The mainframe comes with a standard RS-232 host interface and GPIB (IEEE-488.2) as an option.
All commands and queries to individual modules are routed through the mainframe host interface. Messages are routed based on their port number, with the mainframe seamlessly relaying data to and from the appropriate destination. This messaging system is also used to communicate through the two auxiliary RS-232 ports. Using these ports, any number of SIM900 mainframes and other RS-232 instruments may be controlled through one host interface in the master mainframe.
Each port in the mainframe has independent 511-byte input and output queues. The mainframe also performs any rate adaptation needed when communicating through the ports. A dedicated "eavesdrop" RS-232 port provides an echo of all communications to and from the mainframe to ease debugging of new setups.
The mainframe provides a master 10 MHz clock to the modules which lock their internal timebases to this reference. The master clock in the SIM900 can be either free-running or phase-locked to an external 10 MHz reference through a rear-panel BNC input.
A 4000 byte start-up script is maintained in non-volatile memory in the SIM900. When enabled, the commands stored in the script are executed each time the mainframe is turned on. This provides a simple way to ensure all modules are automatically set to a desired starting configuration.Front Panel
LED indicators provide status information for the mainframe and also monitor communications between the host and installed modules. Indicators display host interface, timebase, communications status, start-up script, and power information.
All modules operate from various combinations of ±5 V, ±15 V and +24 V. These well regulated and filtered DC voltages are supplied by the mainframe power supply which is forced air cooled to maintain stable performance. All supplies are current limited with a trip-off circuit in case of an over voltage or short circuit.