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How did phone companies and companies that made PBX's do switching with discrete logic or simple, non-programmable ICs?
The caller would type in a number, so the system is receiving a sequential code, and based on this code it has connect two wires together. What was the standard electronic strategy to do this?
Just to clarify here: I am not asking about electro-mechanical. I am talking about digital ttl logic, just before it was programmable. So we are talking 1980s in the United States.
For a while, switching was done using electromechanical relays that established interconnect networks. A common setup at one point was the use of stepping relays that would be advanced from position to position under the control of the caller with each rapid pulse in pulse dialing - each digit would plausibly step one of these relays, routing the call from switch to switch until it reached its destination line and caused the remote ringer to ring.
A further improvement on this was the so-called two-axis switch which allowed selecting both an outgoing route and multiple outgoing "banks" from each switch to reach the next, improving the capacity of the exchange. As Wikipedia describes:
These [two-axis switches] were commonly used in telephone switching with ten banks of ten contacts. The coils were typically driven by the electrical pulses derived from a rotary telephone dial. On a two-motion selector, as a digit was dialed, the wipers would step up the banks, then automatically rotate (self-step) into the selected bank until they found an "unused" outlet to the next switch stage. The last two digits dialed would operate the connector switch (final selector in Britain). The second to last digit would cause the wipers to move up and the last digit would cause them to rotate into the bank to the called customer's line outlet. If the line was idle then ringing voltage would be applied to the called line and ringing tone was sent to the calling line.
At each stage, various conditions (such as a lack of outgoing banks) could be detected and reported. An example of such a condition is the "reorder" tone (or fast-busy tone, distinct from the normal busy tone) meaning that the call could not be processed through the network for lack of exchange resources. For example, if one of the above two-axis steppers failed to find an open bank, the caller would get the tone and the call would not be routed further.
This piece of audio demonstrates a pulse-dialing code which reaches various points in an exchange and hears different tones, before ultimately reaching the destination and finding the circuit busy.
At the same time, various signalling schemes such as the use of DC voltages and tones was used to coordinate the state of scarce shared resources such as long-distance trunks.
Mostly it was done with relays.
Before 'touch tone' dialing, phones had rotary dials which sent the numbers as a pulse train of 1 to 10 pulses per digit. Calls were routed either by operating rotary switches with electromagnets that were stepped with the dial pulses, or relay based counters and hard-wired controllers (called 'markers') that computed the route and operated horizontal and vertical bars on 'crossbar' switches to connect the call. The markers in crossbar exchanges had hundreds of relays in them and were very complicated.
Since these systems were already in place when DTMF phones came into use, the existing exchanges were simply supplied with tone decoders that either fed the 2 of 5 codes into the same relay logic circuits that normally took the output of relay counters in crossbar exchanges, or converted them to pulses using some basic digital logic or an MCU for step-by-step and rotary exchanges. The electronic circuitry was confined to a single card per decoder, and the rest of the exchange operated the same as always.
I worked on PBXs in the 1980s and early 1990s when the transition from mechanical to electronic systems occurred in New Zealand. None of our electromechanical systems worked with touch-tone phones. To get around this we had telephones with push button keypads that generated rotary dial pulses inside the phone. These used either a small MCU or a dedicated IC to do the job.
As for 'discrete logic or simple, non-programmable ICs' for switching, except for DIY home phone systems I don't know of any PBX that used discrete solid state logic. In New Zealand we went straight to 'stored program control' using microprocessor based systems (which in some cases were less reliable than the electromechanical systems they replaced, were full of software bugs, and still used relays to connect the lines!).
Before digital switching it was done with "stepping switches" or "step-by-step switches".
Here's a video which describes them:
"AT&T Archives: The Step-By-Step Switch"
Telephone equipment couldn't be changed in one night due the massive amount of it. Every new piece of equipment had to work with the old ones. You want details of a non-existent era. No TTL and discrete transistor era without electromechanical switches in major roles has existed. Computers were already used as brains in some systems say 40 years ago, but still the final switching happened in electromechanical circuits.
