IPC 621 I/O Rack Module Model :- 621-9990 I/O Rack 620-LCS RED The 621 I/O full rack is identical in size to the 19-inch 620 processor racks. It is designed for installation in 8-inch NEMA 12 enclosures or 19-inch instrumentation racks. It holds a maximum of 12 I/O modules, either a Parallel or Serial I/O Module, and a Power Supply Module.