S7-300 FC issue

[Steve G]

I am not very knowledgeable about Siemens but know enough to get around and do a few changes. My question is a couple of days ago I had a drive that stopped working on a system. After checking all hardware, I noticed that the main power relay would not pull in and the enable to the drive would not come on. No faults in the system as I could tell. I went online with the PLC and back traced to the FC block that was triggering the drive contactor and enable output. The trigger was a set of inputs in a & statement that triggered the drive. In the FC block the bits were turning on the off triggering a set and reset bit so as soon as the set was done the reset occurred and never triggered the drive relay, but the inputs that triggered the bits were solid. Thinking there was a wiring issue I check voltage and have a steady 24VDC to the inputs I then though that maybe the card was bad so I changed the inputs in the code to an input that was not used and put 24VDC to the input. We I looked at the FC block it was doing the same thing as before, the FC block bit was turning on then off and the inputs that triggered the bit were steady. With all the hardware and the wiring checked and good I am very confused as to why this is happening. Hope this makes sense and if anyone could give be something to look at further that would be great.

[Groo]

1. How many times is this block called in the software, if more than once then you may not be seeing true data when you are monitoring the block. 2. May help to paste some screen shots of the code in question, include the call, the io parameters and the code.

[Steve G]

It looks like it is call twice in the program there are two identical drive on this system and each one is enabled by this FC Block. I will try and get some screen shot on Tuesday when I am Back in.

[Groo]

FC's should not really be used in this way, they can be but make fault finding more difficult, you have to revert to looking at memory areas to try to see what is going on. If they are called from different FC's, then it is possible to monitor via a path which means you can see real data. FB's are better because you can also stipulate the DB, therefore you would state the IDB that bel;ongs to the FB.

[muotioj]

Make copy of the FC, change other call to that FC. Then you can monitor it. Edited August 16, 2011 by muotioj

[Groo]

This is a possibility but maybe not necessary if the block is called from different blocks, because there is a mode where you can specify a path to the block that you need to monitor. Edited August 16, 2011 by Groo