double coil syndrome

PLC/PAC programming twice the same output: NOT acceptable

Hi


This question is for engineers/technicians that program PLC/PAC and understand the dangerous consequences of writing to an output, whether a coil, or an analog output, from two different places in a program.



This is not a how to question, nor is it a if you can or not question. This is a question as to how to explain to someone that if you do this someone will get hurt, killed, equipment damaged or lost production time will result. Sooner or later.



For this reason, in my 3 decades of industrial projects to do such a thing has been considered illegal.


It has always been my understanding that illegal in this context meant:


1. Not written in law or some standards code, but rather the industry (in North America) standard best practices


2. In the case of an accident involving injury, death, damage or lost production, that such programming is INDEFENSIBLE in a court of law or in a claim to an insurance company.


3. There were enough accidents and claims that Rockwell simply excluded the possibility of such dangerous and senseless programming in RS-5000. However I've been told that Simantic-7 for Siemens S7-300 and S7-400 PAC's issues a warning but let's you proceed with writing to the same output ( bit, or register ) at two different locations in a program.


An engineer I worked with would casually uses such a dangerous practice. What documentation, reports, or reasons Rockwell excluded this from RS5000 can be found to explain to this engineer that the Earth really is round and not flat ?


Thank you for your assistance in providing any kind of documentary evidence. IEEE standard? An article in an industrial magazine on best practices in PLC/PAC programming? . . . ?


Regards,


Christian A Martel, P. Eng.
 
2. In the case of an accident involving injury, death, damage or lost production, that such programming is INDEFENSIBLE in a court of law or in a claim to an insurance company.
what a statement... so when you program that output only once, everything is ok?
If a programming error can cause injury or death there is probably something wrong with the design of the system anyway...

... However I've been told that Simantic-7 for Siemens S7-300 and S7-400 PAC's issues a warning but let's you proceed with writing to the same output ( bit, or register ) at two different locations in a program.
Please enlighten me. Never encountered such warnings, but I don't do PAC's. Well for Siemens stuff, I think only LOGO is LIMITED in using an output only once.
 

This is not a how to question, nor is it a if you can or not question. This is a question as to how to explain to someone that if you do this someone will get hurt, killed, equipment damaged or lost production time will result. Sooner or later.

OK, I get that you feel strongly about this, but try to stay rational. If your code is responsible for injuries and losses, it's your design that is at fault. The poor instruction is innocent.

You may believe that your design preferences are superior to your co-worker's, that doesn't make him criminally negligent.


3. There were enough accidents and claims that Rockwell simply excluded the possibility of such dangerous and senseless programming in RS-5000. However I've been told that Simantic-7 for Siemens S7-300 and S7-400 PAC's issues a warning but let's you proceed with writing to the same output ( bit, or register ) at two different locations in a program.

RSLogix5000 will produce a warning if you use the same tag on more than one OTE instruction. It doesn't "simply exclude it". It took 30 seconds to verify that.

I'm getting the feeling I'm dealing with a troll...
 
Lancie

Why would the value be of by 4? What is the value of N11:0?

Because N7:0/2 being on would contribute a value of 4 to the hex value in N7:0. The binary representation of N7:0/2 would be
0000 0000 0000 0100

If you had 20 in N7:11 the binary breakdown would be
0000 0000 0010 0000

Basically by turning bit 2 on in N7:0 the 2 binary representations would merge
0000 0000 0010 0100 and N7:0 would display 24.
 

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