The firmware in the ZL1BPU interface will not work correctly when the direction indicator voltage approaches zero volts (fully CCW) which is OK for the Yaesu series of rotators which apparently have a residual voltage of about 0.24V in that position, but not the HAM IV.

The indicator voltage on the HAM IV controller is zero volts when fully CCW and it is necessary to reintroduce a 0.24V volt drop for the interface to work correctly.

This is done by adding a 100 Ohm 10 turn variable resistor just before the measuring point for the Ham IV Controller direction meter. The measuring point for the ZL1BPU interface is before the variable resistor the original metering afterwards. This arrangement allows the original indication meter's calibration to be unaffected by the volt drop introduced by 100 ohm variable resistor however you will need to reset the FSD on the meter once the interface is aligned.

The  HAM IV controller PSU voltage is floating with respect to earth and the wiper is connected to earth giving a voltage between pin 3 and pin 1 of 0 to 12V between 0 and 360 degrees rotation.

To align the rotator limits correctly you should you should follow the description given in Murray's instructions. Using Hyperterm. first set up the fully CCW position and adjust R2 until Hyperterm return a value of hex 08 rather than recompiling the code.

Now turn the rotator fully CW and adjust R5 until reads hex 88 if using the ZL1BPU software or hex BC if using Yaesu compatible software. Repeat process until both values are stable as the adjustments affect each other.