The SDM-SIO1A is a serial I/O expansion module for Campbell Scientific data loggers. It is designed to add an additional RS-232, RS-422, or RS-485 (half- or full-duplex) serial port to an SDM-capable data logger for the purposes of interfacing with an intelligent sensor, actuator, or display. Up to 15 SDM-SIO1A modules can be connected to a single data logger SDM port. The SDM-SIO1A behaves much like a native data logger serial port and uses the same familiar serial I/O commands. The SDM-SIO1A is transient and surge protected to IEC61000-4-5 level 4 on the serial port interface, avoiding the need for separate transient protection in most applications.