Autodin (short for “AUTOmatic DIgital Network”) was the Department of Defense’s first computerized message switching system. It was developed to replace a semiautomatic teletype switching system called Plan 55. Like Plan 55, Autodin was developed in the late 50’s, early 60’s by Western Union. The original five Continental US (CONUS) sites were operated jointly by the military and Western Union. Subsequent sites located in other parts of the world were operated by Philco-Ford and military agencies. Autodin remained in service into the 2000’s and has been replaced by the Defense Message System.
I worked as a tech controller in the Autodin AESC (automatic electronic switching center) at Norton AFB, San Bernardino, California, from mid-1966 until 1968. The Norton switch was the first one to come on-line (around 1962, I believe) and maintained a heritage of being the most reliable switch in the system. The last full year I worked there, the switch maintained an uptime of 99.97%. That may not seem amazing, but you have to understand that this was with second-generation computers.
Second Generation Computers
Autodin was initially implemented using second generation computers. These were built using semiconductors (as opposed to vacuum tubes in the first generation) but no integrated circuits. Even simple flip-flops were constructed using discrete transistors, resistors, and capacitors. Usually the circuits were built on small printed circuit boards that were plugged into racks with interconnecting wiring on the back. A large computer typically had thousands of such cards.
Communications Data Processor (CDP)
The primary switching computer for Autodin was called, simply, the CDP. It was a monstrous thing, comprising 19 racks of equipment, each containing hundreds of circuit cards. It was built by RCA and, if I recall correctly, was based on the RCA 501 commercial computer. It was a word-oriented (as opposed to today’s byte-oriented) machine, with 56 bits per word and ferrite core main memory storing 40,960 words. DC power requirements were quite high (hundreds of amperes) and were supplied by large motor-generator sets (AC motors driving directly connected DC generators). It needed a large air conditioner to keep it cool.
There was a wide data bus connecting all the racks together via large cables. Because the signal path was so long, data placed on the bus was parity protected. In fact, the most frequent failure mode was a BTPE (pronounced buhTEEpee), a bus transmission parity error.
It supported a variety of I/O devices. Magnetic drum storage units holding 322 kilobytes were used to store temporary operating data. Console printing was done on Flexowriter automatic typewriters. There was a high-speed line printer for dumps and other large printouts. A bank of half-inch reel-to-reel tape drives provided secondary storage for initial program loading, logging, and off-line message storage. It also had interfaces to multiple Accumulation and Distribution Units (ADUs – see below) and a specialized computer called the Automatic Display Processor (ADP – see below) that drove the system console.
Each switching center had two CDPs, arranged in a hot standby configuration. If the online CDP failed, all I/O devices were automatically switched to the standby processor, which took over with little discernible interruption.
The CDP was notable in the history of computer development because it was microprogrammed. Each instruction in the custom language developed for the Autodin application decoded into a subroutine of simpler operations that the hardware could perform.
The message switching program on the CDP ran in a cyclic manner through receiving, switching, and sending phases. This could be observed easily on the CDP maintenance console, which provided light displays for many of the CPU registers. They were arranged to display 27-bit half words in a 3 by 9 arrangement. Each column of three binary bits could easily be read off as an octal digit, which was used for troubleshooting and debugging purposes. The rapid cyclic flashing of the register displays became very familiar to system operators and usually the first indication that anything was going wrong was a change in the light pattern as the online unit executed a transfer to the standby unit.
Accumulation and Distribution Unit (ADU)
ADUs were the equivalent of what would later be called communication front-end processors. They provided the interface to the data communication circuits connecting the switching center to tributary terminals and other switching centers. Each switching center had three ADUs, with two required for normal operations. The third was maintained in warm standby and could be substituted quickly for one that had failed.
Automatic Display Processor (ADP)
The ADP was a curious little computer used only to drive the System Console, which provided convenient display and control capabilities for operating the message switching system. It was curious in that it used mercury delay lines for main memory. That is, the contents of main memory were kept circulating in a loop through the delay lines, with the processor executing its cyclic program as the instructions streamed by. It was also curious in that its initial program load was from punched paper tape.
Tape Search Unit (TSU)
There was also a stand-alone computer called the Tape Search Unit, which was used to search history tapes to recover messages that had failed to be delivered for some reason. This was the first computer I ever worked with that required manual entry of a bootstrap program to get it started. If the power to the TSU had been turned off you had to manually enter about thirty instructions in binary by punching buttons on the TSU front panel. That bootstrap program could then read the tape search program in from mag tape.
The primary terminal equipment used at Autodin tributary stations was called the Compound Terminal (CT) or, more familiarly as “the cube.” It was called that because the processor unit, made by IBM, was exactly square, about 3-1/2 feet on the sides, and about 5 feet high. It provided facilities for transmission and reception of 80-column punched cards using an automated version of the IBM 026 keypunch. It also could send and receive text messages using a Teletype ASR28, which had a paper tape punch, tape reader, and page printer. Messages were prepared by punching them into either cards or tape, then reading them into the system. The CT then transmitted them into the Autodin switch.
Larger facilities used a faster terminal built by IBM, which could punch paper tape so fast that it would fly four or five feet out of the punch before dropping to the floor. Later there was another high speed terminal built by Univac, called the 1004/DLT6, basically a Univac 1004 computer with special data com equipment (the Data Link Terminal model 6) attached.
Mode 5 Terminals
In an attempt to reduce the cost of Autodin terminal equipment, while still obtaining its automatic acknowledgement of messages by the receiving station, the DOD commissioned Western Union to provide an asynchronous Autodin terminal that could be used with existing Baudot (five-bit code) teletype equipment. The result was an asynchronous system called Mode 5 (the main synchronous terminals were Mode 2).
A Mode 5 control unit could be connected to an existing teletypewriter terminal to provide automatic numbering of outgoing messages, and automatic acknowledgement of incoming messages. With no control characters available in the Baudot code, it transferred control sequences in a novel way–one that was later used with the Smartcom modem control codes to switch from data mode to command mode–the use of idle blind periods. A control sequence consisted of a pair of identical characters that followed an extended idle period on the line. The control unit inserted pauses in transmission so it could send acknowledgements back for received messages, and it automatically detected pauses and control sequences sent by the Autodin switch.
Mode 5 transmissions were not error protected, but there was at least assurance of delivery of each message by the automatic numbering and acknowledgement system.