The RIM II system uses OPTO 22's OPTOMUX digital and analog remote input/output controllers (RIM II assemblies). These operate as slave I/O devices to the TMS3000 computer. RIM II architecture uses master/slave polling protocol (i.e., speak-when-spoken-to). The host computer (i.e., ARTIC co-processor) is the master station, and each PRIM and CRIM controller is a slave station.

An OPTOMUX unit is a 4-, 8-, or 16-point rack that accommodates photo-isolated analog or digital power I/O modules. Each OPTOMUX unit consists of removable brain board and I/O mounting rack. The removable brain board contains a microprocessor to communicate with the host computer and control the plug-in I/O modules located on the I/O mounting rack.

RIM II (OPTOMUX) assemblies are either analog (PRIM w/temp), or digital (PRIM, CRIM). OPTO's design permits any combination of analog I/O modules to be plugged into an analog OPTOMUX unit; likewise, any combination of digital I/O modules may be plugged into a digital rack. To be consistent with the original RIM I design, RTD analog input modules (i.e. flow) interact with a separate PRIM II assembly, and digital output modules (i.e., pump start interact with the CRIM II assembly).

RIM II I/O modules are defined in the TMS3000 system in the METER field. All RIM II meters must have a minimum of three I/O types defined: 1) Flow; 2) Temperature; and 3) output.