Plan 51 Switching Center

A Data Communication Historical Series

By Bob Pollard

PLAN 51:

A semi-automatic punched paper tape data communications-switching center, with patch cord facilities, push-button panels and relay switching matrices.


   All messages received (input) at the center were via a paper tape printer-perforator (punch) (39A), which created a printed and punched paper tape based on the message content. The printer-perforator punched and printed on "chadless" tape. Chadless tape implied that the holes punched in the tape did not result in the hole remains (chad) being scattered all over the equipment or floor. When the holes were punched in the tape only three quarters of the hole was punched which resulted in the chad being retained or connected to the tape by a small section of chad. This allowed the transmitter mechanical pins to penetrate the holes in the tape and read the tape because the chad was pivoted up, but still connected to the paper tape. The Baudot code was used and the character bits representation was punched across (column) the tape, which required a tape width that would accommodate five bits.


   The paper tape from the perforator was fed from a roll of paper tape contained on a reel. After the tape was printed and punched it fed directly into (down) a paper tape chute into a temporary storage bin, then fed under a tape tensioner and across a paper tape transmitter (5032), and then onto a tape storage reel immediately to the left of the transmitter. The transmitter would feed any idle characters between messages automatically and then stop upon detection of the Start of Message/Message address indicators. A white "beehive" Message Waiting Lamp above the transmitter would illuminate to indicate when a message is present for routing, which alerted the operator. In addition a light illuminated the transmitter area allowing the operator to read the routing indicator(s) (address). If the message contained a single destination routing indicator, the appropriate push button would be depressed, which would cause the message to be routed to the proper cross-office (center) destination. Multiple addressed message required a different operator procedure. This cross-office connection was accomplished through the associated electrical-mechanical switching matrices. Once the electrical relay connection was accomplished the transmitter would begin sending the message to the addressed cross-office reperforator associated with the addressed destination. This punched tape was fed into a tape storage bin and then to the sending (output) transmitter. As the tape fed (stepped) through the transmitter it was routed to a paper tape take-up reel. It appears the name "Reperforator Message Switching Center" was derived, from the reperforation (duplication) of messages from receiving to sending positions (cross-office).


   Other procedures and switching options were available to the Operator for the handling of multiple address indicators and other special requirements. Each connected tributary required an assigned receive (input) perforator and a send (output) transmitter position. A tributary could have both send and receive capability, but may only require a send or receive service.


   The Baudot (or similar) code set was used for all messages. Since the Baudot code did not have Start of Message (SOM) or End of Message (EOM) symbols it was necessary to use a sequence of characters for those purposes. Common usage was “ZCZC” for the SOM and “NNNN” for the EOM. When a complete message was received (at the center), EOM present, the message could be routed to its destination, based on the routing indicator(s). Each message would include, as a minimum, a SOM, Address field (indicators) and an EOM.


   Western Union's success with the Semi-Automatic Plan 51 Switching System qualified Western Union as the prime contractor to design, provide the engineering functions and install the Plan 55 Automatic Switching System for the United States Air force (USAF). The Plan 55 System was in operation during the implementation phases of the AUTODIN system and was gradually phased out of operation.