Programmable Communications Interface Bus System

Programmable Communications Interface (PCI) data bus consist of a pair of twisted wires traveling from module-to-module receiving and delivering coded information. Code identifies message and its importance. When multiple messages attempt to access PCI bus at once, code assigns priority ranking.

Twisted wires used by PCI bus system are called bus "+" (positive) and bus "-" (negative). Both wires carry approximately 2.5 volts. Network consists of some modules with "bias" and some with "termination".

Bias is part of voltage divider network which places both bus "+" and bus "-" at 2.5 volts. Termination is part of circuitry required to complete voltage divider network and also provide some electromagnetic protection for PCI bus. Terminations in all PCI applications have approximately 120 ohms resistance.