Run an end-of-line test sequence and see each step pass or fail

The PLC runs a fixed sequence of steps for each part and gives a result for every step. It reads the serial number, closes the fixture, checks the wiring, starts the insulation test, powers the part for a functional test and writes the result. The part passes only if every step is OK.

1. Scan. A barcode scanner sends the serial number to the PLC over RS-232. The PLC checks that this serial has not already passed.
2. Clamp. The PLC closes the fixture and waits for the "fixture closed" sensor. No test starts on an open fixture.
3. Continuity. The PLC drives one wire at a time and reads all the wires at the other end. It finds open, crossed and shorted wires.
4. Insulation. The PLC disconnects its own lines from the part and starts an external insulation tester. It waits for the tester to answer PASS or FAIL.
5. Functional. The PLC powers the part, measures its current and checks that it answers a command in time.
6. Record. The PLC writes one record with the serial, the time and the result of each step, then shows green or red on the tower lamp.

Run the Wiring faults scenario and compare what you see on the wires with the failure text in the table.

The PLC uses the method "drive one, read all". It switches on one output, which puts 24 V on one wire at connector X1. Then it reads all the inputs wired to connector X2. A good wire lights only its own input.

• Open wire: the expected input stays off.
• Crossed wires: the expected input stays off and another input switches on.
• Short between two wires: the expected input switches on, and another input switches on with it.

This check proves the connection map. It does not measure resistance. If your specification gives a limit in ohms or milliohms, add a resistance meter or a dedicated harness tester, and let the PLC read its result.

In a real fixture, a series resistor or diode on each wire start protects the outputs when two wires are shorted.

With 8 wires, the example uses 8 outputs and 8 inputs of the M-Duino 42. For more test points, add a second PLC on Ethernet, expansion modules or a switching matrix. The logic stays the same.

An insulation test puts a high DC voltage, for example 500 V, between the conductors and the shield or the housing, and checks the leakage. That voltage must never reach the PLC inputs and outputs. A dedicated insulation tester makes the measurement. The PLC runs the sequence around it.

1. The PLC opens a set of isolation relays, rated for the test voltage, that disconnect its own lines from the part.
2. It checks that the guard of the test area is closed.
3. It sends a START pulse to the tester and waits for BUSY.
4. When BUSY goes off, it reads PASS or FAIL. It waits for the tester to finish and discharge the part before it reconnects its own lines.
5. If the tester does not answer within the timeout, the step is KO with the reason "tester no answer". The PLC keeps its lines disconnected and waits for a reset of the tester. It does not assume that the high voltage is off.

Many testers can also send the measured value over RS-232 or RS-485. The PLC can store it in the record next to PASS or FAIL.

The guard check in the PLC program is process logic. Protection of people from high voltage needs a safety-rated circuit, separate from this logic, that cuts the tester output when the guard opens.

The PLC builds one record for each test attempt. The record holds the serial number, the time, the result of each step, the reason of the first failure and the attempt number. It writes the record in two places.

• Local copy: one line in a CSV file on the microSD card, with the time of the real-time clock. If the network is down, nothing is lost.
• Plant copy: the same record over Ethernet to your MES or database, for example with Modbus TCP, MQTT or an HTTP request.

Before a test starts, the PLC checks the serial against its records or your MES. A serial that already passed is blocked, so a double scan never creates a second OK. A KO serial can be blocked, or allowed one retest that the record marks as attempt 2.

Run the Insulation and retest scenario. It allows one retest. Run it again with "Not allowed" to see the block.

1. Stop at the first KO, or run every step

If you stop at the first KO, the cycle is shorter and the part goes to the red bin sooner. If you run every step, the record shows every fault of the part, which helps when you look for the cause. Do not run the insulation test on a part with a short that the continuity step has already found, unless your tester and your specification allow it.

2. What a KO part does next

A common rule is that a KO part must go into a red bin with a sensor before the PLC opens the fixture for the next part. This keeps KO parts out of the good flow. You decide if a KO serial can be tested again, and how many times.

3. Where the limits live

You can keep the limits in the PLC program, or read them from your MES for each product reference. With limits in the MES, one bench tests several references, and the record stores which limits the PLC used.

4. Proof that a step really ran

Write a result for every step, also for the steps that did not run. In the table, NOT RUN marks a step that did not run, which is not the same as OK. Check the time of your real-time clock regularly, for example against an NTP server, so the records of different benches stay in order.

This simulator goes with our case study on end-of-line testing with an M-Duino 42.

Read the end-of-line testing case study

What you see here is a general pattern. The part, the number of wires, the limits, the times and the faults are examples, not the design of a real bench.

Related pages: a pneumatic test bench with an Arduino based PLC, to check a sequence before it runs on a machine, and the automated fuse test bench project pack, with test sequences, interlocks and one report per test.

These tutorials and examples cover the parts of this bench on an Arduino based PLC.

• RS-232 with Arduino and ESP32 based PLCs, for the barcode scanner and the tester
• Basics with the SD card in an industrial PLC
• How to store information from the RTC of an industrial PLC
• Sync the RTC clock with an NTP server
• Modbus TCP with Arduino and ESP32 based PLCs
• Modbus TCP register map for a plant (PLC Solution Library)

More examples: PLC Solution Library · all simulators.

Example I/O for this bench

This example uses all 16 outputs and 17 of the 26 inputs. The outputs are transistor outputs, so the supply relay and the isolation relays are interface relays driven by the PLC. Check your loads and input levels against the ratings in the M-Duino family datasheet. Full specs: M-Duino 42 · M-Duino family.