slc500/4 help with latch

Peternjs

Member
Join Date
Mar 2004
Posts
4
Need to set and reset latch by single touch input from several 550 touch screens and scada package. My logic seems to be evading all of my efforts. Input is via a b relay and momentry however the scan seems to reser the latch to quickly. I need to set and reset by one touch for about 90 inputs.
 
Momentary pushbuttons in SCADAs and HMIs are allways a source of trouble. You need the communications scan to be so fast that the normal (natural ?) short press of the button is "seen" by the PLC.

This method works independently of the scan rate:
Use a "latched" pushbutton in stead of a "momentary".
In the PLC, evaluate the "write" tag with a oneshot. When the oneshot is activated, do the action that you want to perform (like changing some state), AND unlatch the tag immediately .
Let the "indicator" tag be the adress that changes state.
 

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