The 621-9940C Serial I/O Module (SIOM) collects data from the I/O modules in the serial I/O rack and
interfaces with the SLM. The SIOM resides in slot N of the full-size I/O rack or in slot H of the I/O half rack. The SIOM allows each I/O slot to be configured for a different module density–0, 8, 16, or 32 points. However, the total number of I/O points must stay within the I/O capacity of the 620 Logic Controller.

Terminating resistors are required on each serial channel to match the cable impedance. Multidrop
configurations require two terminating resistors in the last SIOM of the channel.

The SIOM circuit board has four 8-position DIP switch banks and one 4-position DIP switch. These DIP switches are used to select the module starting address, card fault recognition, communication fault
responses, normal or test mode operation, and the number of I/O points in each I/O rack slot.