AUTODIN Terminals
A Data Communication Historical Series



Data Source: AUTODINERS (Jim Keenan, Keith Jolley, Bob Pollard, Ron Railing,)


   There were many different terminals used in the AUTODIN system. These ranged from the uncontrolled teletypewriters (Mode II) and controlled teletypewriters (Mode V), to the sophisticated computer operating in a fully controlled Mode I. Nearly every type of communications device has been used at one time or another as an AUTODIN terminal.


   The list would include Teletype Corporation models 28, 35 and 37 teletypewriters used to transfer narrative messages; IBM 360's and 370's, UMIVAC 1004 and DCT 9000, the RCA Spectra 70/45, and Honeywell 200 computerized terminals. These systems transferred data from information recorded on cards, magnetic tapes, magnetic disks, and punched paper tape.


   Also there were other message switching systems interconnected to AUTODIN. These included the Automatic Message Processing System (AMPS); a Burroughs D825 computer, the Tactical Automatic Data System (TADS); a Burroughs 3500 computer, and the Local Digital Message Exchanges utilizing various computers such as: UNIVAC 418, IBM 360/40/50, CDC 1700 and RCA Spectra 70/35.


   The teletypewriters operated at speeds from 50 to 300 baud; the Teletype models 28, 35 and 37 operated at 50-75 (28), 110 (35) and 150 (37) Bauds and were used normally for narrative traffic. The ADP terminals (Automatic Data Processing) consisted of peripheral devices and controllers (small computers, such as the IBM 360/20). These terminals could be single units, such as a card terminal or a magnetic tape terminal, or a combination, such as a compound terminal. The speed range for these various terminals were: below 300 bps (low speed), from 300 to 600 bps (medium speed), and above 600 and up to 4800 bps (high speed).


   One of the original terminals that basically caused the origination of the COMLOGNET system (later AUTODIN) was the punched card transceiver machines, which consisted of a card reader/punch, and a refrigerator-sized unit and electronics package made by IBM. Transmission was synchronous using a modified Hollerith code designated the Fieldata code and used internal MODEMS to transmit punched card data over private telephone lines (leased lines) connecting the Air Materiel Command bases. These IBM card transmission-receiver units were frequently called the Compound Terminal or "Cube" by operations and maintenance personnel.


   The Fieldata code set was a variation of the Hollerith code set used for punched card (Data Processing) operations. The punched card was designed to use a 12-bit code set, where the Fieldata code set used 8 bits, including the parity bit. Later, following the implementation of the ASCII code into the AUTODIN system, the Fieldata code was converted to ASCII when sending or receiving over the communications lines.


   AUTODIN terminals operated in three basic modes. The first was Mode I (computerized as mentioned above), which communicated with the ASC either in Fieldata (early) or in ASCII. This mode was fully controlled, and both ends had the ability to stop the transmitter. In addition, both character parity (odd for data, even for control and framing) and horizontal parity was used to guard against errors. This technique was theoretically inferior to the cyclic redundancy checks used in more modern systems, but in practice it served very well. Mode I transmission was in "line blocks". Each line block was comprised of 80 data characters (original IBM card format), and four block framing characters, two on each end. The first framing character was a Start of Message (SOM) for the first line block of a message, and Start of Text (STX) for subsequent line blocks. The second framing character was a DELETE character in terminal transmission and a security character on trunk transmission. The third framing character was an End of Text (ETX) for all line blocks except the last, and an End of Message (EOM) for the last. The last framing character was an addition without carry of the "one" bits of the line block, used for error detection. Error correction was by the receiver transmitting a Non-Acknowledgment (NAK) for a line block in which an error was detected.


   In addition to Mode I terminals, which converted both punched card and teletypewriter code into ASCII, AUTODIN accepted input directly from two types of teletypewriter equipment, in two modes.


   The first equipment type was called "five-level" (modified Baudot code), in which characters and symbols were encoded into a combination of five positive or negative pulses (1's or 0's); these could also be punched into paper tape, the same kind used in the older manual relay systems. When received at the ASC, the "front end" equipment translated the teletypewriter code into the correct internal code for that type of ASC (Extended Fieldata for CONUS or ASCII for Overseas).


   Later teletypewriter equipment employed the ASCII code directly, using a tape wide enough to accept eight positions. It employed parity (8th bit position), and thus had a primitive error detection capability, which allowed the ASC to reject an entire message if an error was detected.


