S7 question

I'm trying to solve a problem at work
Hardware schematics show a brake is activated by Q20.1 !
But in the program when i display the outputs there is no Q20.1
In the symbols it's showed as "unused symbol"
but at the I/o-card Q20.1 is lighted

probably the brake is activated by a masterdrive ? but how does this axplains why Q20.1 is lighted while it has not been used?

Have you check QB20, QW19, QW20, QD17-20, or
PQB20 etc. or is it possibly forced, or controlled indirectly ?

I agree with Seppo,

I think that somewhere in your program, a chunk of outputs are being written to as a group. Sure makes it hard to troubleshoot, doesn't it?

I have an idea - look at your symbol table and find the next lowest output address that is being used. In other words, is the next lowest address Q20.0, or Q19.7, or Q19.6, etc. Then try to find that address in the program. Good Luck!

Kevin

I don't know for sure that this is true for the S7 series, but another possibility is that the output was "forced" high via a programming device and that the output state is retentive.

I have seen this on other PLCs.

Steve

Is the LED on the output module on constantly or only when the brake is actuated?

You mentioned that the hardware schematics show the brake controlled by a PLC output, but the PLC program doesn't explicitly address it. It might be that the design was changed when the equipment was installed, and the wiring to the output module left in place. If that is the case, it's possible that the LED is lit because the circuit is energized. That could happen if the LED in the module is on the line side of the circuit instead of the logic side.

If the LED is constantly lit, it could indicate a faulty output circuit on the module. It might even be the reason the control of the brake was moved from the PLC output to the drive.

Open the "Hardware Configuration".
Look at the I/O addresses. Is Q 20.1 there?
Go "Online" (3rd icon from the left)
Right click on the address and select "monitor"
If it comes up green, set the value to 0 and "Modify"
If the program is writing to the output, it will turn back on.
If not it should go OFF.

okey ! problem solved ! the adressing was indirect !