ASCII String Compare Clear on Powerloss

[JZerb]

Having an issue, this is my first attempt at setting something like this up, and hoping its just something minor im missing. scenario.

have a Panelview800, using the Current User system tag in the Global Connections to write to a Current_User tag that i made in the external tags tab in CCW. That tag writes to a String address in the Micrologix 1400, which then has an ASCII String Compare instruction setup and that in return sets a Boolean bit to tell the PLC whom is logged into the HMI. The issue im having is if someone is logged into the HMI then there is a power loss, that ASCII String Compare instruction continues to evaluate as true when the power comes back and there for a Comm Error pops up on the Panelview800. If i unplug the ethernet cable from the PLC, or the ethernet switch, or the Panelview and then plug it back in, all is well.

 

[Saffa]

There is a first scan bit in the S2 file i think it is, that you could use to clear the string when the PLC powers up.

Why does the PLC lose power but not the HMI?

 

[JZerb]

they both lose power. its on a little test station i have for work.

it actually seems to happen on first power up, and then all subsequent power ups after that, not just when someone is logged into the HMI and then power is removed and turned back on. to make it even more interesting, if i unplug the ethernet cable from the HMI or the switch and plug it back in it doesnt clear the error. it will only clear if i unplug the cable from the switch to the PLC.

 

[OkiePC]

If the PLC loses power, you can use the First Scan bit (S:1/15) to clear the string tag.

 

[JZerb]

i did forget to add that into the ladder, thanks.

but unfortunately that didnt fix the issue. when everything first powers up there is a button on the screen thats visible that should only be visible after someone logs in, the HMI sits there for a bit, then returns an error that says it unable to write to the string in the PLC that holds the data to allow or disallow that button to be visible.

 

[Saffa]

This is why I control my HMIs in all our small pump stations with a relay from the PLC. When the door is closed and no one is on site, I turn off the HMIs. Also saves batteries during power outages.

You could just power the HMI as long as the PLC is running, with an always on output