   In addition to the code differences, the teletypewriter equipment used two Modes, Mode II and Mode V. Mode II was simply uncontrolled teletypewriter. In five-level code, it had no error detection other than the use of channel sequence numbers to guard against message loss. Mode V was a more primitive form of Mode I. It had start-stop capability and, in eight-level teletypewriters, the error detection capability mentioned above. Mode V also employed channel sequence numbers to preclude loss of an entire message.


The following is a brief description of the Mode 1 through Mode V operational functions.



A synchronous transmission mode using the ASCII eight-bit (seven data bits plus parity) code set. The transmitted characters are synchronized bit by bit by utilizing the ASCII sync characters. Other synchronous control characters are used for idle line conditions. A start or stop bit is not required. Various data code elements, such as the ASCII, Hollerith punched card, magnetic tape, computer generated data, facsimile, etc. can be transmitted/received after it has been converted to the ASCII odd parity code. Control characters are used to maintain the proper data protocol and control.


MODE 11:

This mode is an uncontrolled teletypewriter operation with full duplex capability (simultaneous transmission in both directions). Normally uses the Baudot (modified) or ASCII code. In the uncontrolled mode there is not any control over the sending or receiving functions. The transmission of data may begin at any time and the receiving equipment must be in an operational mode at all times.


MODE 111:

This mode is a Mode 1 operation with data transmission in only one direction. This mode is not used in the AUTODIN System.



This mode is an uncontrolled teletypewriter operation with data transmission in only one direction. This mode is not used in AUTODIN.



This mode is an asynchronous operation (transmission) normally using the ASCII or Baudot (ITA 2) code set. A start (space) bit and a stop (mark) bit are required. Each character transmitted begins with a start bit and ends with a stop bit. The start bit is usually one bit long (duration) and the stop bit is from 1.5 to 2 bits long. Therefore an 8-bit ASCII code character would become a 10.5 to 11 bit character. The Baudot (modified) character would become a 7.5 to 8 bit character. The start bit trips the teletypewriter clutch mechanism allowing the following bits to be decoded (character definition) and the stop bit latches the clutch stopping the mechanism. This would repeat for each character received. Normal idle line condition would be a positive (mark) voltage condition.


Mode V may function in either a controlled or uncontrolled environment.


Mode V link for a picture of the terminal

Autodin Ancedote from Doug Eyre

While stationed at RAF Welford, England our Autodin (Terminal) broke once in the entire time I was there.  We were down about 48 hours before a technical representative was dispatched.


This guy had to be seen to be believed - bowler hat, pin striped suit with vest, black shiny shoes and an umbrella.  The only thing he was worried about was the directions to the local post office because he was a stamp collector.


This tech opened up the front of the equipment and for two days all he did was turn schematic pages in the TO and point his little penlight inside the machine. After two days, he received a phone call from European HQ that said there was a problem with another circuit at a higher priority site. So he left with the TO open on the work bench and the equipment open in the rack.


One of our maintainers, a SSgt by the name of Tad Phillips, decided to try tracing the schematic.  To the best of my recollection this was still a discrete component type of machine with transistors but no ICs.


Anyway, Tad traced the circuit and then pulled out a small sub-unit and found a cracked resistor.  We were told that this resistor was a part classed as NRTS (not reparable this station) and that it would have to be sent to the repair depot.


Our circuit had now been down for about five full days. I told Tad to call the local TV repair shop in the small town of Newbury and see if they had a resistor of that value, wattage, and physical size. They did, so he went down town, bought it and installed it.

Voila! -The circuit came up. Did we get a thank you and well done praise from HQ?  Heck no! We were "chewed out" because a   non-milspec part was installed into the unit to fix it!  We offered to remove the part but HQ thought that probably wasn't a good idea. Then all went back to normal!




An uncontrolled Teletype means it operates without any controls over when it sends or receives. An operator could start transmission of a message at any time, whether the receiving device was ready or not. In order to avoid lost messages a start of message (SOM) and an end of message (EOM) sequence, along with message sequence numbers, were used. A typical SOM would be "ZCZC" and an EOM would be "NNNN". This would allow the receiving operators (device) to insure they received a complete message, although, errors could be present in the body of the message.


   Before the ASCII code set (8 bits, including parity) was available the existing code sets (5 bits, no parity bit) did not allow for control characters that would control the activation (start/stop) of the sending and receiving devices. In the automated perforated tape- switching center (Plan 55) the ZCZC and NNNN were used for in center start and stop and to control cross- center transmission.