Restoring a HP9830B calculator.

[Tony Duell]

In many ways the RotEn signal is one of the key signals in the HP9866. I called it that because it enables the feedback gates on the data shift registers, so it causes them to rotate the data. But as a result it indentifies which of the 2 main modes the printer logic is in. RotEn = 0 is the character input mode, RotEn = 1 is the print mode.

Anyway. RotEn = 0 iff the row counter is in state 0. The row counter is U12 on the data path PCB, RotEn comes from U19a. So the next thing I would check is what U12 is doing. Is it counting. If not, why not? Is it held in the reset state? Is it getting clock pulses (produced by the microcode, BTW). This is not too hard to debug.

I assume you have the 2 DataFile articles on the internals of the 9866. I described it all there.