PLC vs MES: Data and Control on a Gas Appliance Test Line

Roles

What the PLC Must Do

The PLC runs the fixed sequence and directly controls the gas isolation, ignition, exhaust proof and the pass/fail gate. It must hold the interlocks locally, so a lost MES connection cannot leave gas supplied or allow a failed unit to release; a lost communication or a failed sensor should produce the safe state, not a pause in an unsafe condition. The sequence should be version-controlled and the same at every station, so a change to a test or limit is applied deliberately rather than station by station. These requirements are covered in G7, with the operator screen in G8.

What the MES Stores

• Work order and product model, so the station loads the correct test sequence;
• Serial number and the measured results, tied to the unit and the time;
• Pass/fail and the repair and re-test history;
• Output, quality and batch trends for management.

Because each result is linked to the serial, the MES is the basis of traceability described in G6. A unit that failed and was repaired must show that history, and the same record should keep the component batch so a field fault can be traced.

Connecting Them Safely

1. Keep safety interlocks in the PLC, never dependent on the MES;
2. Load the correct sequence and limits from the order, but store a local copy so the station runs if the network is lost;
3. Send results to the MES and buffer them locally until synchronised;
4. Reject any unit whose result is missing or cannot be matched to a serial;
5. Control changes to the sequence and log who changed them.

The common error is to let the information system carry the safety function, or to run without local storage so a short network loss stops the line or loses results. A small factory can begin with a local PC that performs the MES role and add the full system later, but the PLC independence and the traceability should be present from the start. Calibration and measurement confidence, which the MES records, are described in G9.

Following One Unit Through the Two Systems

An operator scans a serial and the station asks the MES which model and order it belongs to; the MES returns the test sequence and limits, and the station stores a local copy so it can continue if the connection is lost. The PLC then runs the sequence by itself: it proves the exhaust, opens the regulated gas, reads the pressure and the flue gas, and keeps the interlocks at every step. The MES is not consulted during this real-time sequence, because a slow or lost connection would otherwise delay or remove the safety action. At the end, the station sends the measured values and the pass or fail to the MES and holds them locally until the MES confirms receipt; if a unit failed, the record keeps the failure and, after repair, the later retest. This path shows the division in practice: the order and the history belong to the MES, while the immediate sequence and safety belong to the PLC.

What to Specify When Ordering a Line

• Which safety functions the PLC must hold independently and the safe state for each fault;
• Which orders, models and sequences the MES will store and how they are selected from the serial;
• How long results are buffered locally and how they are synchronised after a network loss;
• Who can change the sequence, with version control and a log of the change;
• How a unit with a missing or unmatched result is held until the record is complete.

Specifying these points in advance avoids the most common integration dispute, which is discovering late that the safety interlock has been placed in the information system or that results are lost when the network drops. A factory that begins with a local PC for storage can grow into a full MES later, but the PLC independence, local buffering and serial traceability should be specified at the first station rather than added as an afterthought.

Why the System Must Have a Defined State for Every Fault

A reliable station is not defined by what it does when everything is working, but by having a defined state for every fault it is likely to meet. A lost sensor, a stuck valve, a broken connection, a power loss, a network loss and a partially completed record should each produce a known response rather than an ambiguous pause, and that response should be the safe one for the hazard involved. The PLC is the part that holds these states because it is directly connected to the gas and the gate and is independent of the network; the MES can report what happened but cannot be relied on to make the immediate condition safe. The same principle should be applied to incomplete results: a unit that finished most of its tests but lost a signal should be held rather than released from the last confirmed pass, because the missing measurement is the one that cannot be claimed. Writing these fault states explicitly before commissioning removes the practice of reacting to each failure for the first time in production, where the response is improvised and the safest option is not always taken. It also gives the test programme a clear set of conditions to induce: each fault is created on purpose and the station is confirmed to reach the defined state. Where the two systems meet, the rules are consistent — the PLC keeps the appliance safe, the local buffer preserves the results, and the MES stores the history only after the measurement is complete. That division is what keeps a short fault from becoming either an unsafe release or a permanent loss of data.