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.