Flexible input/output expansion modules for M221, M241 and M251 Modicon™ PLCs. Includes digital and analog as well as mixed input/output. Also, includes special modules designed specifically for machine safety control. Up to 7 modules can be connected directly to the PLC. Also, the system can be expanded to include an additional set of 7 modules for a total of 14 modules and a couple hundred input/output points.