To fault find a system you need to know EXACTLY how it works
HOW EXACTLY DOES A PLC WORK?
Am I getting the input to the PLC?
The Led on the output card means i am getting voltage out right? does it?
What exactly happens in between? ,theres more than just a program in the CPU
How exactly does it scan the program?
What is this Watchdog Timer? Is it that important?
Can I use the same output twice? That's bad programming isn't it?
A PLC is a logic controller, so use a logical approach to fault find it.
What are the 8 simple test points to check?
The PLC is in RUN, that means theres a program right? does it?
FORCING a bit and toggling a bit is pretty much the same yeah? depends on which PLC
Then you need to Know the specifics
HOW DO I DO THE FOLLOWING?
(some straight forward some not so)
- Check power is ON and PLC is in right mode (RUN or Program)
- Check LEDs for fault definition
- Check and Change Modules if required (with spares and without spares)
- System connection
- Establish link to PLC (RSLinx, a major problem area these days)
- Create a blank project and take a backup (just in case I mess up)
- Open the correct project Off Line and link to PLC
- Interrogate errors in Status table
- Identify if it is a hardware or software fault?
- Identify if it is a PLC or Comms fault
- Change the battery
- Check all settings against a template, Node Address etc.
- Check Hardware Configuration
- Clear Memory and Download program
- Check software against latest copy (Compare function)
- Monitor program
- Searching for specific operands and instructions
- Changing timer, counter values On Line
- Making minor mods Off Line and On line
- Check or create a Custom Monitor table to establish parameter status
- Create a Histogram to monitor address transitions
- Creat a Trend to trend address status or values
- Force a parameter if required
- Call up documentation to assist with software diagnostics
- Printing Cross Reference / Program Listings etc.
Background information also covered
Understanding of the following:
- Number formats, bits,words,
- Binary, floating point, integer
- Program blocks and Data Tables, B3, T4, N7 etc
- How to monitor various blocks
- Understand basic Ladder programs
- Basic Instructions, contacts, Set, reset etc.
- Timers, Counters
- Comparators, Maths
- Altering values in a data table
- Back tracking through a program to establish where power flow stops
- Overall reset procedure
- Fault finding tips