Tube

The 'Tube' interface is a 40 pin IDC connector on the underside of many machines in the BBC Microcomputer family (notably not the Electron nor Master Compact).

Running at the full 2MHz bus speed of the host 6502 its 8 data lines, 4 address lines, and 5 control lines join to a parasite processor board. Often only 3 address lines are used, despite 4 being tracked out to the connector.

At the other end of the connection is a Tube ULA, or an emulation of one; this is a 'gateway' through which the two processors pass messages and send control signals. The Tube specification defines a protocol for the message-passing, so that the same 'Tube host' firmware works with any Tube-compatible processor board.

An asynchronous link and a registered trademark of Acorn.