HP-IL Printer (odd?) page shifting when disabled.

[J-F Garnier]

This is a good question that always intrigued me.

Since the page shift mechanism needs a signal (and a chip pad), wasn't it easier to just use this signal to disable the ROM, instead of moving it to an other address?
As you reminded it, the present solution requires a careful design of the first few ROM words to give up control, since address 4000 is polled at each wakeup from sleep states.

If you look at the "40K ROM" specification at the well-known HP-41 site, you will see that there is a TEST signal to disable the ROM. So IMHO, it would have been better to use it.
But maybe there was a drawback or a potential risk to put the ROM in test mode, so the engineers had to resort to the address selection signals.

Or maybe was the disable feature considered quite lately, something like: "Oh wait, we forgot it, what if a user wants to use his old 82143 printer with the HP-IL module?" and the engineers chose the safest way.

If so I would not call it a clever trick but a workaround.

J-F