The PIO30 modulseries is a decentralised I/O-system for the serial RS232 interface. Only one serial port of the hostcomputer is necessary to use the modules. The modules are connected to one common RS232-cable although meeting the RS232-specifications. Communication is done through an easy-to-use protocoll with checksum. The protocol is open, it can be used on all platforms that allow programming the serial interface. PIO30-series was developed for the use of DIN-rails. All I/O-ports are placed on pluggable connecors and are galvanically separated rom the serial port. Outputmodules use a programmable timeout-procedure that allows setting predefined conditions if the communication to the host is lost. Softwareroutines in Basic, Pascal and C/C++ support the programming of applicationsoftware. Available are modules with digital I/O-ports, relaymodules, A/D-modules and D/A-modules (resolution 12-bit).