The I/O expansion units are used to extend the number of I/O modules.
Each expansion unit consists of a Power supply and a Back Plane with 4, or 5 or 8 or 9 Serial I/O Expansion Slots.
The I/O expansion units, 87K4, 87K5, 87K8, 87K9 are equipped with a 115.2K bps RS-485 bus which links the expansion unit to the main control unit.
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