I have an AD-741, this is how mine works:
Powering off the machine and then holding the reset button while you turn the machine on is how you change the readout from mm to inches.
When you hold the reset button while the unit is powered on already the digital readout is reset to 20.0mm or 0.785in (whichever the machine is set to display at the time).
To calibrate raise the bed and squeeze a 20mm piece between the bed and the rollers, then hold the reset button.
I hope that helps?
-Ameer
