Calculating PLC Input Threshold Values

The old documentation was gathered from 1988 and some class notes for students in 2005, then put through LaTeX in 2011, with a download:
Calculating PLC Input Threshold Values 271 kBytes PLCthreshold.pdf

Before designing input cards we glossed through several manufacturers’ catalogues. They did not tell much, and some even showed a series resistor and an opto-isolator without any threshold sensing as an equivalent of their inputs. Their input cards were probably not too different, as sensors with a few milliamps of leakage current happened to trigger the PLC inputs. Threshold values were all over the place. We bought a couple of opto-isolators and ran a few experiments to confirm that they would switch on widely varying currents. Then we read an application note1 from Hewlett Packard (now Avago technologies) on their threshold sensing opto-isolator, the HCPL-3700. Again, obvious when a decent application note is available. Threshold sensing was not that obvious to students who appreciated the extra notes after a few PLC lessons. Many designs used zener diodes to guarantee the threshold (our first one included). Now we would use threshold dividers and a transistor switch as described in the download, or simply put in comparators. Cost is always an issue, but the prices of comparators dwarfs the prices of PLC vendors’ input cards.

K60 board

We will be placing active devices on the field side of the isolation barrier and then isolating CAN. The cards are still paper designs with dynamic pulsing to reduce power dissipation and also using low cost micro controllers with on-board ADCs to see where they are in a threshold region (going up or down) and state machine based. 100MHz ARM devices are a few dollars so no need to pay thousands for trivial PLC automation vendors’ I/O cards.


1 Hewlett Packard (now Avago Technologies), “Threshold Sensing for Industrial Control Systems with the HCPL-3700 Interface Optocoupler”, Application Note 1004, not sure of the year but over 20 years ago. The new download was 5953-0406E, May 4, 2010.