UsingMicro820as a simple On OFF timer

I want to replace a Grasslin timer
with something that Communicates via EtherNet/IP™
I keep getting schedule changes that force me to go out and mannually change the timer settings.
I was thinking of using a Micro820
http://www.plctalk.net/qanda/attachment.php?attachmentid=58442&stc=1&d=1622562114
Anybody done something like this, or have a link to a sample so I don’t have to reinvent the wheel?

A Micro820 seems like a bit of a sledgehammer to crack this nut; do network-capable programmable timers not already exist e.g. like HVAC thermostats, Google nest, etc.*?


Assuming a PLC, there are several approaches, each too simple to warrant complete examples. One could model an analog Grässlin with an array of 96 bits, each representing one 15-minute (quarter-hour) period of the day. The PLC continually checks the time of day to to calculate an index into that array, and sets the output based on the value of the bit at that index. That should be a fairly trivial program, with the meat of it like this:

depending on the language of choice

The parts that will take more time and reading of the documentation to implement will be:

• getting the time of day (e.g. seconds since zero epoch modulo 86400, plus time zone and DST correction)
• convert that time of day to the index in the range 0-95
  • e.g. divide time of day in seconds by 900
    • dealing with any rounding in the quotient
• ensuring the PLC internal clock keeps synchronized with external time to some desired accuracy
  • an NTP client may be built-in
  • if clock drift is small and accuracy requirements large, then a manual adjustment once per month might be adequat
• dealing with daylight savings time, if an NTP client does not handle it
  • e.g. if changing the time settings is a regular occurrence, then manually adjusting the internal clock for DST is only two more steps per year
• dealing with power outages

I think one would spend far more time on the user interface to choose the array of bit states, although if the Micro820 has a web server, then it might be possible to use that alone and no custom interface would be required.


* Cf. here, probably overkill.

Check Out the TOW (Time Of Week) Instruction (Link Below). I have used this in the past to reset production counters at shift change. Only issue is keeping up with Daylight Savings Time.

https://www.youtube.com/watch?v=ffhFMKAdCR8