atcomsystems.ca/forum Forums Other TDM Phone Systems 1A2 and Comkey Telephones & Systems 400D KTU Issue 14/15 schematic needed

Nice! I think you have half of a QST article written already!

Thanks for posting your solution. I may need that for ideas someday.

-Nelson

George -

Quite a production! Good for you.

To me though, the big questions are - why does the Magic Jack disconnect? Is that momentary open from the line card enough to knock it out? And why is the line card momentarily opening the line circuit in the first place?

But thanks for posting. I've been thinking of adding a MJ to my 501 cabinet. I dropped the 2nd line when the tariffs just got out of hand and while I now have CW from FIOS I still toy with the idea of a second line.

Sam

Sam,
The MJ works fine with the 400G line card alone. I'm using a 2nd MJ on another computer connected to the 1A2 system as line #2 and all is fine. The problem showed up when I connected the FAX/answer machine to my main line #1. I tried connecting it before (CO side) and after (Subscriber) the line card - made no difference. It still did the strange disconnect. Something in the MJ hardware/firmware. Both MJacks responded the same way. I reached a point where I just wanted to make it work. I think there's line loading with both connected (line card series RLC resonance, line rly, 1.1 uf, & 6.8k ??) and the MJ maybe thinks the tel line is somehow shorted and disconnects to protect itself from blowing out it's ringing gen output. Just a guess. It's hard to understand that ~50k added across the line during ringing causes the problem. Hmmm, I never tried just adding a 47k resistor across the line (with the 400G) to see what happens. Another experiment for another time. Just to be complete, the FAX/Ans machine has a similar ring detector ckt with an optoisolator just like I built up. YMMV.

George.

I'd guess that the MJ's answer detection is not very robust. A line port on a switching system or anything that emulates one must provide substantial amounts of AC current to operate ringers while responding to smaller amounts of DC to detect answering. The AC is applied with superimposed DC so that DC current will flow when the handset is lifted. Before answering only AC current flows through the ringers since they have capacitors in series.

Poorly implemented devices cannot distinguish properly between the AC ringing current. They respond as though DC answering current were flowing if the AC current exceeds what they can handle without false ring trip detection. It's not really a matter of protecting itself against damage.

Of course as soon as the AC is removed in response to the false answer detection, disconnect occurs because there is no DC answer supervisory current flowing after ringing is removed.

Does MJ have a published REN (Ringer Equivalence Number)? What is it?

A 400D card actually has a pretty high REN. 2 IIRC. I've seen FAX machines that had pretty high RENs too. When the FAX machine was added to a 1A2 system line the REN of the FAX machine alone caused the A-lead control device to chatter at 20 Hz during ringing. This drove the 400D card bonkers, interfering with proper lamp flash signaling of incoming calls.

In your case the combined REN may exceed what the MJ can handle rather than the limits of an A-lead control device

Reducing the effective REN of the 400D card using an optocoupler may bring the sum of the two down enough for the MJ to be able to handle it. I wonder though how well FAX works over a MJ in the first place and whether the end result will be worth the trouble.

Generally it's preferable not to modify equipment if at all avoidable. There are ways of raising the effective REN that a device such as MJ can handle such as by putting a Viking Ring Booster in front of it. Of course this costs money but it's a compact and easy OTS solution. Another solution is to put a Tellabs or Wescom DLL (Dial Long Line) card ahead of it. This would accomplish essentially the same result. It would require a DC and ringing power source though so it's a much bulkier solution. The stuff is generally available on eBay pretty cheaply though.

George, Steph -

Very interesting ideas from both of you. I was trying to remember if we had fax machines on 1A2 systems, and I think not. I do 't remember seeing them in any quantity till the mid '80s and by then most of the 1A2 systems were either gone or going. When we did put them on, they generally had their own line. I do remember answering machines and modems all coming standard with A-lead control, but not fax machines.


Sam