This is the version of the High Level PIU I/O Bus Controller used in early PIU releases, including R200, R300, and R310/320.

This version of the I/O Bus controller is used in controllers with the 4DP7APXPM11, 4DP7APXPM122, and 4DP7APXPM133 PROM/RAM boards.

The I/O Bus Controller card adapts the microprocessor bus to the signal and timing requirements of the PIU's I/O bus and A/D converter. It also performs diagnostic functions on the I/O section of the PIU, to prevent incorrect I/O card or point selection, to verify correct operation of the I/O clocks, to detect shorts between in the I/O lines, and to check the integrity of I/O address and data lines.