DECwriter

The DECwriter series was a family of computer terminals from Digital Equipment Corporation (DEC). They were typically used in a fashion similar to a teletype, with a computer output being printed to paper and the user inputting information on the keyboard. In contrast to teletypes, the DECwriters were based on dot matrix printer technology, one of the first examples of such a system to be introduced. Versions lacking a keyboard were also available for use as computer printers, which eventually became the only models as smart terminals became the main way to interact with mainframes and minicomputers in the 1980s.

There were four series of machines, starting with the original DECwriter in 1970, the DECwriter II in 1974, DECwriter III in 1978, and the final DECwriter IV in 1982. The first three were physically similar, large machines mounted on a stand normally positioned above a box of fanfold paper. They differed primarily in speed and the selection of computer interfaces. The IV was significantly different, intended for desktop use and looking more like an IBM Selectric typewriter than a traditional printer. Most models were available without a keyboard for print-only usage, in which case they were later known as DECprinters.

The DECwriter's were among DEC's best-selling products, notably the II and III series.

DECwriter

The original DECwriter was introduced in November 1970 at the Fall Joint Computer Conference.[1][lower-alpha 1] Also known by its model number, LA30, it was one of the earliest dot matrix printers to be introduced to market, only months after the seminal Centronics 101 that May at the Spring Joint Computer Conference. At the time, most small computer systems were accessed using surplus teletype units, and the LA30 was intended to be used in the same general fashion. As such, its only computer interface was a 30 mA current loop, as used on teletype machines, with the explicit goal of "having been designed to replace the standard Teletype Model 33, 35 and 37 KSR."[2]

The LA30 used a 64-character ASCII-based character set, lacking lower-case characters and printing them in upper-case. It used a 7-pin print head with glyphs in a 5x7 grid. It normally printed 80-column lines on standard ​9 7⁄8 inch wide tractor feed paper. It could print up to 30 characters per second (cps),[2] matching the maximum interface speed of 300 bps (30 cps assuming one start and one stop bit). The interface could also run at 110 and 150 bps. However, carriage returns required ​1⁄3 of a second,[3] during which time the host computer had to send data it would know would not be printed, the so-called "fill characters" that were common on printers of the era.[3]

Mechanically, the machine was 2 by 2 feet (0.61 by 0.61 m) and came mounted on robust legs that raised the keyboard to standard desk height with the top 31 inches (790 mm) from the ground.[3] Normally, a box of fanfold paper would be placed below the printer mechanism and feed upward though a slot in the bottom of the desk. The case around the keyboard was curved, somewhat similar to the ADM-3A. The entire front cover lifted upward to provide access to the printing mechanism, both for basic maintenance and for feeding in new paper.[4] DEC suggested leaving 16 inches (410 mm) behind the system to provide enough room to swing it fully open.[5]

In June 1972, DEC introduced two new versions of the DECwriter, the LA30A which lacked a keyboard and was used as a dedicated printer, and the LA30-E which added an RS-232 interface option, the "E" standing for the new name for the port, EIA-232.[6] A later addition was the LA30-P, the "P" referring to the addition of a parallel Centronics port, which had become an almost universal standard by the mid-1970s.[7]

DECwriter II

A replacement for original line was announced in August 1974 with the introduction of the DECwriter II line and its first model, the LA36.[8] The LA36 used the same basic printing mechanism as the LA30, and was physically similar although smaller and more rectangular. Like the LA30, the LA36 was also offered in a keyboard-less printer-only model, in this case known as the LA35.[8]

The primary change was the addition of a data buffer that allowed it to store characters. This meant the terminal could continue accepting data from the host computer while the machine was performing carriage returns or other time-consuming operations, and then pick up printing once that was complete. When that occurred, it began printing characters as fast as possible until the buffer was empty again, at speeds as high as 60 cps. This had the added advantage that the host computer did not have to insert fill characters, which in turn led to simpler interfacing requirements and reduced device driver complexity.[8]

There were numerous other changes as well. The character set now included a complete 128-character ASCII set, including upper and lower case as well as various control characters. The character set was stored in read only memory (ROM), and optional ROMs for Katakana and the APL symbols were also available. The 63-key keyboard followed the ANSI X4.14-1971 typewriter layout, and included a further 19 keys for numeric input and various controls. The tractor feed was much larger, with a fixed section on the left and a movable one on the right, allowing it to feed paper from 3 to 14.875 inches (76.2 to 377.8 mm) wide and print up to 132 columns. The print head had enough force to print through six pieces of paper, allowing it to print using carbon paper or copy paper forms.[8]

The systems were so popular that several 3rd party companies introduced add-on cards to give the systems more functionality. The Intertec Superdec offered 1200 bps support, double-wide characters, APL characters and even user-defined character sets.[9] The Datasouth DS120 was similar, lacking the character sets but adding bidirectional printing.[10] The Selanar Graphics II add-on offered bitmapped graphics support as well as increased speeds to 9,600 bps.[11]

