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One of my favorite things: a 207-type stepper relay. This relay was located in the panel sender and was electrically in series with the fundamental circuit. As the panel selectors hunted for a terminal, they sent pulses back to the sender. Those pulses caused this relay to open and close once for each pulse. This relay in turn operated the relays in the counting chain that counted the number of pulses that had been sent back. When the counting chain reached ‘0’, the sender was satisfied and the selector up-drive was disengaged, and the selector was held in position.

Because it had such a critical role in making selections, the 207-type relay had to be adjusted to extremely tight tolerances. If it mis-registered a pulse from a selector, the selector would then hunt to the wrong terminal, and the subscriber would get a wrong number, even though they dialed correctly. As such, this relay was adjusted to a non-operate value of 9mA and an operate value of 10mA. That’s just .1 milliamp of difference between operating and not operating!

You can also think of this relay as a simple repeater. It took weak pulses coming back from a distant selector (perhaps miles away) and as it pulsed, it sent strong pulses back to the counting chain, which did not have to be quite as sensitive. This “repeater” functionality is quite clear when comparing the design of the 207 relay to a telegraph repeater from the early 1900s. They’re very similar! It shows how much they relied on existing tried and true technology and applied it to different problems to come up with a solution that worked.

(Above: telegraph repeater relay. Early 1900s. Note similarity to 207 relay!)

#switching theory #relays #panel system #fundamental circuit

Circuit Fundamentals of the Fundamental Circuit

In order to understand panel witchcraft, you must first understand the fundamental circuit.

Before I scare you with circuit diagrams and other assorted spells and charms, I should probably explain what the fundamental circuit is, and why its so, well, fundamental to the operation of the panel system.

In short, the fundamental circuit is a loop that runs from the sender in that the originating office, to the incoming frame in the distant office. You could call it a “trunk”, and that would be correct, though it would be oversimplifying it a bit. Like a normal telephone trunk, it is a 2-wire circuit (tip and ring), and it carries the signaling and voice between two telephone offices.

However, that’s where the similarities end. Though it is still a connection between two offices, the thought process behind the design differed substantially from trunk circuits that were present in more modern systems. Take the crossbar, for example. To communicate from an originating office to a terminating office, a trunk is taken off-hook, and when the far-end equipment signals that it is ready, the telephone number dialed by the subscriber is sent outward into a terminating sender. The terminating sender takes that information, and (with the terminating marker) controls the setup of the call from that point onward. The originating office, having completed outpulsing just has to wait for the other office to do its job to connect the remainder of the call. The only real connection between the signaling and receiving elements is the tones played towards the terminating side. There’s an added layer of abstraction there--the two sides of the call are free to behave as they want to.

In contrast, the panel fundamental circuit can be thought of as a very long chain of relays all operating together over the same DC circuit stretched out across the miles between offices. It’s almost as if they took a circuit that could fit into a cabinet and extended it as far as they possibly could. Because the relays in the circuit are all operating on the same DC current loop, the tolerances and timing have to be extremely precise, and well-orchestrated. Something that happens on the far end, will have a direct, and almost immediate impact on the relays in the near end, because they are inextricably linked in series with each other.

This is the fundamental circuit. Its beauty is sublime.