From: Tony Duell <>
Date: Thu Aug 28 17:07:00 2003

> At 12:29 PM 8/27/03 -0400, **vp wrote:
> >On the section on the 9825 it says:
> >> [The 9825T] also has a large warning label on outside top warning the
> >> user not to install any ROMs other than the 98211 Matrix ROM or the
> >> 98217A Flexible Disk Drive ROM. It says that installing any plug-in ROM

AFAIK these are the only 2 ROMs that are not already built-in...

> >> that is already built into the calculator may damage the machine. I
> >> have never seen this warning on ANY other HP machine.

The 9825T is a bit strange. There's some bank-switching circuitry to
allow 64K of RAM _and_ all the ROMs (the CPU can only address 64K, and
ROMs and RAM are in the same address space). This involves 2 special
memory boards (one with the ROMs and 32K RAM, the other with the
bank-switching circuitry and the other 32K). There's no OS ROM cartridge
on the RHS of the machine either (in fact the CS line for this cartridge
is hard-wired to be deasserted.

Watch out if you're moving boards between machines -- there are a couple
of trace cuts on the CPU board in the 9825T (to disable some chip select
lines that would have gone to ROMs, etc). On my machine they're hidden
under the CPU hybrid...

You may also have guessed that the ROM modules for the 9825 each have
their onw address (in some of them, it's selected by a jumper on the
PCB!), so that's why putting 2 indentical ROMs in at the same time is a
bad idea...

> IIRC I've only seen that on ONE 9825T and I have a bunch of them.

Mine's a 9825T...

Received on Thu Aug 28 2003 - 17:07:00 BST

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