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

[Diego Diaz]

Hi all,

I've been asked about why the HP-IL Printer ROM is shifted from page #6 to page #4 when it is Disabled by the bottom switch.

First of all, I must admit I don't have a documented answer; so I provide my best guess in the hope that someone else may disprove or confirm this hypothesis.

Here are my thoughts (at risk of showing my ignorance on the subject :-)

Since the ROMs in the HP modules are built using CMOS technology and they are connected to the CPU by a serial bus composed of clocks, sync and data/address lines; disconnecting a ROM chip from the CPU cannot be achieved by simply removing the Vcc line (and/or GND for that matter).

As long as any line is high and any other line is low the ROM will receive enough power to keep on running and responding to the CPU.

So, the only way to effectively remove this ROM from the bus will require all (or most of) the bus lines to be disconnected. A multipole switch suitable for the task would hardly fit in the scarce real estate on the HP-IL shell.

The (clever IMHO*) trick of switching just the ROM's address mask bit A13 from 1 to 0 (changing its native #6 to #4) solves the problem since page #4 is not (was not until Ángel's LIBRARY4) used by the "normal" HP-41 functions.

*Careful selection of the XROM and FN words allows the ROM to return control to the OS as soon as it's called after wake-up.

As stated, just an educated guess.

Any input will be very welcome.

All the best from the Caribbean.

Diego.