OMNIBUS

The OMNIBUS is DEC's bus for the later PDP-8's; it was introduced with the PDP-8/E, in 1970. It provided three kinds of cycles:

• Programmed data transfers, in which the CPU reads or writes data to the device controller;
• Program interrupt transfers. in which the controller interrupts the CPU;
• Data break transfers, the PDP-8 term for DMA.

The OMNIBUS was physically implemented as a large backplane, into which were plugged both the CPU, and the device controllers; device controllers did not have dedicated slots.

In analog electrical terms, it was very similar to the UNIBUS; mostly bi-directional transmission lines, with a few uni-directional control lines. The termination and voltage levels of the two were the same, and shared driver and receiver circuitry.