Powerflex 527 STO Safe Torque-Off hardwire

Hi all,

I am having issues with a Powerflex 527 STO. I am hard wiring a safety relay (guard master MSR27TP) to S1 and S2 and also the common to 01 as per the manual. I also set up the Powerflex configuration connection to "motion only."

I am still getting an fault/error on the powerflex saying "INHIBIT M05 – SAFE TORQUE OFF." Is there something I am missing that I didn't wire up correctly or a configuration parameter I forget to enable/disable? Any help would be appreciated.

Make sure you have 24vdc on S1 and S2 from your safety relay

Are you using the source voltage S+ terminal from the drive into your S1 and S2 inputs via the safety relay? Or are you providing an external 24v signal to S1 and S2 through the safety relay? I'm pretty sure these are tested outputs so S1 and S2 have to have the signal from S+ terminal on the drive.

I haven't used the 527, but have done a number of STO applications on 525s. You can use an OSSD signal or just straight 24V as long as the 24V source shares a common. The OP's point about connecting common to terminal 01 implies that they're using an external 24V supply.


Double check the voltage on S1 and S2, like DarrylR suggested. You may also need a fault reset signal once the STO is re-energized. In the 525, there's a parameter that enables/disables the STO fault. By default, it faults the drive when STO is removed. If the parameter is changed, it doesn't fault the drive, basically making it an "auto reset" safety function.

I measured voltage at S1 and S2 and I am getting 24v, and yes you are correct I'm using an external 24V power supply. I believe I might have found my issue. These drives were previously used on another system that used safety over ethernet, I found that you could have these weird inhibit faults on the drive and the only way to clear them is to factory reset the drives and to do that you reset the safety ownership. I am currently trying to do this and will get back to you guys if this the fix. This is the article I found on Rockwell website that might potentially fix my issue: https://rockwellautomation.custhelp...fety---reset-to-out-of-the-box-configuration-

Just wanted to update everybody and that resetting the ownership to all the VFDs that were faulted via the article knowledge base that Rockwell provided cleared the fault/issue. If you happen to use a powerflex 527 using safety over ethernet then have to switch over to hardwire please make sure to reset the ownership and keep the module inhibited until you have switched the connection to motion only