E&M Telephony

It has been a long time since I designed any telephony equipment. I know the E&M circuits are simple conceptually, but there are any number of real world issues that impact the design, which I can't remember much of.

The battery voltage is approximately 48 volts and the circuits are current limited. Control is done with relay contacts in the old days and sensing was done with a relay coil. Now I'm sure that has all been replaced with a few transistors. I tried looking for ICs that would handle this, but came up empty handed. Is there anything out there that is designed for this? I know there are chips for the SLIC and such, but I don't see anything for E&M.

This is not for a circuit to a CO, so there won't be exposure to lightning or AC wire crosses. This will be between a PBX and networking equipment (circuit to packet). I'm not sure it even needs 48 volts since the interface is really current driven. Anyone know if E&M typically uses a lower voltage these days like 12 volts?

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Rick C
Reply to
rickman
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It's been 30 years since I've done any circuit design for telephony applications. Weigh my responses accordingly.

Is the audio channel on E&M wet? Or are you talking about the signaling wires?

I'm surprised there's modern telco systems out there that would prefer to run a bunch of E&M trunks someplace rather than just a fiber -- I have this vision of fiber run to a little beige box just outside the building, then 20 feet of trunk lines into a PBX that has no excuse not to have a fiber connection.

I suspect that most people use solid-state relays, or perhaps they roll their own SSR circuits. I'd like to say just use a MOSFET, but that assumes that every telco gets the polarity right: it's probably a good idea not to count on that.

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Tim Wescott 
Control systems, embedded software and circuit design 
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Reply to
Tim Wescott

Oh -- and I'd design for at least 48V, just to be on the safe side. If there's one thing I remember about working with telephony, it's that almost nothing you can do will break those expensive wires stretched from hither to yon, but getting good consistent operation out of the thing depends on sticking to the BelCor standards (or whatever is used today).

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Tim Wescott 
Control systems, embedded software and circuit design 
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Reply to
Tim Wescott

"E&M is considered an obsolete technology for new installations, which generally use Basic Rate (BRI) or Primary Rate (PRI) digital interfaces."

NT

Reply to
tabbypurr

Signalling.

That's what this is, a connection from the PBX to a network that will carry the calls over Ethernet by whatever physical medium you have.

My boards go in the existing products this company offers, but they are changing their business model from a hardware/software company to just a software company. They are handing the hardware business (for this product line) to another company and will only sell the software. Strange but true! So my boards will no longer be used.

My contact with the company suggested I might be able to get a little consulting business with the hardware company since they know nothing about telephony. Unfortunately I don't remember much myself.

I think they get the polarity right... or you can just use a diode bridge to connect the current sensor. In some configurations power comes from the PBX only, in others both ends source power. They don't mix up the polarity, but it will change depending on the configuration, type 1, 2, 3, etc.

Yeah, thanks.

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Rick C
Reply to
rickman

I guess they aren't selling this into new installations...

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Rick C
Reply to
rickman

There were numerous telephone projects in Popular Electronics and Radio Electronics. Some were impressive phone systems. One was a doall board that had all governmentally required features to serve as an interface for whatever project the reader had in mind. You may be able to find these on the internet and, if so, use them as a model and a reminder.

Hul

rickman wrote:

Reply to
Hul Tytus

Pull out the databooks, already must know keyword jargon. Search on: "LCA" line-card access, basically the solid state relays. Also the audio chip, "DAA" direct phone-line connection. I immediately hit these via digikey:

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I recall that back in the 1990s (industrial sensors job) we were buying scads of CP Clare "Optomos" ssrs in dip-6 and dip-4 packages, which were cheap because of mass phone company buys.

CPC1000? Doesn't exist now, try CPC1002. If it's 4-pin surface mount, opto-isolated relay 60VDC half-amp mos output with LED input, it was probably developed as solid state version of a phone company mechanical relay.

Reply to
Bill Beaty

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