1A2 Phone System Description ============================ This diagram a simplified view of a typical 1A2 phone system; a single phone line coming into the Key System Unit, or KSU (right) connected to one of the 5 lines on a single 1A2 Western Electric 2564 key telephone set (left). An actual 2564 phone set manages 5 separate lines, but each of the 5 lines is handled the same way, so for simplicity we're only showing one of the 5 lines. The major 1A2 control signals are shown: "T" (Tip) and.. "R" (Ring) are the wire pair from the phone company. "A" (the "A lead") indicates a line has been picked up, "B" (Battery) is A lead ground for all lines. "L" (Lamp) controls the lamp under the line button. "LG" (Lamp Ground) is the ground for the lamp. The connector pinout for these signals for Line #1: A is pin #27, T is pin #26, R is pin #1, B is pin #2 (and is the common ground for ALL the line's A leads), L is pin #3, LG is pin #28. The lamp circuit uses a separate 10 VAC supply, and is typically in one of 4 possible states: 1. Off -- The line is idle/available 2. On -- The line is "in use" with a call 3. Flash -- Incoming call (Flash is 1 Hz / 50% duty cycle) 4. Wink -- Call is on Hold (Wink is 2 Hz / ~80% duty cycle) The "Flash" (LF) and "Wink" (LW) signals are typically generated by an "Interrupter" in the KSU, consisting of a series of rotary switches driven by a motor that output the different signals, the motor turned on when any line is ringing or on hold. These separate signals are then switched by a series of relays in the line card to drive the lamp. The more modern KSUs (1970s and up) used solid state timing circuits for generating the "Interrupter" signals. Also shown are the RR and RT ringer signals which the KSU will supply AC power to when it wants to ring any phones programmed to ring when a call comes in on this line. Unlike normal phones, 1A2 phone's ringers are NOT connected to the T/R signals; this is so that the KSU can cause the phone to ring from any line, and so the KSU can program which phones ring on which lines. The ringer pair is almost always on the Yellow/Slate pair (pin #20/#45). Not shown in this diagram are the wiring of the buzzer. The "Tip" and "Ring" at the far right are the phone lines directly from the phone company/central office (CO). The state of the phone set shown in this diagram is with the phone 'on-hook' no line selected. Here's a simple animation showing the phone in the various states for a phone call; (1) on-hook/at rest, then (2) someone picking up the receiver (off-hook), then (3) selecting the line, and finally (4) hanging up (back "on-hook"). RING If a call were to come in, the KSU would ring all the phone sets whose ringers are programmed to ring. The KSU will also flash the line's lamp on all the phone sets with access to this line. This way someone looking at the phone can tell which line is ringing, and answer it. ANSWER To answer an incoming call, push the blinking line and pick up the handset. Same for picking up a call on hold. When the line button is selected, this closes all the terminals in the line button's switch. And when the receiver is lifted, this closes all the terminals in the "Hook Switch", grounding the A lead as a way to signal the KSU the line was selected. With the two multipole switches closed, this will: 1) Ground the "A" lead (used by the control circuit to determine a line in use) 2) Connect Tip and Ring from the phone company to the phone's voice circuit (the "hybrid") When the voice circuit is placed across Tip and Ring, the phone company sees this and stops sending the ring signal, and connects the call. In this state, the KSU turns on the lamp (no blinking) for this line on all the phone sets that have access to this line, indicating the line is "in use". In general, the phones are all wired in parallel; all the line lamps, A leads for each line, and Tip and Ring for each line all go out to the sets the same way; if line 1's lamp is lit, it's lit on all phones for line 1. (The exception is if the sets are programmed differently for special use) HOLD To put a call on hold, press the "Hold Button". While the button is down, this disconnects the "A" lead from ground, but keeps the talk circuit for the phone engaged (unlike a hangup which disconnects both at the same time). This causes the KSU control circuit to realize someone pushed the Hold button, causing it to short a resistor ("hold resistor") across Tip and Ring, effectively putting the call on hold. When the Hold Button is released, it mechanically releases which ever line button is down on the set, removing the talk circuit from Tip and Ring. The call is now on hold -- the phone company sees current flowing through the "hold resistor", and keeps the call connected. (The hold resistor simulates the current flow through the phone's voice circuit, such that the phone company keeps the call active) Optionally, the KSU may connect 'music on hold' to Tip and Ring via a voice coil/isolation transformers (in place of the hold resistor) for any calls that are on hold. This gives an audio indication to the person on hold that they are on hold, and weren't hung up on. When a call is on hold, the KSU "winks" the line's lamp (2 blinks per second, 80% duty cycle) on all the phone sets that have access to this line, visually indicating that line has a call on hold. At this point any phone set with access to the line can pick up the call, causing the