TTY interface

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The TTY interface is an older and technically relatively simple serial interface . It works asynchronously and uses a current loop with typically 20 mA line current for data transmission . This is why it is also known as the 20 mA current loop interface or the English 20 mA current loop interface .

It was developed in the 1960s to control electromechanical teleprinters over longer distances. The term TTY is an abbreviation of the English term "Teletypewriter" or "Teletype" for teleprinter, derived from the name of Teletype Corporation, Inc. , an American company that was a leading developer and market-dominant manufacturer in the teletype segment for decades. The TTY interface was subsequently transferred from communications technology to the computer sector as an interface to inexpensive peripheral devices for data input or data output. The interface spread particularly successfully wherever galvanic isolation of the coupled devices is required. It is still sometimes used today in long-life measurement, control and regulation devices, mostly to connect log printers.

The interface is not standardized , but is largely handled uniformly as an industry standard .

history

It was developed and introduced with the teletypewriter models Teletype 33 and 35, which were produced by Teletype Corporation from 1963. Both differed from the teletypes that were typical until then, which were operated with a higher current of usually 40 mA at approx. 60 V open circuit voltage and Baudot Code used for data transmission. In the case of the two new models and their successors, Teletype switched to 20 mA and instead of the Baudot code used the ASCII that she herself helped to develop . The teleprinters were initially used in the Teletypewriter eXchange Service (TWX), the teletype network of the United States that was introduced in 1931 and is continuously being developed .

The devices became well-known primarily through their massive use as input and output devices on computers. Teleprinters were considered robust and reliable. In many companies they were already available and due to mass production they were significantly cheaper to buy than input terminals and printers specially developed for use on computers. Both the TTY interface and the ASCII quickly established themselves as the standard for computers. It was not until the end of the 1970s that TTY was replaced by RS-232 as the standard serial interface, which was also developed in the 1960s, but from the outset for use with computers.

technology

A definition of the TTY interface can be found in the CCITT recommendation V.31. In Part 1 of the draft of a DIN 66258, which appeared for the first time in 1981 and was revised again until 1986, a "20 mA interface" was described in more detail. The draft was then no longer approved as a binding standard. Electrically, the TTY or 20 mA current interface was handled more or less uniformly worldwide. With devices from different manufacturers, however, it often happened that the contact pins were assigned differently.

Only two lines are required for the interface. In the idle state, a constant current of 20 mA flows, which is interpreted as a logical "1"; every interruption of the current flow stands for a logical "0". Only one of the two connected devices may feed power into the data line, i.e. it may be "active", the remote station is necessarily "passive". In the computer sector, four lines were soon common, two lines as a transmit current loop and two as a receive current loop, for simultaneous sending and receiving (duplex operation). Teletype intended a maximum transmission rate of 110  baud . For more modern interface designs, 4800 baud is sometimes considered to be the technically sensible maximum of the transmission speed. According to other sources, 9600 bauds are also possible with a maximum distance of up to 1000 m between the connected devices.

Further areas of application

The interface was transferred to numerous other areas of application, but modified more or less significantly.

Because data transmission via a current loop is significantly less susceptible to interference than voltage-controlled signals such as RS-232, TTY interfaces were also used in the industrial sector. As a rule, however, a different data format was used (instead of 7 data bits, 1 parity bit and 2 stop bits, mostly 8 data bits and 1 stop bit) and higher data transmission rates than the maximum intended by Teletype Corporation. The Siemens - Programmable logic controllers (PLC) of the Simatic -series "S5" were equipped with TTY as a programming interface. To connect the devices to computers with an RS232 interface, the so-called "Köster Box" was available as a common accessory for implementation .

The TTY interface was found particularly frequently in measuring devices because there, especially in the medical field, electrical isolation or galvanic isolation of the coupled systems is required. With current loop interfaces, the requirement can be met without any special effort by using optocouplers as transmitting and receiving modules. In some cases, very durable devices in this area still have TTY connections to this day. Usually simple, but particularly robust log printers are connected via it.

See also

Individual evidence

  1. ↑ Harald Schummy: Measurement technology with the personal computer: measurement data acquisition and processing . 3. Edition. Springer-Verlag, 2013, ISBN 978-3-642-93528-2 , pp. 28 ( limited preview in Google Book search).
  2. ↑ a b c Bernd Schürmann: Computer connection structures: bus systems and networks . Vieweg & Sohn Verlag, 1997, ISBN 978-3-528-05562-2 , p. 212 ( limited preview in Google Book search).
  3. ↑ Rainer Scholze: Introduction to microcomputer technology . 3. Edition. Springer-Verlag, 2013, ISBN 978-3-322-94098-8 , pp. 263 ( limited preview in Google Book search).
  4. ↑ Friedrich Wittgruber: Digital interfaces and bus systems . 2nd Edition. Springer-Verlag, 2013, ISBN 978-3-663-01615-1 , p. 53 ( limited preview in Google Book search).
  5. ^ Ekbert Hering, Klaus Bressler, Jürgen Gutekunst: Electronics for engineers . 4th edition. Springer Verlag, 2001, ISBN 978-3-540-41738-5 , p. 580 ( limited preview in the Google book search)