Looks to me like you should be addressing coils, not holding registers.
Try [Test Unit]51.C3801 and [Test Unit]51.C3805.
Edit: Make sure device name (Test Unit) and unit id (51) are correct as well.
Looks to me like you should be addressing coils, not holding registers.
Try [Test Unit]51.C3801 and [Test Unit]51.C3805.
Edit: Make sure device name (Test Unit) and unit id (51) are correct as well.