A telecommunication control unit (TCU), line control unit, or terminal control unit[1] (although terminal control unit may also refer to a terminal cluster controller) is a Front-end processor for mainframes and some minicomputers which supports attachment of one or more telecommunication lines.
[2] TCUs free processors from handling the data coming in and out of RS-232 ports.
The TCU can support multiple terminals, sometimes hundreds.
Many of these TCUs can support RS-232 when it is required, although there are other serial interfaces as well.
The advent of ubiquitous TCP/IP has reduced the need for telecommunications control units.