing signal dropout. DUH no wonder. None the less they now want an extension cable built for the handheld unit out to 250 feet and to 1000 feet for two new applications. They say that the cable can have a goiter on each end (Think of the snake that just ate the rabbit) that contains the necessary re-generation circuits to make the signalling reliable. Redesign of the handheld unit is out of the question as of this point because already several 1000's have been or are in the process of being built.

Does anybody here have direct experience with repeater circuits for a half duplex signal of this type. It has to simultaneously re-generate the signal in both directions because there is no signal wire that specifies the direction on the cable at any given time. This would be similar to an Rs485 repeater if such a thing existed. Are there ready made chips for this job?

Michael Karas