The Serial I/O Module (SIOM), models 621-9938R 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.

Serial I/O redundancy requires the model 621-9938R redundant Serial I/O Module (SIOM) in each serial I/O card file. This version of the SIOM 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.