The Peculiar Marietta Crossbar Machine

The Marietta No. 5 Crossbar machine was installed in 1963 in a building addition, upstairs from the existing No. 1 Step-by-Step office. The new machine provided service for new residential and business customers on the new "422" code and also provided Centrex CO service for Lockheed on the "424" code.

The machine had a very strange setup that forced customers to dial "6" to reach most metro numbers so that dialing would be the same as the existing Marietta Step-by-Step office. This created 8 digit dialing.

These were the dialing instructions for Marietta Customers in 1963:

To call Marietta 422, 424, 427, and 428 numbers,
Smyrna 432, 435, and 436 numbers, Dallas 445 numbers, Woodstock 926 numbers, Austell 948 numbers,
Douglasville 942 numbers, Powder Springs 943 numbers, and Ackworth 974 numbers ….. dial the 7 digit number.
To call all other Metro Atlanta calls ….. dial "6" plus the 7 digit number.

Configuration of the Machine for "6 Plus"

A No. 5 Crossbar normally expects local numbers to be 7 digits and performs route translation on the first 3 digits only. A No. 5 can do more complex translations involving the first 6 if required on long distance calls, using a Foreign Area Translator but this would not have helped in this situation.

When placing a call in the No. 5 Crossbar, your digits are recorded in a device called the Originating Register. At end of dialing, the digits are passed to another device called the Completing Marker. The Completing Marker is the "brains" of the machine. It has wired translations to convert any possible 3 digit code into a route to a destination. In a full sized No. 5 office, there would be 8 Completing Markers and as many as 150 Originating Registers. It would not have been an easy feat to change the behavior of the marker to handle this unusual dialing pattern but they found a way to make it work.

The No. 5 was able to handle 8 digit local calls from its original design back in the 1940s when there were still manual central offices around. These manual offices used party line letters. So the Panel system, No. 1 Crossbar and No. 5 Crossbar machines were designed to accept an extra digit at the end of a 7 digit local number for a party line. So back then you could have a number like PE6-2474-J. The crossbar office would route the call to the PE6 office and send it "2474J". This made it possible to put together the kludge that they used in Marietta.

There were two things that had to be done. First, the marker's had to route all possible local numbers on the first 3 digits. So calls dialed with a "6" in front of an office code, such as "6-349-4201" would look to the marker like "634-9420-1", as if there was a party line digit of "1" at the end. In a case like this, it would choose a route, based on the digits "634" to the Ben Hill Office (code 349), delete the first 3 digits dialed and send the last 5 to Ben Hill ("9-4201"), which is what Ben Hill would expect.

The second requirement was to use Pre Translators. Not all local numbers would have a "6". For example, the Austell "948' office was dialed without the prefix. When the Originating Register was recording the dialed digits, it needed to know whether to consider dialing to be complete after 7 digits or 8 digits. So the Originating Register used the services of a Pre Translator. On the 3rd digit dialed, the register would connect to the Pre Translator and pass it the information. The Pre Translator wiring would tell the register how many digits to expect.

Dialing a call to an office that was supposed to have the prefix and not including the "6" would be blocked. So, in the Ben Hill example, the code "349" is a valid code but it would be blocked and go to a recorded announcement.

All of this would have been enough if it weren't for the requirement to switch tandem calls, which created conflicting codes.

Tandem Switching of Calls for Dallas, Ackworth and Woodstock Offices

The Marietta machine also served as a tandem for the three smaller offices in Dallas, Ackworth and Woodstock. Those customers dialed "8" to call all of Metro Atlanta. The digit "8" put the call through the Marietta crossbar machine. Since these customers couldn't be expected to dial "8" plus "6", the Marietta machine handled these calls on a 7 digit basis, like it would have done for its own customers if not for the weird setup.

So the machine was able to route calls to the same destination for its own customers with the "6" code and for the tandemed offices without the "6" code. And it had to enforce the rule based on who was dialing.

This was done using Class of Service. The incoming tandem calls from the three offices above were one class of service and the local Marietta callers had a different class of service.

