I/O Expansion Unit:
The I/O expansion units are used to extend the number of I/O modules. Each expansion unit comprises a power module and a back plane with several I/O slots for plug-in Serial I/O units. A PC or Main Control Unit can link to the unit via different interfaces. The I/O expansion units, 87K4, 87K8, are equipped with a 115.2K BPS RS-485 bus which links the expansion unit to the main control unit.

Also available are RF-87K series Wireless versions which are based on a direct sequence spread spectrum and RF technology, operating in the ISM bands with a Frequency Range of 2410.496MHz~2471.936MHz, with a Channel Spacing of 4.096MHz.

The 8KE4/8KE8 Ethernet Expansion Units devices are similar to the 8430/8830 Main Control Units except that they have a faster processor but no RS485 expansion connection. They are designed to just connect to a ethernet point. Unlike the other expansion units though, they support both 8k Parallel and 87k Serial Plug-in units.