This is a 1024 I/O Control Module.
The Input/Output Control Module coordinates communications between the processor and the 621 I/O system and formats the data flowing between the I/O system and the processor. It also monitors individual I/O module fault diagnostics. The IOCM works in conjunction with the Parallel Link Driver Module and/or Serial Link Module to control data
flow to I/O. The IOCM is installed in slot G in the 620-25 and slot I of the 620-35.