This arrangement created conflicting codes. In the Ben Hill example above, the customer dialed "6-349-4201". Another customer from Woodstock could conceivably dial "634-9420" in the Toco Hills office. For this reason, the originating registers in Marietta used a feature called "stations delay" that came from the old days of Manual Offices with party line letters. For 3 digit codes that might or might not have an 8th digit, the Pre Translator would tell the register to wait for 5 seconds after the 7th digit to allow for an 8th digit. So in this case "634" would have had a stations delay.

This situation even occurred on long distance calls because the originating registers could not use the initial "1" for a direct dialed call to determine the number of digits to be dialed. If a Marietta customer dialed a Calhoun number, by dialing "1-629-2234", the register did not consider the "1" to be a part of the number and therefore the code "629" could be an office code itself or could be part of a number like "6-292-2341". So it added the stations delay on these calls. The marker probably recognized that a "1" was dialed and used the long distance route to reach "629" in Calhoun instead of the local route that would end up going to the "292" number.

Here are a few examples of how calls would be routed
Via Digits
233-1234 233 Buckhead Step Direct 31234 Blocked1 Ok
432-3347 432 Smyrna Direct 23347 Ok Ok
445-2214 445 Dallas Direct 2214 Ok Ok5
451-1011 451 Chamblee Atlanta XBT 4511011 Blocked1 Ok
6-233-1234 623 Buckhead SxS Direct 31234 Ok Blocked5
6-237-1234 623 Buckhead SxS Direct 71234 Ok Blocked5
6-236-1234 623 Buckhead SxS2 Direct 61234 Ok Blocked5
6-261-9011 626 Buckhead 5XB Direct 19011 Ok Blocked5
6-521-4275 652 51 Ivy 5XB #1 Atlanta XBT3 5214275 Ok Blocked5
6-522-4275 652 51 Ivy 5XB #2 Atlanta XBT3 5224275 Ok Blocked5
6-523-4275 652 Auburn Ave SxS Atlanta XBT3 5234275 Ok Blocked5
6-445-2214 644 Dallas Atlanta XBT 4452214 Ok Blocked5
6-448-5785 644 Norcross Atlanta XBT3 4485785 Ok Blocked5
6-475-2323 647 Alpharetta Atlanta XBT3 4752323 Ok Blocked5
6-478-2323 647 Jonesboro Atlanta XBT 4782323 Ok Blocked5
6-629-1174 662 Calhoun (toll) Atlanta XBT 6291174 Failed at XBT6 Blocked5
1-629-11749 629 Calhoun (toll) Atlanta Toll 6291174 Ok Blocked5
629-11749 629 (ambiguous) Atlanta XBT 291174 Failed at XBT10 Blocked5
6-292-3232 629 Indian Creek Atlanta XBT 2923232 Ok Blocked5
627-5676 627 Lakewood Atlanta XBT 6275676 Blocked1 Ok
6-627-5676 662 Lakewood Atlanta XBT 6275676 Ok Blocked5
6-349-1111 63411 Ben Hill Atlanta XBT 3491111 Ok Blocked5
634-9111 63411 Toco Hills Atlanta XBT 6349111 Blocked1 Ok

1 Call requires a "6" when dialed from Marietta but not from Dallas. The Dallas customer already has dialed "8" to get here.
2 A vacant code is not caught by the crossbar translation so it gets sent to the destination office.
3 Direct trunking would probably have been used if not for the limitation.
4 The Dallas customer is able to incorrectly dial a "8" when calling inside of Dallas, causing the call to go through Marietta and back.
5 The Dallas customer should not be dialing the "6" prefix into the Marietta office after dialing "8".
6 Attempting to call Calhoun as a local call with a "6" prefix, would take the same route as a 627, going to the downtown tandem. It would fail on the tandem and go to an announcement.
7 The Dallas customer can't dial "1" into the Marietta crossbar.
8 The call is translated properly but blocked because it requires a "1"
9 A time delay occurs after the seventh digit is dialed to see if an eighth digit is dialed.
10 The call fails at the XBT because there is no 291 code and also there are only 6 digits sent.
11 This code dialed as 7 digits takes a different route than the code dialed as 8 digits.

The inability to translate on 4 digits, with the extra "6" made it impossible to take different routes to places having the same first 2 digits. It also meant that the machine couldn't detect vacant codes on some offices and ended up sending the call anyway.

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