The Serial Link Module (SLM), model 621-9939, controls serial I/O data transfer to and from remote I/O. This module has a separate processor with its own data table and operates asynchronously to the processor module. One or two SLMs can be installed in slots L and/or M.

Each SLM controls two serial I/O channels. Each operating channel must be terminated at the extreme ends using the terminating resistor provided.