Is it modbus TCP or RTU ?
Screenshot of localisation the edged bit you are talking about ?
BTW, communications are almost always async. If so, then, most of the time, data is buffered before any send/receive. So planning to do some treatment on data to spare a communication cycle feels wrong.
If you run TCP, cycle time should not be a problem. I will assume you run RTU modbus. Then, you may want to do some priorities in your data. For exemple, ON/OFF orders, may be send at each cycle, while setpoints could be sent once every 10 cycle.
One more tip, Don't forget to group your requests : doing a read on 2 words is faster than doing 2 reads on 1 word. Meaning you may want to use a block read, with not used data in it, just because it is faster than doing several reads.
Same tip applies to bits.
I speak about ModBus RTU in standard realization in Codesys as native device. There is an option for ModBus Slave Channel: write on rising edge trigger.
So my question is how this trigger works internally in Codesys.