atcomsystems.ca/forum Forums Other TDM Phone Systems 1A2 and Comkey Telephones & Systems Can anyone tell me why...

Matt, I must disagree with the folks who keep telling you to put the SLT T&R before the line card. It was never done that way on 1A2 key systems in Bell System territories. Other equipment could use that scheme, but in general practice, it was never done.

In shoebox installations, (551 series KSU's) there was no ringing generator, and small installations had the key sets wired internally for bridged line ringing. This scheme was not detrimental to any line cards that I ever worked on, and I probably worked on several tens of thousands of them in thirty years.

SLT's need to be wired for A-lead control, and wired to the KSU just as if they were one button of a multi-button set. A third pair (T&R and A leads being pair 1 & 2) is for the CMB. There is no reason that the ringer in the phone cannot be wired on the output leads of the 400-D KTU. I have many practices showing exactly that scheme. The WE 400-D ktu, which was the most prevalent one in use, had no problem controlling ringers after the KTU.

For you to understand how this works, and why you're getting weird results, here's the scoop: To put a call on hold, two things happen, and they happen in a particular order. The A-leads open (this happens on the down-stroke of the red Hold button) the line key releases the T&R (this happens on the upstroke of the Hold button, just as it hits bottom, releasing the line key mechanically.) This sequence, which is tightly timed, is done with relays (later IC's) in the line card. The A relay is associated with the T&R and the H relay is associated with the A leads.

Hanging up in a normal manner, whether by releasing a line key, or just hanging up, releases the T&R before the A-leads open. Therefore, no Hold condition. If for some reason the contact (switchhook or line key) sequence gets fouled up, you'll get a false hold.

See more discussion below about false hold.

This sequence is critical to putting a line on Hold. That's why, in a previous post, I said to install a N/C push button in series with the A leads on your 554 to use as Hold button.

Another admonition: If there are many ringers bridged across the output T&R leads of a 1A2 key system, there is a possibility of a false hold upon hangup. The capacitors of the ringers will essentially cause the A relay to be slow-release, so that by contrast, the A-leads are opening first. This will put the call on Hold. The remedy is to eliminate a few bridged line ringers.

If you try to create a homemade A lead controller, it must behave just like the real thing, meaning that the sequence must be correct.

In the long run, you are better off converting your SLT's to A lead control.

Arthur -

Sam

You can wire T/R ahead or behind the line card...

This is a new one on me. I have always understood the operation of the 400 line card to be open the A/A1 before T/R and the line gets put on hold. Open the T/R before the A/A1 and the line goes on hook.

The logic involves the sequence of two relays on the card, the line sensing relay in series with the CO line and the A-lead relay. If you connect before the card (the CO line directly) the relay in series with the line will not operate when the T/R opens and closes on the hookswitch. I can't see that doing anything other than a false hold.

-Hal

Just remember that an SLT device will not be placing a call on hold, only keysets will be placing calls on hold. For this reason, only keysets need to have their T/R connected behind the line card. A combination of off-hook current flow sensed on the station side T/R plus the A-lead going open is what causes the line to go on hold.

Current flow on the station side is ONLY needed for this purpose. SLT devices need not be drawing current through the station side because they have no means to open the A lead simultaneously while in use.

Then, we need to refer back to the potential for bridged ringer load affecting the line card's reed relay's current limitation.

Western Electric's 400H line cards were designed to address this argument (among other things). Any device going off-hook on the station side T/R without A leads present will immediately result in an on-hold condition when using this card.

Funny, that's exactly where I learned this, from New Jersey Bell, NY Telephone and C&P Telephone installation practices in the mid 1970's through the 80's.

Also, many brands of line cards included polarity guard for the station side (Some San/Bar, Brand/Rex and ITT400TPL and WE400H). Ringing voltage on the station side would be unavailable. This was an FCC thing to prevent DIY's from getting shocked with consumer-level 1A2 system sold over the counter. Yes, there were phone stores that sold 1A2 systems and San/Bar made them, designed for DIY use. Sales of these required fully-protected line cards where the T&R were electrically isolated from ring voltage and the line's 48VDC.

If I am so far off the mark, I urge anyone here to explain the difference between an RJ12 and RJ13 connection. I'm fairly confident that you will find that I am correct.

I have printed documentation if anyone is interested in a fax. I don't have it stored in a format where I can e-mail it.

Well, if I contribute nothing else... I sure am good at starting lively discussions!

Regardless of how I end up finishing this thing off, I'm really enjoying this debate.

PS

The homemade A1 control is working perfectly... Well, except for the false hold condition, but that is nothing to do with the A1 control.

And anyway I like the flashing light....

Basically everythig is working how I want.... I'll sit back now and watch the debate about how things should be wired......

In any event, Thanks for all the discussion!

Ok, I see what you are getting at. If you think about it I answered my own question.

Open the T/R before the A/A1 and the line goes on hook.

If you put a SLT T/R before the card the T/R after the card will ALWAYS be open regardless of whether the phone is on or of hook, won't it? So the A/A1 will ALWAYS open AFTER the T/R because it is always open.

-Hal

I agree that converting the SLT's to A lead control is the way to go.

The disadvantage in my mind is I can't do it to every SLT I own, for example, the pay phone. Plus if I add another SLT, I have to go through the rewire process again, which isn't that big of a deal, but I tend to like to keep things configured as they were when they left the factory.

I gotta say It's very satisfying to just plug in an SLT, pick up the receiver and watch my 2565's light up.

I'm pretty much sold on how I have it setup, but the "before or after line card debate" has my interest.

Things sure seem to work better by putting it all after the line card, but I can see why this could cause problems for the cards. I have about 8 spare K400E cards if I should end up toasting one.

Having said all this, until I can figure out the noise that is present when wiring SLT's before the cards... I'm opting to leave everything after the card.

And now, in the red corner with blue trunks... Ed...

In the green corner, with red trunks... Arthur...

Have a clean debate and no hitting below the belt....

We always did it the way Arthur said: All Telephone set T&R, whether Key or SLT, were cut down (with their A Leads of course)on the output of the card.

I have used 6Bs & Melcos for a variety of circumstances where a lead conversion was not possible (modems and fax machines especially). Never had a problem with them.


Matt, when you have all this A-Lead stuff mastered, there is another challenge for you -the ultimate uber-Alead application, that which struck fear into the hearts of telephone men everywhere for a generation (maybe two):

The dreaded REMOTE HOLD.

I can say no more. It's too frightening just to think of it.

Sam

Matt:

I'll just concede on this one. I lack the experience to continue the debate. If you would like me to fax the docs on the differences, I'll be happy to do so. Just PM me a fax number.

Well, down but not out. The card I made is apparently closing A/A1 before T&R make it. So... hold doesn't work.

I'll be getting one of sandman's devices one day. But for now, I just wired my homemade circuit to A/A1 of line 3; so if the line 3 is lit, line 1 is in use. Not perfect, but hey, I need to find a use for all the extra keyset bulbs anyway.

The saga will continue, I'm sure....

Meantime I've learned one heck of a lot about how the cards function. Thanks everyone for your comments and tips.

- Matt