KSU to disconnect the hold resistor from across the line, and stops the winking lamp, returning the lamp to "steady on". If the person on the other end hangs up while the call is on hold, the phone company will drop the call, opening Tip and Ring briefly (zero voltage) which causes the KSU to disconnect the hold resistor, extinguishing the lamp, and freeing up the line. HANGING UP To hang up the call, put the receiver back in the cradle. This causes the "Hook Switch" to open all its terminals: 1) disconnecting the phone's voice circuit from T and R 2) disconnects the "A" lead from ground ..effectively hanging up the call. The KSU will turn off the lamp for that line on all phone sets with access to the line, indicating the line is available for use. MORE ABOUT "HOLD" -- HOW TO TELL "HOLD" FROM "HANGING UP" The "A" lead signal changes from ground to an open circuit in two situations: 1) When someone hangs up 2) When someone presses the Hold button The only difference is that when someone hangs up, the A lead and T/R signals are all disconnected at the same time, due to the ganged contacts of the hook switch. Whereas, when someone pushes and releases Hold: while Hold is pressed, the A signal is disconnected but Tip and Ring are still attached to the voice circuit (the line button remains down) keeping the call active. This triggers the hold circuit to place a resistor across Tip and Ring (the "Hold" resistor), which will keep the call on hold. Then when the Hold button is released, the mechanics of the Hold button release the selected line button, disconnecting the phone's voice circuit, releasing Tip and Ring from the phone. The hold resistor now keeps the call from dropping until someone picks up the line. This small delay between A becoming open and Tip and Ring being released are the basis for detecting the difference between a Hold and Hang Up condition. How this is done varies based on the hold circuit in the KSU's line cards. The detection has to be done within a short time window, roughly 1/10th of a second; quickly enough to happen before the user releases the Hold button, but not so quickly as to mistake the small delays in the hook switch contacts opening (when someone hangs up) for a Hold condition. If the Hold circuit delay is too slow, the call can be dropped when someone presses/releases Hold quickly. Too fast, and the call might accidentally go on Hold when someone hangs up slowly. (The hook switch's mechanics should prevent this) This is why it's possible to drop a call if one pushes + releases the Hold button too quickly; the Hold circuit will think it's a hangup when the line buttons pop up to quickly, dropping the call. It is for this reason (I think) that the Hold button mechanics are loaded with a very strong spring, making it hard for someone to press and release it too quickly. MULTIPLE PHONE SETS The above diagram is a simplification in that it shows only one line on a single phone set. But in a 1A2 phone system, each set will have several lines, and the sets are all usually wired together in parallel (or "bridged", or "chained" or in some of the Bell System Practices documents, "multipled") either with 66 punch blocks or with C-P-C connection blocks. Still, the above description applies even in the presence of many phone sets, since all the lines and phone sets operate the same way. In a simple configuration, let's say there's two incoming lines and 10 phone sets. This means there will be two line cards in the KSU, one to manage each of the two lines. And let's say the KSU is wired so that all 10 phones can access both lines as Line #1 and Line #2. The phone's lamps, A leads, and Tip and Ring will all be wired in parallel; if any A lead on line 1 is grounded at a phone set, the KSU sees the A lead for that line grounded. If a call comes in on line #1, all phones will show line #1 blinking because the line 1 lamp on all phones are wired together in parallel. Every phone's line 1 lamp blinks. And one or more people can pick up line #1 because the Tip and Ring wires are also wired in parallel. At the KSU, each line has its own 'line card' which handles the four different states for that line: 1) Idle 2) Ringing (incoming call) 3) Answered/in use (off hook) 4) On hold The one line card only manages a single line. The KSU must contain at least one card for each incoming line from the phone company. The lamps on 1a2 phones give a visual indication for each line's state: 1) Idle -- lamp is off 2) Ringing -- lamp blinks at 1 flash per second, 50% duty cycle (500ms on/500ms off) 3) Answered/In Use -- lamp stays on 4) On hold -- lamp "winks" at 2 flashes per second, ~80% duty cycle (400ms on, 100ms off) It's up to the people sitting at the 10 different phone sets to use the system correctly; don't pick up a line if the line is "in use" (its lamp is "on"), and usually a ringing call (lamp is "flashing") is only answered by a secretary (to prevent more than one person answering a call at the same time). And if one wants to make an outgoing call, only pickup a line that's idle (lamp is "off"). A call on Hold (lamp is "winking") should probably be treated like a line that is in use; ignored by everyone except the person who put the call on Hold. There may be an agreed apon rule that if a call seems to be on Hold for a long time, someone can pickup the call to see if the person on Hold is getting frustrated waiting too long for the person who put them on Hold.