RS-422 vs. Telco E&M Signalling

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.

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gnuarm.deletethisbit
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and receivers to provide a telco E&M signalling interface. The wiring is within a building, so no need for handling of extreme voltages, transients, etc. My problem is with the various interface implementations I might encounter and 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 something might be squeezed onboard. Otherwise this will be done in the interface 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 termination resistors have been incorporated in the cable before.

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 contact closure between these two lines. The last has a remote switch closure to ground and the near end a -48V referenced sensor.

RS422 should be capable of driving a SSR, but an optoisolator driving a BJT darlington style would probably work too, 1V drop is probably close enough to a hard closure for most purposes.

I once ran a latching relay off a _RS232_ signal using only 3 parts. max switching rate was something like 0.4Hz, it was a mid sized relay (250VAC 15A contacts) a smaller relay could proably go faster.

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Reply to
Jasen Betts

to the RS-422 input. My concern with the polarity of current through the r emote contact closure. I don't know that all equipment actually uses relay s. 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 opposit e for the two cases while my circuit would essentially be a pull down in bo th cases resulting in the current flowing in the same direction in each cas e. If the contact closure is provided by a bipolar transistor it won't wor k with this passive circuit in the one type of interface.

I am not asking how my end should be implemented. I'm asking how these cir cuits are usually implemented. I have a board in production with an RS-232 driver/receiver on it. That would work ok I think for receiving a signal since a -48 volt swing is easy to attenuate down to trip the differential i nput of RS-422. My concern is that my circuit has to operate in two differ ent modes where the current should flow in opposite directions and if the f ar end is driving with bipolar devices they won't work with my inputs. If they use relays then all is golden I would think. But I expect few units b uilt these days will use actual relays. I'm asking what they actually use to implement the drive circuits.

Rick C.

Reply to
gnuarm.deletethisbit

On Tuesday, June 12, 2018 at 9:00:05 PM UTC-4, snipped-for-privacy@gmail.com wrot e:

eceivers to provide a telco E&M signalling interface. The wiring is within a building, so no need for handling of extreme voltages, transients, etc. My problem is with the various interface implementations I might encounter and being compatible with the minimum of circuitry. The existing I/O boar d has very little room for additional circuits, but if it is small enough s omething might be squeezed onboard. Otherwise this will be done in the int erface cable in some manner, either in the existing shell or with a small p od inline. Not entirely sure of the ramifications of that aspect. I know termination resistors have been incorporated in the cable before.

types in the three E&M types. One requires a local closure to ground for a remote -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 w ith the OC output on the RS-422 driver. I can't see a way to protect the d river output (+7 to -0.5V) while providing a closure for a -48 V sensor wit hout an active circuit. So this will use an active device although it can be not much larger than a passive.

or where the remote end switches between ground and -48V. Another is a loc al ground referenced sensor and a -48V output on the SB line with a remote contact closure between these two lines. The last has a remote switch clos ure to ground and the near end a -48V referenced sensor.

the RS-422 input. My concern with the polarity of current through the rem ote 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 th e circuits pull down to -48V but the other only has a ground contact with t he sensor referenced to -48V. So the current direction should be opposite for the two cases while my circuit would essentially be a pull down in both cases 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 with this passive circuit in the one type of interface.

closures are usually provided? There is no possibility of adding a relay to this design even externally I think. See anything I'm missing that woul d simplify this?

I found some very tiny optos, cleverly disguised as solid state relays. I guess I don't really get the distinction other than the SSRs use MOSFETs to get bidirectional current flow on the switch side. They are only 2.5x1.5 mm which might fit on the existing board if I can remove some apparently un used functionality. This board is a blivit anyway. I was barely able to l ay it out when I revised it last time. I will see if we can make this a ve ry small board incorporated into the cable.

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Anyone know of similar units that are a bit less pricey? $3 won't break th e bank, but $1 would be nicer.

Rick C.

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gnuarm.deletethisbit

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