Terminal Displays
Introduction
This page features a collection of DEC Display Terminal guides.
Before CRT terminals
were widely available, the Teletype was the king of computer to human interface.
Teletype Corporation's Model 33 terminal, introduced in 1963, was one of the most popular
terminals in the data-communications industry. Over a half-million Model 32s and 33s were made by 1975.
In 1976, a new ASR-33 cost about $1000. CRT based terminals using semiconductor electronics and microprocessors
were lower cost and Teletype machines were gradually replaced by
dot-matrix printers and CRT-based terminals in the middle to late 1970s. Teletype Corporation discontinued Model 33 production
in 1981.
The design objective for the Model 33 was a machine that would fit into a small office space, match with other office equipment of the time and operate up to two hours per day on average. Since this machine was designed for light duty use, adjustments that Teletype made in previous teleprinters by turning screws were made by bending metal bars and levers. Many Model 33 parts were not heat treated and hardened. The base was die-cast metal, but self-tapping screws were used, along with parts that snapped together without bolting. As another cost-saving measure, the paper tape equipment was dependent on the keyboard and page printer mechanisms. The computer interface was a serial 20 milliamp (ma) wire connection. Most DEC terminals provided both RS232 and 20ma interfaces to be compatible with the Teletypes. In this manner it was to replace a Teletype with a DEC terminal. Early DEC computers featured 20ma serial interfaces as the Teletypes were the most widely available interactive terminals.
The Model 33 used the seven-bit upper-case only ASCII code, also known as CCITT International Telegraphic Alphabet No. 5, with one even parity bit and two stop bits, with symbol rate of 110 baud. It was geared to run at a maximum speed ten characters per second speed; 100 words per minute, but other speeds were available: 60 wpm, 66 wpm, 68.2 wpm, and 75 wpm. There were also many type wheel options.
The Teletype Model 33 contained an answer-back mechanism that was generally used in dial-up networks such as the TWX network. At the beginning of the message, the sending machine could transmit an inquiry 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. The answer-back feature was built into most DEC terminals to remain compatible with the Teletype functionality. At one company I worked at the answer-back mechanism was used by a VAX based film retrieval application to define which VT220 terminals were connected to a micro-fiche machine and which were standalone terminals.
The text above was adapted from Wikipedia.org.
