The Serial I/O Module (SIOM), models 621-9938R or 621-9940 , are installed in slot N in the remote Serial I/O card file. SIOM controls the Serial I/O card file and interfaces with the model 621-9939 SLM in the processor card file. The model 621-9938R SIOM is used in redundant serial I/O configurations, but can be used in nonredundant applications. Terminating resistors (200 or 300 ohm resistors) are required at each extreme end of the serial channel to match the cable impedance. Two terminating resistors are required in each SIOM that is at the extreme end of the link.

Serial I/O redundancy requires the model 621-9938R redundant Serial I/O Module (SIOM) in each serial I/O card file. This Serial I/O Module has dual communication ports so that it can attach to the redundant serial I/O lines. The optional model 621-9928R Serial Link Selector (SLS) is available to switch a communication link so that it is always connected to the active processor. This serial link would normally be used for the Loader/Terminal.

Configure the SIOM by using the four 8-position and one 4-position DIP switch modules located near the top edge of the SIOM circuit board. These DIP switches select the module starting address, card fault recognition, communication fault responses, normal or test mode operation, and the number of I/O points located in each I/O card file.

DIP switch module SW1 selects the absolute starting address for each card file. The starting address is selected from a range of 0 through 2,032, in increments of eight. During startup, the SLM queries all connected SIOMs for their starting addresses and the number of allocated addresses in the card file. The SLM also determines the sequence in which the I/O card files on the channel are accessed, based on the absolute starting address of each SIOM. Overlapping SIOM starting or I/O addresses and addresses higher than 2,032 are prohibited. Be sure to never install an 8 or 16-point I/O module in a 32-point slot.

DIP switch module SW2 determines rack output states in case of a fault and allows for the option of auto-test or normal operation. DIP switch modules Switch3, Switch4, and Switch5 determine the number of I/O points per card file slot.

The 'R' suffix indicates the module can be removed or inserted into an I/O rack while the rack remains powered. Other I/O modules in the rack continue to control the machine or process.

The 'C' suffix indicates models that conform to CE directives 73/23IEEC, the low voltage directive, and 89/336IEEC, the EMC directive.