RS-422 vs. Telco E&M Signalling

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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.

Re: RS-422 vs. Telco E&M Signalling
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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.
    

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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.        

Quoted text here. Click to load it




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.

--  
     ?

Re: RS-422 vs. Telco E&M Signalling
On Wednesday, June 13, 2018 at 3:31:09 AM UTC-4, Jasen Betts wrote:
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.com> wrote:
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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.  
Quoted text here. Click to load it

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.

Re: RS-422 vs. Telco E&M Signalling
On Tuesday, June 12, 2018 at 9:00:05 PM UTC-4, snipped-for-privacy@gmail.com wrot
e:
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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.  
Quoted text here. Click to load it
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.  
Quoted text here. Click to load it
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.  
Quoted text here. Click to load it
 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.  
Quoted text here. Click to load it
 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.  

https://www.digikey.com/product-detail/en/toshiba-semiconductor-and-storage
/TLP3419-TPF/TLP3419-TPF-CT-ND/6605329

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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