A serial transmission system for controlling a network of input/output devices is provided. The serial transmission system includes: a three wire serial link; a master unit for controlling communication over the serial link; multiple input remote units for receiving information from the input devices; and multiple output remote units for controlling the output devices. The master unit includes a programmable optically isolated microcontroller adapted to initiate a synchronous data link control (SDLC) communications protocol with the remote units. In addition the master unit includes memory tables for storing input/output, error, diagnostic, status and node availability information. This information can be shared with a PC or host controller. In addition, the PC or host controller can include programs for directing the master unit and remote units to initialize, reset, scan, or maintain an idle condition.