The 621-0004 has been superseded by the 621-0021RC Enhanced Diagnostic Module.
The 621-0004 System Diagnostic Module (SDM) provided an additional measure of system monitoring. The SOM operates essentially as an external system monitor. It acts as a watchdog timer, monitoring various control and data lines.

The SDM, upon detecting a system malfunction,
de-energizes an onboard relay that can in turn remove
power from the system’s output modules. This provides an effective method of halting the controlled
machine or process.