Monday 13 April 2020

RS-232 (1960)

Mouse using a serial interface
Introduced 1960

Known to many as just a “serial interface”, RS-232 is a surprisingly old communications standard that was once everywhere and even today it refuses to die. A simple way to connect a computer with a peripheral or other computer, the RS-232 interface could be used – with a bit of effort – to connect just about anything to just about anything.

The RS-232 interface is a “serial” interface because it transmits data one bit at a time in two directions, compared with contemporary “parallel” interfaces that typically transmitted 8 bits at a time in one direction.

The standard defines one end as the DTE (Data Terminal Equipment – e.g. a computer) and the other as the DCE (Data Circuit-terminating Equipment – e.g. a printer) connected together using a cable with 3 or more wires and terminated using 25-way D-type connectors. The number of wires in use varied depending on how many features you wanted to use, and many implementations ditched the large 25-way connector for something smaller, typically a 9-way connector on a PC.
RS-232 Breakout Box

Connecting DCE-to-DCE or DTE-to-DTE would require a special cable, and indeed it was possible to make custom cables quite easily depending on your needs, or you could simply use one of many adapters between different types of connector or equipment. Typical transfer speeds could vary between 50 baud (bits per second) to 9600 baud, 19200 baud or even higher.

A typical early use of RS-232 would be to connect a terminal to large computer system, or in later days a PC to a printer, modem or mouse. Indeed, on the PC the serial port lingered into the 2000s until the more versatile but fundamentally similar Universal Serial Bus (USB) became the de facto standard for connecting things together.

Because most computers either had a built-in serial interface or one available as an add-on, if you had (or made) the right cable then you could transfer files between utterly different and usually completely incompatible systems. The first challenge was to acquire a copy of an application such as Kermit and apply it to both systems, but once you had that it was a reasonably trouble-free way of moving data about.

Although RS-232 interfaces are less common than they used to be, they can still be found on industrial and laboratory equipment, communications gear and even home entertainment systems. But perhaps the RS-232’s most peculiar lasting legacy is this song

Image credits:
Konstantin Lanzet via Wikimedia Commons
MdeVicente via Wikimedia Commons - CC0 1.0

Spitting Image - RS232 Interface Lead 

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