Just wanted to check that we are talking about the same type of EHT power supply (PSU).
My interpretation of the use of this type of PSU’s is that the ‘IN’ terminal is used to set the maximum output current that is required for the job in hand (all tubes have a maximum current, which should not be exceeded, which is in turn related to their wattage). This maximum current can be set by applying a DC voltage (0 – 5 Volts) or by applying a PWM signal (0 – 100%). The best method is a matter of choice and I just happen to have chosen a DC voltage but there is no reason why PWM should not be used. The actual tube current is set and or varied by the ‘TTL’ terminal using a variable duty cycle (0 - 100% mark space ratio). This can be achieved either by a Pulse Width Modulated signal or by a Pulse Repetition Frequency signal and again it is a matter of choice which is used.
To use the Mach3 Laser / Engraving plugin the maximum tube current should be set to suit the material then the actual tube current is varied, as necessary, by the PRF output from the Digital Trigger. The output pin used to link the Digital Trigger to the TTL terminal must not be mapped to any other function within Mach. The output pin used to link the speed PWM to the IN terminal must not be mapped to any other function within Mach. From here on it is just a matter of fine tuning to get the best results.
Tweakie.