atcomsystems.ca/forum Forums Other TDM Phone Systems 1A2 and Comkey Telephones & Systems Greetings and a Request for Some Info on How to Setup Small Keyset System

Before A.C. was invented, batteries ruled the world. Therefore, by tradition, any supply that is used is called battery except the ringing supply (nominally 90Vac @ 20 Hz), which is called generator. Before ringing machines (motor generators) were invented, the subscribers and operators turned a crank on a magneto (generator) to produce ringing current (A.C.)

COTM's (Crusty Old telephone Men) refer to the paths through any circuit as "battery & ground" when trouble shooting. "Battery & ground make the world go 'round" is our mantra.

You will notice that the interrupter is a clever little goober. It has an ST lead ("start") a steady ground connection, and a steady battery (10Vac) lead. Each 400-D card has an ST lead that, when grounded, starts the interrupter motor on a single revolution. If the ST lead is still grounded, the interrupter motor will continue to turn, being fed a ground via its own internal contact. When the ST lead is opened, the motor will continue to turn, via its "windshield wiper" circuit, until it lands on its home position. Then it stops.

You can force this to happen by taking a test lead from ground to the ST lead and touching it momentarily. The interrupter will start, travel around one rev, and then stop at home.

It needs to stop precisely at its home position so that the next time it's needed, its contact cams will start at precisely the right point. On an incoming call, it is necessary to get the ringing cycle started as soon as possible to avoid "glare". (More on glare later.) Another reason it needs to stop precisely is that all of its contacts, in the home position, are closed. The lamps contacts and ringing contacts always are energized right as it starts. In the event of a power failure in the 10 Vac supply to the interrupter, (like your blown fuse) or a motor failure, the lamps will light steadily, and the ringers will ring steadily. It is done this way so that incoming calls would not be missed.

You will note that CO (Central Office) ringing is sent at the cadence of 2 seconds on, 4 seconds off. That ratio was developed by the Bell Labs eggheads as being just urgent enough to get people off the couch to answer the phone, but not so annoying that they took down the Remington Model 31 and blew the damned phone off the table. This cadence creates 10 rings per minute.

In key systems and PBX's, (generally used in office environments) the ringing cadence is 1 second on, 3 seconds off, making the number of rings per unit time higher. This creates more of a sense of urgency, and Ma Bell, being no dumb broad, recognized that an answered phone call is a billable phone call.

In large key systems, where there are many interrupters on many KSU's, the cacophony of many different cadences, starting and stopping at random times, would be an untenable situation. Therefore, in a particular system, one main interrupter would drive slave relay units, (412A KTU's) so that all lamps and ringers would be synchronized.

Glare: During the incoming sequence of a typical phone call from the CO, you will agree that if the incoming call were to be initiated anytime during the 4-second silent period, there would exist a statistical probability that an unsuspecting person might pick up the phone during that period. In a residential situation, that would be a bit unsettling, but would cause no great harm. Person A was calling person B, and B picked up the phone before A heard a ring. No big deal.

However, in an office environment, not everybody who USES a phone is authorized or trained to ANSWER the phone. A mailroom clerk is allowed to call out, but he is not really prepared to answer and direct incoming calls as would a receptionist.

The event of colliding, unintentionally, with an incoming call is called "glare." On manual PBX's ("cord boards") glare was reduced by instructing operators to use the highest-numbered trunk jack (usually at the right side of the row of trunk jacks) for outgoing calls, working their way down toward the left end of the row, as the incoming calls would come in from the lowest numbered jack, (left end of the row) followed by each succeeding numbered jack towards the right side of the board. This policy also had the effect of making the mechanical wear & tear on jacks (made of brass) more even, so that jacks would not need to be replaced so quickly. This instruction was always given to key system users, too. One should always use the highest numbered key on a set to initiate outgoing calls.

On key sets, ABR (automatic button restoration) was invented to help avoid glare. It forced a situation wherein users would pick up the handset before selecting an outgoing line. This avoided the sequence of picking up a handset on a line with an already-pushed line key, causing either glare or an interruption of a call in progress.

On PBX's it can be almost 100% eliminated by the use of ground-start trunks, rather than loop-start ones. I'll let Sam explain GS trunks, as my typing finger is getting tired.

Wow...how do you remember all of this information? I have been reading this thread and continue to learn something every time I look at it.

Jeff it's what COTM did before Tv; learned the trade, which you are trying to do and getting a good background in telephony.

Jeff, a COCP (crusty old college professor) told me that in order to learn something thoroughly, teach it. I have been teaching telephony either officially at the phone company, or unofficially to folks like you who just want to know these things casually, for 30 years.

I can describe every blue-collar operation there is, because I have done those jobs. From power technician to switchman to splicer, to installer/repairman, with side trips into management, business office, and Safety & Security.

