make me understand

mug · Sep 27, 2005

hi! guys, im new in plc programming, i am currently reading some PLC program and i dont understand why certain bit, i think its an integer bit (ex. N10:11/10)how it turns on? when i cross reference i found no OTE to make it on? can somebody help me to understand this? or maybe you can give me something to read to enhance my knowledge on this? thanks

Eric Nelson · Sep 27, 2005

A few ideas off the top of my head...

Bit could be turned on/off remotely by something like an HMI (this seems the most likely in your case)
Bit could be indirectly referenced with a pointer Example: N10:11/[N10:3] when N10:3 contains the value 10
Bit might turn on when certain values are moved to N10:11. Look for N10:11 in the cross reference list

I'm sure there are a few other ways that others will add.

-Eric

mug · Sep 27, 2005

your correct on the third one, i found it in a MOV function, if i am not mistaken N10:11 is composed of 16 bits? how does it works?

bernie_carlton · Sep 28, 2005

The 'N' registers are most often used to hold a number, interpreting the entire 16 bits as one unit (N10:11). But it can be interpreted as individual bits as you have seen (N10:11/10). In an opposite manner the 'B' registers are most often interpreted by a single bit (B3:5/2) but can be interpreted, using all 16 bits, as a number (B3:5). The SLC500 - Micrologix series are very flexable in this respect. The reasons for using a location in both ways, the MOV transferring all 16 bits as a unit, then the usage of N10:11/10, are totally dependent on the program and the programmer. We hope that the programmer commented the program sufficiently to provide the reason for this usage.

dalem · Sep 28, 2005

Just curious if the program you are looking at has all the comments/descriptions located above each item in the ladder/rungs or it looks like a bunch lines and characters and no text? (As in an application that has just been uploaded from the PLC? --[ ]------(L)--.)

I've picked up a number of used PLCs and uploaded the program already in them before deleting and that it seems almost impossible to figure out what someone was doing with the unit previuosly.

TimothyMoulder · Sep 28, 2005

This is typical of AB systems, where comments are not saved to the PLC on download. Some PLCs offer the option to save comments to the PLC, others (like Unitronics) do it as a norm. It depends on the manufacturer.

Some people view it as code protections - darned hard to reverse engineer the logic when you have nothing to go by but a list of IO. It can still be done in some cases, but why? If you have an IO list, as Terry would say, "roll your own."

TM

Rich1955 · Sep 28, 2005

I have to roll my own alot because I can't read Japanise and boy is it fun.

mug · Sep 29, 2005

thanks for your views,the program has all the details, i just still cant find out how this bit turns on/off, i cant see the conditions which turns it on thats why i find it hard to troubleshoot the machine, is it some programming technique where you can hide the conditions? thanks to all maybe i just have to keep reading more on the manuals.

Eric Nelson · Sep 29, 2005

If you can post the .RSS file, we'd be glad to take a look at it for you.

-Eric

jkrom · Sep 29, 2005

As Bernie explained earlier, the N registers are normally used for integers (at the word level), but the individual bits can still be looked at for various reasons.

For instance, if you use MOV to set the integer value of N11:10 to 1024, the bit N11:10/10 will now be set "high". I'll use this technique often to change the state of multiple bits at once (i.e. MOV -21846 N7:0 will set the "even" bits low and the "odd" bits high)

Hope that helps

Ron Beaufort · Sep 29, 2005

Mug, you said:

i just still cant find out how this bit turns on/off, i cant see the conditions which turns it on

Eric Nelson MIGHT have nailed it for you when he said:

Bit could be turned on/off remotely by something like an HMI (this seems the most likely in your case)

is there an HMI (PanelView, RSView, or some other type of operator's input device etc. etc.) connected (networked) to this system? ... if the answer is "yes" then check to see if the operator device is capable of turning this particular bit off or on ... you'll have to examine the program IN THE OPERATOR'S DEVICE - and NOT inside the PLC ...

note that this is a VERY common way to have operator input devices control the PLC's logic ... secret handshake: if this IS indeed the way the bit is being controlled, then NO AMOUNT of checking the PLC program or reading the manuals is going to help ...

and as Eric said: if you'll post your .RSS file, we'll be glad to look at it for you ...

dalem · Sep 29, 2005

I was hoping to get the oppertunity to look at the RSS file, but wondered if it was unethical to ask for such. I would not see why, but perhaps some companies may have issues with doing such.

If interested in allowing me to look at the RSS file for/with you, email to [email protected] .

Always willing to help where I can,
Dale

mug · Sep 30, 2005

guys thanks for the help, i just cannot send now the file, but jkrom can you please or can somebody explain this principle further, because the program is working like what you've said. so thus this means that the value of N10:11(composed of 16 bits) being moved, how will i know the value, should i convert the 16 binary bits to its decimal equivalent,or vice versa, am i getting it??? please correct my thoughts i wanted to learn this. thanks

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