“Tags” are Ignition’s gateway-internal data monitoring objects. OPC “Items” are addresses understood by an OPC server to access external data. Ignition’s OPC Tags are gateway-internal data monitoring objects that are subscribed to an OPC server’s item address. Ignition receives external changes to that item from the OPC server and passes them on to Vision windows’ property bindings, tag events, tag changes scripts, etc. Writes to the OPC Tag from the Ignition side are passed to the OPC server.
The system.tag.* functions work with Ignition’s internal data monitoring objects, including non-OPC objects like memory tags and expression tags. The system.opc.* functions bypass all internal data objects and go directly to the OPC server to read or write an external value.
OPC Tags are subscribed with a requested update pace from the tag’s scan class. This causes the OPC server to produce regular read requests to the external device to obtain the current data. Good drivers for OPC servers optimize multiple subscriptions to adjacent or nearly-adjacent PLC data items into combined requests, depending on the protocol’s flexibility. The calls to system.opc.* are not part of subscriptions, so are generally not mixed with other requests.
Some confusion occurs because OPC Items – the addresses – are often called OPC Tags, since they generally point at external data that is in fact a PLC Tag. I try hard to use “Item” in the right places. /-: