The Teletype Model 28 is a product line of teleprinters, 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 Corporation built, this line of teleprinters used an exchangeable type box for printing and sequential selector "Stunt Box" to mechanically initiate non-printing functions within the typing unit of the teleprinter, electrically control functions within the teleprinter and electrically control external equipment. The Teletype Model 28 is more rugged and more expensive than later Teletype Model 32 machines. The Teletype Corporation introduced the Model 28 as a commercial product in 1953 after being originally designed for the United States military. There are three versions of the Model 28 teleprinter:
the Model 28 ASR, (Automatic Send and Receive), which has a built in 5-level paper tape reader and 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 paper tape reader/punch.
History Teletype Corporation's Model 28 line of communications terminals was first delivered to the US Military in 1951 and commercially introduced in 1953. This series of teleprinters and associated equipment was popular in the various branches of the United States Armed Forces, and commercially in the financial and manufacturing industries. 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 1981.
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 run at 100 words per minute with less maintenance than that required by a contemporary teletypewriter running at 60 words per minute. Additional design criteria included the requirements to run quieter and be lighter than previous teleprinters. The Model 28 equipment was also designed to successfully operate in a wider range of temperatures and operate in moving vehicles. The Model 28 equipment adjustments are made by turning screws and not by bending metal bars and levers as is done in the later Model 32 and Model 33 series of teleprinters. The Model 28 printing unit frame is lighter due to the use of stamped sheet metal instead of cast iron. 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 an incoming message. One of the design advances in the Model 28 is the use of a compact and lightweight type box. In the Model 15, the moving carriage assembly weighs slightly over five pounds. The carriage assembly in the Model 28 weighs eight ounces. This weight reduction allows for a faster carriage return, necessary for 100 word-per-minute operation. The lighter carriage assembly effectively eliminates the effect of gravity on operation in the air and on the sea where level operation in not practical. The type box is easily removed, without tools, for cleaning and there are many type box options. The Teletype Parts Bulletin 1149-B. lists sixteen available Model 28 type box options. Another design advance in the Model 28 was the use of a new all-steel clutch that uses internal expansion to minimize wear and reduce the need for lubrication. The Model 28 series was also modular. The keyboard unit, the printing unit, the perforators and the transmitter distributors are self-contained and swappable to aid in troubleshooting and maintenance. The Model 28 printing units contain a sequential selector, known as the Stunt Box, which monitors the received line and the keyboard signals for receipt of single characters and sequences of characters. Forty-two functional control positions are provided, some of which as used for functions such as carriage return, line feed, figures shift, letters shift, unshift-on-space and the signal bell. The removable Electrical Service Unit (LESU) is an integrated chassis housing the line fuse, convenience receptacle, terminal blocks for wiring options and mounting facilities for optional sub-assemblies. The Model 28 is usually geared to run at maximum speed ten characters per second speed, i.e. 100 words per minute (wpm), but other speeds are available. These speeds include 60 wpm, 61 wpm, 65 wpm, 66 wpm, 67 wpm, 71 wpm, 75 wpm, 81 wpm, 88 wpm, 100 wpm, 106 wpm, 107 wpm and 200 wpm (on some paper tape equipment).
Teletype Model 28 line of equipment
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