"A" lead & hold control with 2nd phone off hook?

[5415551212]

Question for those running key systems:
Say you have two regular key phones on the same extension off hook and one presses the hold button and hangs up a single phone what happens?
Can you place a call on hold if another key phone is off hook?
The "A" lead from what I understand allows a call to be placed on hold and tells the lamp for that line to wink.

If a phone extension goes off hook the line card senses loop current thru the card and "A" lead connects to DC 'ground' (call in progress) via the hook switch in the phone (or via an "A-lead adapter" for a non A-lead compatible phone).

The 3 states of hold as I understand it are
1) When hold is pressed for a moment on a key phone it disconnects the "A" lead from DC ground but not the tip and ring .The line card still senses normal 'loop current' but now A lead is floating. However this would only work if the all the "A" leads were a series circuit correct? If not then any other phone off hook would keep the A lead grounded.

2) When the hold is released, it mechanically releases which ever line button is down on the set, removing the loop current. But with another extension off hook there would still be loop current flowing thru the line card so the call would not go on hold ?

3) With all phones hungup the "A" lead is floating or has a voltage potential to ground (via a relay coil or whatever circuit in the line card).
Thanks

[poplar1]

Question for those running key systems:
Say you have two regular key phones on the same extension off hook and one presses the hold button and hangs up a single phone what happens?

Line will not go on hold since at least one phone is still off hook = A lead is still shorted to A1 lead at the line card.

Can you place a call on hold if another key phone is off hook?

No

The "A" lead from what I understand allows a call to be placed on hold and tells the lamp for that line to wink.

Pressing the hold key removes the short between A lead for the active line and the A1 lead common to all lines on the set. This must occur *before* tip and ring are disconnected. (When hanging up on a call you don't want to place on hold, the tip and ring disconnect first. This happens with the hookswitch contacts only. If the hookswitch contacts are out of sequence -- that is, the A/A1 opens first -- then the line will go on "false hold" condition since this was not the intention of the user.)

If a phone extension goes off hook the line card senses loop current thru the card and "A" lead connects to DC 'ground' (call in progress) via the hook switch in the phone (or via an "A-lead adapter" for a non A-lead compatible phone).

Correct

The 3 states of hold as I understand it are
1) When hold is pressed for a moment on a key phone it disconnects the "A" lead from DC ground but not the tip and ring .The line card still senses normal 'loop current' but now A lead is floating. However this would only work if the all the "A" leads were a series circuit correct? If not then any other phone off hook would keep the A lead grounded.

In 1A1 or 1A2 Key, the A/A1 and Tip/Ring are separate. The A leads for each line are in parallel to whichever phones pickup that line. Yes, any phone off hook on the same line will keep A lead grounded.

2) When the hold is released, it mechanically releases which ever line button is down on the set, removing the loop current. But with another extension off hook there would still be loop current flowing thru the line card so the call would not go on hold ?

It would not go on hold, not because of central office line current going through the line card, but  because A is still shorted to A1 in at least one set.

3) With all phones hungup the "A" lead is floating or has a voltage potential to ground (via a relay coil or whatever circuit in the line card).
Thanks

With all phones hung up, the short between A of a particular line and A1 is missing. Therefore, the line/busy lamps are dark on all sets.

[5415551212]

Thanks for confirming my understanding of the A-lead.
I am finally getting back to building up my key shoebox system along with one of Gregg's 'Seriss' 1A2 boards, and revisiting my idea for designing a modern line card, call it 400i (as I think H was the last revision).
I have ideas for about 5 different cards, but first want to make the most basic linecard.
However I am thinking of adding just one feature;
the card could also detect when other non- key phones 'upstream' from it go off hook, and then I was thinking it could also detect downstream by more precisely measuring loop current and changes in loop current.