Excellent, thank you! I printed out some of those diagrams and studied them last night.
I take it that the line cards went through different versions over time (e.g. the 400E appears to dispense with the A relay of the 400D) and that third-party vendors offered their own versions which were directly compatible.
A few of the options pose some questions. The 400D KTU diagram shows a 30-second ring timeout option by omitting any resistor from the 1/2/3 terminals. Was this just a consequence of giving maximum flexibility by having a very high value resistance on the card, or a deliberate design feature to allow such a long delay? 30 seconds seems awfully long given a standard 2 on/4 off ring cycle. The only place I can think of where one might want such a long timeout would be if the C.O. line was on a manual board with non-machine ringing which might be of variable timing.
I see the BR link options to have the L relay operate from normal ringing on the C.O. line, but when/where were the remote A.C. and remote D.C. ringing options used?
