People (operators) will intentionally lie = "I didn't do anything" vs. "I pressed the On, Start and Stop buttons at the same time to see what would happen".
Machines (logic) will unintentionally lie = self-diagnostics is only as good as the sensors and programming. A blown fuse in the encoder circuit may result in a 'failed motor' error message.
Operators, technicians, and ENGINEERS, will come up with almost any excuse sometimes or straight up deny something when they feel their job is on the line. It is human nature and is something we have to live with.
Whenever I write a program, I try to plot every possible scenario that could happen in run mode from an operator input stand point of view, such as what happens if they press both of these buttons at the same time, what happens if they push this when the machine is running (interlocks and one shots), what happens if they change these setpoints on the HMI, etc.
What I like about RSLogix is that you can compare two PLC programs and see what changes were made. What I would do if there's a call for a PLC acting funny is compare the current program in the processor to the original you have saved on your laptop. If there are differences, you can debug a lot faster to see what is going wrong. You can save the current program, download the original fresh copy, and observe the changes. You then have the failing copy on your hard drive to debug at your desk. Sometimes people like to wire up new I/O, nothing is documented, and they add new logic. Comparing the two programs is a great way to see what was changed and how it could affect the logic.
once you've got it running right – then THINK:
what will happen if the power fails while the machine is in Step 1 – in Step 2 – in Step 3 – etc. ??? ....
and then (more importantly) ...
what will happen when the power comes back on again ??? ...
How right you are! It is important to think this every step of the program. When I write a program, I look for 5 possible things that could happen at any time during my program...
What happens to the program/machine if there is...
a.) loss of power (upon power up, the machine should need to be restarted agian, not restart automatically)
b.) loss of communication such as a bad node, a cable is broken, loss of power to node, bad COMM card or board, COMM cable unplugged(do you leave outputs on, do they all turn off, etc)
c.) processor goes into program mode or a processor fault (software failure)
d.) a sensor, field device, or component in the control panel fails (aux contact, motor starter, photo eye, etc)(hardware failure)
e.) operator presses stop or start or the machine has a fault such as a jammed photo eye or a tripped out conveyor chain prox switch(make sure all necessary logic is reset before operator restarts machine, and that the machine will not start in half way of sequence and instantly without annunciator start up time delay warning)
There is also the 6th, UNKOWN thing that can happen. That is when a lot of troubleshooting takes place.
a.)power failure
b.)COMM failure
c.)program mode or software failure
d.)hardware failure
e.)operator input to machine / machine fault
f.)???
When checking a 2300 volt (and higher) main breaker to see if it is off, do not assume that the wires on the bottom are the load side. 999 times out of 1000 you will be right, but you only get to be wrong 1 time in your life. Sometimes people have wired the incoming line to the bottom of the breaker (because the main was replaced and the old leads were too short to reach the top of the new breaker). I know of at least 3 men who would still be alive if they had not made that assumption on a 12.6 KV switch.
You are also 100% correct. Never assume anything until you verify it, especially in a new work envirnoment or unfimiliar turf (different plants, different machines, different power distribution systems, etc).
I am used to seeing blue colored wires as 24VDC, red as 120V control voltage, black as line voltage, white as neutral, green as ground, etc. Before I get too confident digging around in control panels with all blue wires, I take the time to measure them out with a voltage meter and identify the power sources in the panel (and then after that, still treat everything as if it's live.)I've seen blue control wiring as 120V, red wires as 12VDC, and white wires that should be neutrals miswired as line power. I have also seen circuit breakers wired on the bottom side as line, and top side as load --- VERY UNSAFE.
it's just the way it is here).
