Siemens S7 PLC Fleet → SPC via OPC UA — Production-Grade Setup

The conversation I have most often with quality managers at Siemens-heavy plants goes like this: they have S7-1500s on the new lines, S7-300s on the lines that have been running for fifteen years, and they want SPC across all of it without touching SCADA, without a new historian project, and without telling the line operators anything has changed. The OPC UA path is the right answer for the 1500s — it is already there, it just needs to be enabled and pointed at the right data blocks. The 300s need a gateway, but once that gateway is in place, from the SPC engine's perspective there is no difference between data coming from a 1500 and data coming from a 300 via gateway. They both look like OPC UA nodes. The mistake I see most often is plants that solve this for the new hardware and leave the legacy lines out of the quality program entirely — which means the oldest, most variable process equipment is the least monitored equipment on the floor. That is precisely backwards from a risk management perspective. When you get all of it into one namespace and start seeing I-MR charts from the 300-series lines for the first time, you typically find things that have been drifting for months that nobody noticed because the data was never in a form anyone could trend.