DECprinter I, new DECwriter IIs

The DECprinter I, model LA180, was introduced in September 1976. This was essentially a simplified version of the LA35, offered as a printer only using the Centronics port to provide up to 180 cps.[12] In November, the same basic mechanism was used as the basis for new versions of the LA35 and LA36, differing primarily using serial ports which made the easier to connect to DEC systems. These went on to become one of DEC's best-selling products.[13]

DECwriter III

January 1977 saw the introduction of the DECwriter III, or LS120.[14] This was a cost-reduced version of the LA36 that supported only serial input out of the box, lacking the former current loop interface. Three new versions based on the LS120 were introduced in November 1978, the print-only LA120-DA DECprinter III, the LA120-RA which replaced the LA36 terminal, and the LA120-RB, otherwise similar to the RA but able to print on up to nine-thick copy paper as opposed to the normal six-thick of the base model.[15]

The LA120's were similar to the earlier models mechanically, with only minor changes to the layout of the printer and the floor stand. Internally, the primary change was the addition of a 1 kB buffer, which allowed it to store many lines of text. The printer examined the data, skipping over blank areas at high speed, and printing in both directions by reading backward through the buffer where appropriate. The overall speed increased to 180 cps In addition to the character sets of the II series, the III added new character sets with national replacements for Finland, Denmark, Sweden, Germany, Norway, and France. It also offered eight options for character width (narrow or wide) and double-strike for bold.[15]

The LA120s normally used only a RS-232 interface, but the LA12X-AL add-on kit provided a current loop for those who needed it, while the LA12X-BB added parallel, and LA12X-CB for Unibus. The LA12X-DL expanded the buffer to 4 kB.[15]

DECwriter IV

The first complete redesign of the DECwriter line was introduced in June 1982[lower-alpha 2] with the DECwriter IV. In contrast to the earlier models, which were large stand-alone units on their own floor-standing mounts, the IV series were small desk-top systems that looked like contemporary electric typewriters, notably the IBM Selectric. They were slow, at 30 cps, and were not intended as outright replacements for the III series, which were more suited to computer-room operation.[16]

Two models were offered, the LA34s which used a typewriter-like roller feed mechanism, and the LA38s which added a tractor feed mechanism, which could also be purchased separately for the LA34. Both fed paper in from the top, like a typewriter, and did not need any room below them for paper feeding.[16]

The DECwriter IV series also introduced optional support for DEC's sixel graphics format, allowing it to produce black and while graphics output. This worked by sending characters using only 6 of the 8 bits of the printable character set and using those six bits to directly control six of the seven pins in the print head.[17] This way, graphics data could be sent over 7-bit links. The printer could expand the data horizontally to several different characters-per-inch settings.[18]

Letterwriter 100

Otherwise similar to the IV series, the LA100 series used a nine-pin print head and offered three different printing speeds to provide what DEC referred to as draft, memo or letter qualities. In draft mode it printed at 240 cps, while in letter quality it used a 33 by 18 dot matrix that reduced the printing rate to 30 cps. As before, the LA100 was offered as the print-only Letterprinter 100 or a variety of Letterwriter 100 terminals.[19][20]

The internal character set ROM was further expanded to support British, Finnish, French, Canadian French, German, Italian, Norwegian/Danish, Spanish and Swedish. More interesting was the addition of plug-in ROM cartridges containing the actual glyph data for the characters. The system could support two plug-in cartridges and as many as three internal ROMs (bare chips) to allow five character sets at a time.[21][20]

DECwriter Correspondent

The LA12 DECwriter Correspondent was a small form-factor 20 pound[22] terminal for portable use. Various models offered built-in modems or other interfaces. The system was otherwise similar to the IV series in features.[23][22]

Notes
  1. The LA30 manual is dated 1971, which suggests it was not available until that year.
  2. In this case, the manual is from October, pre-dating this release date. Some investigation is warranted.

References

Citations
  1. Present 1978, p. 21.
  2. DEC 1971, p. 1.1.
  3. DEC 1971, p. 1.2.
  4. DEC 1971, p. 3.4.
  5. DEC 1971, p. 2.2.
  6. "Memories Cut Cost of PDP-8 Systems". ComputerWorld. 12 July 1972. p. 15.
  7. "Decwriter LA30". 23 December 2019.
  8. Present 1978, p. 47.
  9. "Intertec advertisement". Computerworld. 22 May 1978. p. 49.
  10. "LA36 Made Interactive". Computerworld. 19 December 1977. p. 30.
  11. "Board Adds Features to Decwriter LA35, LA36". Computerworld. 4 December 1978. p. 37.
  12. Present 1978, p. 59.
  13. Present 1978, p. 60.
  14. Present 1978, p. 76.
  15. Terminals 1983, p. Chapter 14.
  16. Four 1981, p. 1.1.
  17. Four 1981, p. 5.22.
  18. Four 1981, p. 4.21.
  19. Letterwriter 1982, p. ix.
  20. Terminals 1983, Chapter 10.
  21. Letterwriter 1982, p. ix, x.
  22. "DEC Introduces New 'Correspondent'". Hardcopy. April 1982. p. 13.
  23. Terminals 1983, Chapter 13.

Bibliography
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