On an interview for another facet of my life, I was once asked "What do you know about how the phone company works" and I said, rather flippantly, "I know how they take electrons and make them into pennies."

When I immersed myself in telephony, it was still being provided by the Best Telephone Company In The World. You can see from even my short description of the lowly interrupter how the whole system was designed to cover any situation. It truly was One System. The company would strive to avoid any unanswered call, so that revenue would be maximized, but they also would strive to avoid any foul-up that would diminish their reputation. Collisions, glare, wrong numbers, and delays in completing or answering a call were all to be avoided.

Contrast this dogma with just one item of telephony that you are familiar with in today's telephone environment: Caller ID. It seems like a great innovation, but I'll wager that it causes more confusion, missed calls, misdirected calls, and general frustration than any other "improvement" that has been put out there.

I have a cell phone (yes, gasp, it's true) and when I call people I remind them that the best way to reach me is at my business number. In a small town like mine, we just go by the last 4 digits, as a quick way of announcing our numbers. (There's only one exchange, SHelterisland 9 -- 749 -- and we dial all seven digits) People ignore what I say, and because THEY are using a cell phone, they take the easy way out and just hit "redial" and try to reach me via MY cell number. This screws up my operation, because now I have no record at my office of call details, and many messages don't get through to me due to the Babel Tower of so many competing cell companies and general mismanagement of the cell industry.

So, an innovation has caused a dilution of the old Bell System quest for perfection.

Stick around, and if you really want to know how the whole Big Machine works, guys like Ed, Ken, Sam, myself and the others will explain it. (Still waiting for SilverSomeone to chime in on Ground Start Trunks...)

Just remember, if you learn about it, we expect you to go home and hook up a small 1A2 system, so that the tradition will continue. To paraphrase a famous saying, "Once you go 1A2, you never go back."

Alright Arthur, I confess, I've been a little lazy. Probably still groggy after all that turkey.

Ground Start Trunks:

Loop Start Lines (Standard Telephone Circuits) were/are fine for most purposes. But for critical business applications they fell a little short.

It used to be (I don't know if it still is, everywhere) that if you didn't hang up when you finished your call on a loop start circuit, you'd get dial tone back again. This is especially bad on pay phones, because it lets you make another call(s) for free. It's a problem on PBXs when you conference someone in to an existing call and the third party hangs up and now you're conferenced in with Dial Tone.

It's also a problem with Glare (collision between incoming and outgoing calls).

Instead of just going off hook and completing the loop to get Dial Tone, you (the PBX) have to apply a momentary ground to the Ring side of the line. The CO recognizes a request for DT and returns a ground on the Tip side of the line. The PBX recognizes the confirmation signal and connects a load to the circuit, completing the loop. When the distant end hangs up, the CO opens the Tip side of the line and the PBX recognizes the disconnect signal and hangs up the line.

This "handshaking" almost completely eliminates Glare, the problems with third-party conferencing and people cheating on pay phones.

If I wasn't clear enough, please let me know and I'll try to elaborate it more clearly.

Also, there were other signaling methods that the phone companies came up with - Wink-Start, E&M (2 & 4 wire), Delay Dial and others. All these were part of the tapestry of Telephony that made the American Telephone System the best in the world.

Sam

Sam, exactly the correct explanation. We just need to remind the readers that we are talking about two-way CO trunks.

The one thing that made GS trunks glare-proof was the fact that as soon as they were seized at the CO end (for an incoming call towards the PBX) the CO circuit sent a ground out on the (normally open) Tip side, as you said. This grounded tip remained until the call was over; then it opened again.

The trunk circuit in the PBX saw that ground, and immediately made the trunk busy to any outbound attempt (an extension user dialing "9" for instance) by the PBX. This occurred either during the ring period, or the silent period, so that the 4-second interval of silence was never an issue: The moment that the trunk was seized, it was busy, and therefore no glare.

Arthur, I have room on my wall for a shoebox KSU

I just sold a couple on E-U-Know-Where. If I have another, I'll email you.

I owe someone a dial card retainer kit for a 500 set. Was that you?

It was for a 302 set...let me know if you have one.

"Once you go 1A2, you never go back."

I think that I am arriving at the point where I can say that I whole heartidly concur.

Arthur, I am gobsmacked to say the least by your knowledge of the trade. You understand the system, which for the most part, does not exist anymore (the understanding of the system I mean).

Techinicians today, are for the most part, parts replacers. Nothing gets "fixed" anymore, just tossed and replaced. It's like in IT. You have someone come in with a problem, and the technician concludes it's the motherboard. Well, you know the motherboard is a big place. What is it exactly on the motherboard that is causing the problem? Today, only engineers can answer that question, maybe. So, the motherboard gets tossed with 98% functionality, and the customer pays 700$ for a job that could cost 5 bucks, minus the man hours.

One more question if I may, and please pardon my ignorance, what exactly is the role of the interruptor?