Bench rig at school, TIA V18. CPU 1214FC DC/DC/DC and an ET 200SP with an F-DQ 4x24VDC/2A PM HF. I'm trying to switch a small 3RT2 contactor with a 24VDC coil off output 0 as a safety contactor exercise.
The moment the output goes TRUE the channel passivates. Buffer says "Read-back error" on channel 0 and the QBAD bit sets. The contactor clicks in for a fraction of a second and drops out again.
So I tried a second contactor out of the drawer, same thing, then output 1, same fault. A video said to put a diode across the coil, so I did that too and it made no difference.
Does the coil pull too much for the module? It's a 3RT2015, the coil is tiny. Or have I wired it wrong somehow, I've only got two wires so I'm not sure how. Could someone explain what the F-DQ is actually reading back while the output is on?
Part number of the module, the full 6ES7 one, and list me the wires: A1 to which terminal, A2 to which terminal. Exactly as they are, not as you meant them. And take the diode off until the manual says you want one, it isn't what's tripping this.
6ES7136-6DB00-0CA0. Wires: A1 to terminal 1 on the F-DQ, which the label says is DQ0 P. A2 to the 0V bus on the bench, the same bus the CPU and the 24VDC sensor supply sit on. The M terminals on the F-DQ I didn't use, I thought M is just 0V and the bench already has 0V so thats what I did. Diode's off now, same fault.
There's your fault, and it isn't the coil. That module is PM switching. It switches the P side and the M side of the load, and it reads both back. Your coil is meant to float between DQ0 P and DQ0 M of that channel. With A2 on the bench 0V the M side is tied to ground permanently, so the module switches its M output, sees it not doing what it expects, and passivates. The read-back is right, the wiring's wrong.
Move A2 off the bench 0V and onto the M terminal for channel 0. Nothing else on that pair. Check the wiring example in the module manual for your exact order number, because if I remember right the terminal numbers differ between the PM and PP variants.
Moved A2 to the M for channel 0 and the output holds, contactor stays in. Sorry if this is basic, but why does the module care? I get that it switches both sides. I don't get why a 0V on the bench is different from its own 0V.
Because the M side is a safety channel, not a return. If A2 sits on the bench 0V and someone shorts A1 to 24VDC, the coil stays pulled in and nothing can switch it off. With the coil between P and M the module opens the M side too, and it checks that leg works every time it switches. That check is the read-back you were tripping.
Ran it all morning, output on and off two hundred times off a test bit, no passivation, contactor follows every time.
The module was right and my wiring was wrong. A2 belonged on the channel's own M terminal and I had it on the bench 0V, which meant the M side could never do the thing the module checks it does. 24VDC coil between DQ0 P and DQ0 M, nothing else on that pair.
I still don't know whether the diode goes back on, there's a paragraph about it in the manual I haven't understood yet. Thanks petra66 and larsdk.
Wiring for next time: A1 to P, A2 to M, nothing else.