I am looking at adapting an existing board which has RS-422 drivers and rec eivers to provide a telco E&M signalling interface. The wiring is within a building, so no need for handling of extreme voltages, transients, etc. M y problem is with the various interface implementations I might encounter a nd being compatible with the minimum of circuitry. The existing I/O board has very little room for additional circuits, but if it is small enough som ething might be squeezed onboard. Otherwise this will be done in the inter face cable in some manner, either in the existing shell or with a small pod inline. Not entirely sure of the ramifications of that aspect. I know te rmination resistors have been incorporated in the cable before.
I need to support three E&M types, I, II and V. The E circuit is of two ty pes in the three E&M types. One requires a local closure to ground for a r emote -48V sensor while the other requires a closure to the SG line with a remote ground. This may require a FET of some type rather than working wit h the OC output on the RS-422 driver. I can't see a way to protect the dri ver output (+7 to -0.5V) while providing a closure for a -48 V sensor witho ut an active circuit. So this will use an active device although it can be not much larger than a passive.
The M circuit has three varieties. One is a local ground referenced sensor where the remote end switches between ground and -48V. Another is a local ground referenced sensor and a -48V output on the SB line with a remote co ntact closure between these two lines. The last has a remote switch closur e to ground and the near end a -48V referenced sensor.
I think I can use simple bias resistors to provide a switching voltage to t he RS-422 input. My concern with the polarity of current through the remot e contact closure. I don't know that all equipment actually uses relays. I would expect FETs or even bipolar transistors might be used. Two of the circuits pull down to -48V but the other only has a ground contact with the sensor referenced to -48V. So the current direction should be opposite fo r the two cases while my circuit would essentially be a pull down in both c ases resulting in the current flowing in the same direction in each case. If the contact closure is provided by a bipolar transistor it won't work wi th this passive circuit in the one type of interface.
Anyone worked with this interface before that can tell me how the contact c losures are usually provided? There is no possibility of adding a relay to this design even externally I think. See anything I'm missing that would simplify this?
Rick C.