The serial fieldbus I/O systems I-87K and RU-87P series consist of a basic housing that offers 1 to 9 slots for plug-in modules, depending on the series. Data is transmitted via the RS-485 bus and the DCON protocol is used for communication. I/O modules can be inserted into the slots in plug-in design, which include both multi-channel analog input/output modules and digital input/output modules.