Siemens 505 Illogical ladder

[Wootts]

Hi guys, first time posting on this site. I'm a Maintenance Electrician and am having trouble understanding some ladder logic on one of our printing presses. The issue is they are using internal coils with the same address on multiple runs of logic. The way I understand this type of programming is the last coil scanned in the program will be the state of all associated bits at the start of the next scan. But when we are monitoring the program online we can never see the internal coil change states. but the outputs that the internal coil is controlling do go high. The rung appears to be in a illogical state. Is it just that we can never see the change in state of the internal coil as it happens multiple times every scan?

Internal coil C1821/C1822 is the one being used in multiple rungs.

Output Y137/Y138 Is high Even though coils C1821/22 display low

 

[Dravik]

So, are you outputs used in more than one place as well?

Because yes, W/ that processor, it will scan the entire ladder, last output state wins(disregarding SFP here for a moment, as they could be used there too) and THEN the I/O table gets updated.

 

[Wootts]

The outputs are not used in any other location. Everything is working on the machine just trying to grasp the program. So even though the internal coil shows itself as OFF in the ladder and data window, the output will remain on as long as the state of the internal coil preceding it is high?

 

[Wootts]

Sorry, mis read your response. You're saying that the output tables are only updated after the entire program is read. So unless these outputs are used in a SF that rung of logic does not make sense to me

 

[Dravik]

Only if there was another rung of ladder(or SFP) that was evaluating true and setting the output coils later in the program.

If it is JUST those 2 rungs, Those coils should not be energized unless they are forced somewhere.

Edit: Yep!

 

[Dravik]

If you're allowed to edit that program online, Throw an 'always off' coil in front of the ladder and see if the output stays on. If it does, you've got something odd going on elsewhere.

 

[Wootts]

Okay I will try that on a down day sometime soon. Thanks for your help in clarifying how the PLC scans the program

 

[alive15]

Usually if you have two of the same outputs in a ladder, one rung will attempt to turn the output off while the other rung will attempt to turn the output on, causing on/off flickering on that output.

There are small # of cases where having two of the same outputs in different locations will still let the program, but generally, you just want one output instruction for every physical output connected to your PLC.

 

[Wootts]

So the output is working fine, it is going on and off when required. There is only one location in the program for the physical output (Y137). It is not activated in any SFP and when I do a address cross reference it comes up with just the one location for the output.

I still do not understand how the PLC is reading this particular rung of logic. It appears that the multiple use of the internal coil (C1822) is making the first examined contact low but yet the rung reads true.

Is it possible that the PLC is reading the internal coil as high in a preceding rung, creating a high contact, then in a following rung it reads it as false thereby displaying the contact as low?

 

[rdrast]

I seem to remember that on the 505's, internal coils are updated immediately. Only physical outputs are updated after the complete program scan.

Because of that, the internal coil is used as a temporary storage bit for logic in the same or nearby rungs. I use the same technique all the time for in-run logic, or just to spread 50 xic/xio instructions into an easier to read rung with sub-rungs. It saves having to use a thousand bits for temporary memory storage.

 

[Wootts]

I seem to remember that on the 505's, internal coils are updated immediately. Only physical outputs are updated after the complete program scan.

Because of that, the internal coil is used as a temporary storage bit for logic in the same or nearby rungs. I use the same technique all the time for in-run logic, or just to spread 50 xic/xio instructions into an easier to read rung with sub-rungs. It saves having to use a thousand bits for temporary memory storage.

That makes sense. They utilize a similar technique throughout the program whereby inputs are used to activate internal coils (C111-116) then those bits are moved to a V-memory location then they clear those internal bits to make ready for use further down the ladder. That is easier to follow then this particular run of logic. It just makes it very confusing to monitor when you have a rung that appears to be in an illogical state but is working fine.

 

[dwoodlock]

Could the output byte or word be being written elsewhere in the program? That would be another thing to check.

 

[sigmadelta]

Did you cross reference C1821 and C1822 to see where the coils are for them?

 

[Wootts]

Did you cross reference C1821 and C1822 to see where the coils are for them?

Yes, They are turned on a few rungs up. This logic run is to turn on an air solenoid for one unit. The machine has 8 units so the logic is basically copy and pasted with only the physical input/output addresses changing. So when we monitor the program the internal coils display an off state, while the output shows true.

 

[sigmadelta]

No coils for them below these 2 rungs you posted?