DSC's KeyBus is a 4 wire bus for connecting keypads and modules that has been around since the PC5010/PC1555. It has power, ground, data and clock lines with a maximum distance of 1000' from the panel. It is very similar to the 4-wire bus DSC used from the beginning with the PC1000 but enabled DSC to introduce modules to expand the basic functionality of the panel. DSC later created ComBus for the MaxSYS serves the same purpose but the two systems are totally incompatible with each other.