Exclusions and SPDT Relays

For switch devices with SPDT—also known as Form C—relays, a physical connection always exists between common (COM) and either the normally open (NO) or the normally closed (NC) pole.

When using NI switches, calling niSE Disconnect with an SPDT relay results in a disconnect in software, but the relay position does not change in hardware until niSE Connect is called.

Therefore, to guarantee the state of an SPDT relay, you must connect either the NO or the NC pole to COM using niSE Connect. NI Switch Executive verifies that there are no exclusion conflicts before connecting a relay, but does not verify exclusions when disconnecting relays using niSE Disconnect or niSE DisconnectAll.

niSE DisconnectAll physically resets all relays to their default positions. Because NI Switch Executive does not verify exclusions on disconnect, excluded connections in default relay positions could pose problems for SPDT relays.