Thursday 3 November 2011

Computer Diagnostics with PLC

Look inside nearly any industrial complex these days and you will find a miriad of computers
 all busy crunching numbers. One look at a server with all its blinking lights and you could be forgiven for thinking that they, the machines, have taken over. Examine the factory floor and most certainly there will be some computing usually in the form of a PLC with interaction to a PC. Open a cabinet that contains an operating PLC and its blinking lights are its way of saying, I'm working, each light corresponding to a particular sensor and output. programming these machines can leave the novice programmer scratching their head wondering how to complete a logic circuit but all it needs is a methodical approach with safety and efficiency being key elements in the operation.

Look at any electrical circuit and it will usually look like a ladder. Each rung of that ladder contains instructions and are followed in sequence down the ladder and then repeated continuously during the operation. On those rungs the instructions can be either on or off. A completed rung where the elements are true will output a go function. That leads to another process. And moves forward in that way. This process can get quite complex and worries me at times when I know that critcal aspects are controlled by minute but very essential programs.

Here are some tips

1. Trial and test and double test every program in the field. Run all the cycles so that the system is foolproof.

2. Do not assume that a functioning program that works is going to be safe. It might be, and might not!

3. Be  aware that as the law requires people to be responsible for their work. So check and recheck.


So your program is written and and its time to load. Press the load button and it doesn't work. Great! That's what I need, an uncooperative computer, or rather multiple computers. So now its time to diagnose why its not working. A frustrating experience for the novive under pressure.

Here a some basic hints

1. Does the PLC have power. It is turned on

2. Is the PLC in program mode or run mode. A toggle switch on the PLC changes this around.

3. Is the ethernet cable properly connected. The sockets need to be inserted tightly so that they make good contact.

4. Ensure all other necessary cables are connected.

Having a preference to use the mouse with my left hand I'm constantly bring the mouse from  public computers over to the left side because I always find them on the right.  Sometimes the mouse cable pops out and there is an error  on the screen. Don't panic.  Look back at the last thing that was done and check it. Its happened to me few times. Cable always seems to be too short to reach the left side. So alwsays check the cables.

Software or Hardware Problems

Once the external hardware is firmly fixed, swap with working models if not, malfunctioning  computers can have a miraid of problems like blue screens, beeping, black screens and just plain dead screen.

Lets assume here that PLC program written with say Actwin or Allen Bradely software has been configured correctly and the computer set up is still not working. The easiest way from here is to ask for a computer tech because its getting away from the main task of programming the PLC and distracting your attention from making that PLC program work well. Here is a few tips if  a tech is not available

1. Beeping computers usually indicates a hardware fault. Check the beep codes and rectify

2. Blue screens sometimes just go away after rebooting. If not  run in SAFE MODE and use the diagnostic facilities avaliable there

3. Black screen with lights or minimal light is usually a power supply failure. It could also be in power saving mode.

It can be exasperating at times when multiple problems occur at once making diagnostics difficult. Tackle the simplest tasks first. Remember to always backup data and maintain a strong anti virus anti malware system. Reduce the stress and ask for advice from an expert when needed. If a problem can't be found in a set amount of time, extra time spent on that task becomes a waste of your time. 

Computer Beeping Problems

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