I'm not sure I understand how/why a different interrupt would be helpful, there is code checking if a value is available from FreqCount before doing any of the math to find the torch voltage. I did make some more changes last night so the outputs are only modified if the change is different from current status, before it was attempting to set an output every loop even if it was already set high/low.
Todo:
I'm going to add an output for ArcOK and rework the PCB to add another transistor as this seems to be a common feature on other units to support plasma cutters without that output. I'm thinking that I will set the output if the incoming frequency is 5% of the THCAD range above the low. So if the range for 0-10v is 100k-900k, the ArcOK output would go high at 140k or in the case of a Hypertherm with 50:1 divider the torch would be at 25 volts. I'm sure this could be lower but that seems like a pretty safe value.
Also going to attempt to get the Modbus working on a separate Teensy and try serial communication between the two and depending on what the whole Modbus code looks like I may try to integrate it with the THC code. I would really like to keep the loop time as quick as possible and let the software (UCCNC in this case) act on it accordingly but I should be able to remove all the LCD code once Modbus is working.