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An overview of the computer control of an ammonia-based External Active Thermal Control System (EATCS) for the International Space Station Alpha (ISSA) is presented. The control system relies on the use of Honeywell-provided Multiplexer-Demultiplexers (MDMs) and code produced using the Matrixx rapid-prototyping tool. The functions of the EATCS (collecting, distributing, and disposing of thermal energy) are managed by an MDM designated as the external (EXT). The EXT MDM validates commands and initiates control sequences in response to operator commands by sending separate commands to lower-level MDMs. Two of the lower-level MDMs are the S1-1 and P1-2 (otherwise known as the S1/P1 MDMs), named after the ISSA truss segments on which they arrive (Starboard segment 1 and Port segment 1).