A product line of send and receive 5-level Baudot code page printers, typing and non-typing tape perforator and tape reperforators, fixed head single contact and pivoted head multi-contact transmitter-distributors, and receiving selector equipment; regarded as the most rugged machines Teletype built. Used an exchangeable type box for printing. Used a sequential selector "Stunt Box" to mechanically initiate non-printing functions within the typing unit of the page printer, electrically control functions within the page printer and electrically control external equipment.
The Teletype Model 28 is a line electromechanical teleprinter equipment designed for heavy-duty office. It is more rugged and more expensive than later Teletype Model 32 machines. The Teletype Corporation introduced the Model 28 as a commercial product in after being originally designed for the US Military. There are three versions of the Model 28:
- the Model 28 ASR, (Automatic Send and Receive), which has a built in 5-level paper tape reader and tape punch;
- the Model 28 KSR (Keyboard Send and Receive), which lacks the paper tape reader and punch;
- the Model 28 RO (Receive Only) which has neither a keyboard nor a reader/punch.
History
Teletype Corporation's Model 28 line of communications terminals, introduced in 19YY, is one of the most popular teleprinter in the data-communications industry. Over Model 28 and 33's were made by xxx,
A Model 28 cost about $XXXX, much NNNN than other teleprinters at the time, such as the Creed and the XXYYXX. Teletype machines were gradually replaced in new installations by dot-matrix printers and CRT-based terminals in the mid to late 1970s. Basic CRT-based terminals which could only print lines and scroll them are often called glass teletypes to distinguish them from more sophisticated devices.
Teletype Corporation discontinued Model 28 production in 19YY.
Model 28 ASR vs. ASR-28
While the manufacturer called the Model 28 teleprinter with a tape punch and tape reader a Model 28 ASR, many users, specifically computer users, called this equipment an ASR-28. The earliest known source for this Teletype Corporation equipment naming discrepancy comes from Digital Equipment Corporation documentation where the September 1963 PDP-4 Brochure calls the Teletype Model 28 KSR a "KSR-28" in the paragraph titled "Printer-Keyboard and Control Type 65". This naming discrepancy continued from the Teletype Model 28 to other Teletype equipment in later DEC documentation.
Technical information
The design objective for the Model 28 was a machine that would... Teletype made adjustments by turning screws and not by by bending metal bars and levers as was done in the laetr Model 32 and Model 33 series of teleprinters. Model 28 parts were heat treated and hardened. The Model 28 ASR allowed the user to operate the keyboard to punch tape while transmitting a previously punched tape and to punch a tape while printing something else.
The Model 28 used the five-bit Baudot code, also known as CCITT International Telegraphic Alphabet No. 2.
The Model 28 is usually geared to run at maximum speed ten characters per minute speed, i.e. 100 words per minute (wpm), but other speeds are available: 60 wpm, 66 wpm, 68.2 wpm, and 75 wpm. There were also many type box options. The Teletype Parts Bulletin lists XXXX available Model 28 type box options.
The Teletype Model 28 contained an answer-back mechanism that was generally used in dial-up networks such as the Telex network. At the beginning of the message, the sending machine could transmit an enquiry character or WRU (Who aRe yoU) code, and the recipient machine would automatically initiate a response which was encoded in a rotating drum that could be programmed by breaking off tabs. The answer-back drum in the recipient machine would rotate and send an unambiguous identifying code to the sender, so the sender could verify connection to the correct recipient. The WRU code could also be sent at the end of the message. A correct response would confirm that the connection had remained unbroken during the message transmission. Finally the sending machine operator would press the disconnect button. Note that the receiving machine did not need operator intervention. Since messages were often sent across multiple time zones to their destination, it was common to send a message to a location where the receiving machine was operating in an office that was closed and unmanned.
The Teletype Model 28, including the stand, is XX inches high, XX inches wide and XX inches deep. The Teletype Model 28 weighs XXX pounds, including paper. This machine requires less than XX amps at 115VAC 60Hz. The recommended operating environment is a temperature of 40 to 110 Fahrenheit, a relative humidity of 2 to 95 percent and an altitude of 0 to 10,000 feet. The printing paper is a 8.44 inch by 4.5 inch diameter roll and the paper tape is a one inch by 1000 foot roll. Ribbons are 0.5 inch wide by 60 yards long with plastic spools and eyelets for proper ribbon reverse operation.
Interface
The Model 28 could accommodate an This was replaced by a blank panel on units operating on a current loop. There were different kinds of Model 28 XXXYYYs that included Rotary Dial, current loop and polar signalling.
Related machines
The Model 35 line used a similar mechanism
Historical impact