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www.thinkSRS.com Communications Examples for SIMs Introduction The Small Instrumentation Module family was designed to support simple communications between a user's computer and a collection of instruments. While it is possible to communicate directly with a SIM module, this note will only consider the case of communication through a SIM900 Mainframe. The SIM900 has two host computer interfaces, RS-232 and GPIB, one of which is active. To switch interfaces, use the rear-panel piano-style DIP switch (position 3): up is for RS-232, down is for GPIB. The 5 right-most switches are interpreted based on the host selection, and determine either default baud rate (RS-232) or instrument address (GPIB). Only one host interface can be active, and the selection is determined at power-on time for the SIM900. The simplest style of communication through the SIM900 is the "connection" model, where a single bidirectional I/O stream is managed. Upon power-on, this stream is initially directed to the SIM900 itself, so that, for example, an identification query will result in the SIM900 ID string as a response back to the host computer. Using the SIM900 "CONN" command, the user can steer the I/O stream to one of the instrument ports of the Mainframe (1