plc safety signature

James Mcquade

Member
Join Date
Oct 2007
Location
Nashville, Tennessee area
Posts
3,640
I was recently asked the question "what is a plc safety signature"?
this facility is the first location where i worked that used safety plc's.

i didn't have a clue - so what is a plc safety signature?
thanks in advance.
james
 
Rockwell, GuardLogix controllers require a safety signature. I suggest you parse one of the GuardLogix manuals. Or a "S" controller manual. There is step by step procedure on generation, and explains the function.
 
It's a checksum for identifying the current safety-related software (logic) and hardware configuration. Used on the paperwork certifying machine safety has been checked and verified.
 
It's a checksum for identifying the current safety-related software (logic) and hardware configuration. Used on the paperwork certifying machine safety has been checked and verified.
The key here being that if the checksum changes, it means SOMETHING safety relevant changed in the programming. Could be a new safety input card was added, could be the code was changed, COULD BE SOMETHING WAS BYPASSED.


Regardless, undocumented changes to the safety system are a big no no at places that care.


Some safety PLCs I've seen just have 1 overall checksum, or 1 software 1 hardware. I've seen other safety PLCs that have checksums breaking the software down much more.
 
The key here being that if the checksum changes, it means SOMETHING safety relevant changed in the programming. Could be a new safety input card was added, could be the code was changed, COULD BE SOMETHING WAS BYPASSED.


Regardless, undocumented changes to the safety system are a big no no at places that care.

Exactly. And it's important to note that undocumented changes to the safety system are a big no-no to the PROSECUTOR when Bubba's hand gets ripped off.
 
I think the 4 manufacturers I have seen is AB, Siemens, Asi manufacturer, Profibus Manufacturer: Most have special software and training so that you know what your doing on estop programming. I thought it complicated the estop circuit, so now your not just verifying hardwired devices for correct functions, all that wiring that was in a physical panel is now in the estop programming. So now estop is not just wiring but wiring, network setup, and then the programming. Too much fun and sometimes easy to bypass if you dont know what your doing and sometimes easy to wipe out the programming.

I would talk to your distributor see if they cant give you some training or let you get your hands on their trainer if that is possible.
 
I think the 4 manufacturers I have seen is AB, Siemens, Asi manufacturer, Profibus Manufacturer:


Fun fact: ASi and Profibus are actually network protocols, not manufacturers.



Most have special software and training so that you know what your doing on estop programming. I thought it complicated the estop circuit, so now your not just verifying hardwired devices for correct functions, all that wiring that was in a physical panel is now in the estop programming. So now estop is not just wiring but wiring, network setup, and then the programming. Too much fun and sometimes easy to bypass if you dont know what your doing and sometimes easy to wipe out the programming.

You are correct that it complicates things, if there is only one safety circuit. However, it starts being simpler when there are multiple safety functions and zone, and door switches, and manual/teach/service modes to deal with.


If your drive supports it, you can do networked safety directly to the drive, and not need to wire up any local safety terminals. This saves lots of time.


Having a networked safety PLC also makes it easier to have things like safety signals shared to wireless units like AGVs, which would not have been possible before, or would have required a secondary system in place.
 
I thought it complicated the estop circuit, so now your not just verifying hardwired devices for correct functions, all that wiring that was in a physical panel is now in the estop programming. So now estop is not just wiring but wiring, network setup, and then the programming.
If you only have a simple system for example consisting of a single E-Stop relay, a safety PLC does not simplify things. But if you have anything slightly complex, a safety PLC can simplify things considerably.
As a matter of fact, in order to reach the highest safety levels, a safety PLC can be the only way to achieve it.
Too much fun and sometimes easy to bypass if you dont know what your doing and sometimes easy to wipe out the programming.
It is harder for a maint without the software to bypass the safety. If you have RFID sensors instead of more simple switches, then it is very hard to bypass.
Even if you do have the software and the program and can change the safety program, then you cannot hide that you have made a change. That is what the 'signature' that James Mcquade mention is used for.
A safety PLC also makes it easier to test and document the system. This is getting more and more important. The days of an uneducated noob slapping a control system together, stick an E-stop relay in there somewhere, and then all is OK is over.

Here is a tip for James Mcquade:
Since you are going to work with safety, and since you are in the US where you can land yourself in jail if you get to be blamed for a serious accident, be sure to document everything.
Risk assesment, verification, validation, protocol everything.
 
Last edited:
Safety signature multiple processors

Hi guys,

Quick question, if you sign of a safety plc. Can that program then be used on multiple machines? I am just wondering as there could potentially be different hardware on other machines even though they are the same model machine.


Or will the safety signature be bound to that specific backplate and processor
 
In Siemens land, part of the overall signature is tied to the hardware, but it tied to part numbers (and parameters) and not serial numbers. So if you have the exact same hardware you could use the same program with the same signature.
 
I think I'm 99% correct in the Rockwell side. The hardware software & hardware configuration plus the time stamp. Is how the safety signature is created. So even if I created the same hardware and software the likelihood that I would create the signature to the same nano second isnt very possible.

Not only look at the safety signature but look and see if it is locked? We all know a lock keeps the honest man honest. If the safety system isnt locked anyone with the correct software can change it. Yes you will know that the software has been changed but you wont know what until you do a comparison.

We added 1 emergency stop to a system and had to re-validate the system. Why because we made changes and being human a small fat finger of the keyboard could be dangerous. That small mistake may not be seen until years later.

Yes I've found one Emergency stop that was mistyped. It it was a few years later. Luckily they where not putting a safety demand on the system when I found the problem. The customer presses the Emergency stop button whe they go home at the end of the night. The other half of the system wont run. So i got a call to find out why. The original programmer mistyped a 5 instead of an 8.
 

Similar Topics

So when going through creating logs of all our safety signatures for tracking purposes. I found one PLC without a safety signature. This one is...
Replies
7
Views
8,834
I've ready through the the previous posts, and we've worked with safety design for a long time. In the past, we worked with Pilz directly, and...
Replies
9
Views
430
Hello, I have been programming with Logix for over 2 years now, my current job just put me on a job where we had to change some of our original...
Replies
7
Views
900
So I'm still a little new to LC muting, and currently do not have a test bench. THE SITUATION My LC function is set to manual reset. I mute the...
Replies
3
Views
925
Hi guys, This my first time posting hope I get help in my issue.. I have Allen bradley control logix safety PLC (1756-L61S) connected with...
Replies
11
Views
2,258
Back
Top Bottom