How I broke the target position
At the beginning of the dilution run, I pressed "Go Home" without first changing the offset on the "Move Target" screen to 0. This moved the target past the location the computer was expecting and caused a fault, which cuts off control from PDP. Note: we're not sure if the Amp Fault light was on before or after I did this, because I didn't make a note of it before trying to change the target position; the fault could have happened the last time someone sent the target to Top.
How to reset the target position
We took an escorted access and Dustin first called the counting house and made sure the Move Target controls were closed. There can be problems if we manually move the target while the PDP controls are open. He then pressed the "Esc" button (has a red octagon) on the manual controller and then had the counting house try to change the position again. It did not move, so they closed the GUI (again), and Dustin toggled the position manually with the up (ᛏ) and down (ᛎ) arrows. He again asked the counting house to try moving the target, and it worked, so we left.
How to prevent this from happening again
Before moving the target, record the temporary offset in the logbook. Then, set the offset to 0 and click "Go". Then move the target wherever. Use the drop down menu and "Go" instead of the "Run Home" button because this has caused some difficulty. You don't need to invoke the offset when the target is in the home position, so just leave it to 0 while at home. Keep the offset 0 until the target is back to ammonia (or cross or wherever). Then type in the recorded offset and click "Go", watching the monitor to make sure the crank actually moves.
It is worth mentioning the Amp Fault current goes red whenever the target is moved to Top and the offset is invoked. There doesn't seem to be a way around this, so we are going to do another controlled access and reset the position controller again the next time we have to move the target.