PLCs, SCADA, historians and the MES, and how they relate to ERP.
3 questions in Manufacturing & Industrial IoT.
A PLC runs the machine on a deterministic scan cycle and holds the truth about plant state; SCADA gives operators a view and supervisory control; a historian archives the resulting time series; an MES turns a production plan into work orders and a batch record; ERP sits above with the commercial view.
The two systems are not counting the same thing: one counts units completing an operation, the other counts stock received into a location in a posting period, and scrap, packing units, partial confirmations and backflush assumptions each open a gap. Reconcile at the boundary daily, with a named owner.
The line survives on cached work orders and locally queued events, so the difficulty is the merge, not the outage. Reconciliation has to be idempotent, ordered by source timestamp rather than arrival, and honest about units that passed a gate nobody could evaluate. It also connects degraded mode to the point an interviewer is testing.