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Posted: 2022-10-2808:38 PM
An EStop is a component of a safety system. You should engage an adequately competent and certified professional to assess the required safety integrity of the system, and to validate that the as-designed and as-built system meets the functional safety requirements.
The logic that you are looking for is something of a rising edge trigger, and then a hold in, with a drop out.
Here's a slightly horrible example in Ladder Logic (where it didn't have an RTrig or similar.. so I had to create similar from scratch, as 'AND NOT PrevRunSwitch')
NOTE: This logic is not adequate to meet ANY required safety integrity requirement. Additional consideration and assessment would be required to determine the adequacy of the logic above for any purpose.
The reset of the EStopOK circuitry would be handled by a separate safety related system, which would ensure adequate levels of integrity around the EStop, and device inhibit on EStop. It would also perform the reset of the EStop (because the EStop should only be allowed to Reset if the safety related components have been confirmed as being 'healthy')
Lead Control Systems Engineer for Alliance Automation (VIC). All opinions are my own and do not represent the opinions or policies of my employer, or of my cat..