Collecting Data from Legacy Machines Without Replacing the PLC

You can run legacy machine data collection without replacing the PLC, and often without touching its program. The route depends on what the machine offers: a communication port, accessible signal outputs, or nothing beyond a stack light. This walkthrough shows how to decide and deploy, starting with whatever delivers results fastest.

Before you start: decide what to measure

Do not try to collect everything. For most small plants, three data points already change decisions: machine status (running, stopped, in alarm), the produced-parts counter and the reason for stoppage. Start there and expand later.

Also note the PLC brand and model, firmware version and available ports, and whether there is any risk to the warranty or operation. Any intervention should be agreed with maintenance.

Step by step

1. Take inventory. List each machine, its PLC (or lack of one), communication ports and whether the program is available.
2. Pick a reading route per machine. If the PLC has an Ethernet or serial port with an open protocol, read directly. If it has a proprietary protocol, use a gateway that translates it. If there is no access, use external sensors.
3. Install the gateway or sensor. An industrial gateway connects to the PLC and publishes data in a standard such as OPC UA or MQTT. External sensors, like an inductive sensor on a mechanism or a signal taken from the stack light, provide counts and status without touching the PLC.
4. Isolate electrically. Use interface relays or isolated inputs when taking field signals, so the machine is not disturbed.
5. Connect to the collection system. Middleware reads the gateways, stores the data with timestamps and feeds dashboards and reports.
6. Validate in parallel. For a few days, compare the automatic counter with a manual count and tune filters and stop rules.
7. Set up a maintenance routine. Define who looks after each gateway, who receives lost-connection alerts, and where configurations are stored.

Options by situation

• PLC with Ethernet and a known protocol: read it directly, usually the cheapest option;
• PLC with serial such as RS-232 or RS-485: a serial-to-Ethernet converter or gateway;
• PLC with a proprietary protocol: a gateway that translates to OPC UA;
• machine with no PLC or no access: an external counting sensor, an energy meter or a stack light reader;
• stop reason: a push button or simple screen for the operator to log the cause.

What to measure at the end

After a few weeks of data, look at machine availability, the duration of each stop type and the gap between produced and planned. That is how you discover, for instance, that the biggest loss comes from many small stops rather than a few big breakdowns.

If you need to build the collection system and dashboards, our automation and production data work covers everything from sensor selection to visualization.

Frequently asked questions

Can I collect data without changing the PLC program?

In many cases yes, using gateways that read memory or external sensors. Agree it first with maintenance and the manufacturer to avoid warranty and operational issues.

What is OPC UA and why does it show up in these projects?

It is an open industrial communication standard. Gateways use OPC UA to deliver data from PLCs of different brands to the same collection system.

Are external sensors reliable for counting parts?

Yes, when properly installed and validated against a manual count. Position adjustment and filtering prevent double or missed counts.