Setting Termination & Pull-Up/Pull-Down Resistors in a stack of SeaI/O Devices

A “stack” or “chain” of SeaI/O modules, connected via the pass-through connectors or screw terminals on the left side of the enclosure, communicates via an RS-485 bus, which must be properly terminated to work correctly. A set of three dipswitches is located on the left side of enclosure, next to the “ADDR” rotary switch. These switches control line termination and the RS-485 pull-up and pull-down resistors.

The pull-up and pull-down resistors ensure that the input ports are at a known state when not being driven by the RS-485 line. In most cases, all three of the dipswitches on each SeaI/O module should be in the down position, except the two end modules. The first SeaI/O module in the chain should have all three dipswitches in the up (enabled) position, and the last SeaI/O module should have only the termination dipswitch in the up (enabled) position.

Make sure that only the first and last SeaI/O modules have line termination enabled (up position). Improper termination settings can result in invalid data or communication failures.