Vision 570 PWM Output not working

[32Racer]

Looking for a little help on a Vision 570-57-T20B and a snap V200 18E62B.. I have this setup running a packaging machine. I did not do the programming of the unit. I am also not a programmer. Here is the problem that I am having and hope someone can give me a little assistance. I am using the PWM output to control a Solid State Relay. I am sending pulses to the relay to control heat on a resistance Wire to seal poly bags. The PLC has no input to turn the PWM output on. It comes on when the machine is powered up. The program was written and machine installed and has been running since October 2012. Now it seems that the PLC is not sending any pulses out from one of the PWM outputs. The other one is working correctly. I have confirmed that the wiring from the Plc to the SSR is correct. So it is like the PWM output isn't sending any pulses.

Another strange thing, After the program was written and machine was tested I bought another exact same model Unitronics PLC and downloaded the exact same program into it. This was a different packaging machine but when I turned on the PLC everything in the program except the 2 PWM outputs worked. I went in Visilogic and can turn on the outputs manually and the relay will activate. But when I run the program I get no PWM outputs. This is the same program that has worked since October in the other machine. Can someone please tell me how to look in the program and see the pulses or any other test I can do?? Could it be possible I need relay outputs to control the SSR? Any help will be greatly appreciated.

[Gabriel Franco]

Have you checked operand values for frequency, duty cycle and MB that turns PWM on?

If those operands are all set to 0, PWM will not work at all. It can be the cause your second unit doesn´t work either.

[Simon]

Be aware that the program files holds only the structure and the logic of the program. The other component is the values in the MI and MB values. As Gabriel has mentioned, the PWM output is controlled by 2 MI values and an MB value. If these have lost the original values that the machine was set-up with, then the output won't work.

Likewise if you program a new PLC with the existing program, the values will be zero unless something in the program (or a manual setup procedure) changes them.

If you go to the "HW Configuration" (hardware configuration) you can view the settings of the PWM output.

[32Racer]

Thanks Guys!! One question How do these values get changed in a machine that has been running for 6 months? Also is there anway I can lock them where they will not get changed. Thanks Chris

[Simon]

Firstly, those values don't normally change without some intervention:

* power interruptuions and electrical noise can corrupt these values.

* fiddling from machine operators and service people.

* a dead backup battery (check the value of SB8)

You can provide a fall-back state by making them "power-up Values". This means they can be changed while the PLC is running, but as soon as you cycle power the values get reset to the "power-up value".

With a bit more work you could lock them in more rigidly. You will note that the PWM values can only be linked to MI and MB values. However if you use a "Store Direct" you can link those MI values to contstants by storing a constant (#) value to the MI values every scan. Likewise you can force the run MB to be always on by linking it to SB1 (Always on) via direct contact and coil. Both these methods will prevent the PWM values being changed while the program is running, and the constant values are saved into the program where they cannot be edited online.

I hope this makes sense.