Reading a masked ROM the hard way (was Re: Wanted: ROM images from HP 9895, 82901/82902, and other HP-IB disk drives)

Eric Smith spacewar at
Mon Sep 21 19:53:41 CDT 2015

Someone was kind enough to mail me the masked ROM out of his HP 9895
floppy drive. I've read it and mailed it back. It's an MK36000 series
24-pin 8KB masked ROM, with the same pinout as the Motorola MC68764
EPROM, so reading it with an EPROM programmer set for the Motorola
part should work fine. In read mode, no programming voltage is
applied, so there should be no risk of damage to the part.
Unfortunately my Data I/O Unisite does too good a job trying to
protect devices against reverse or misaligned insertion or incorrect
device configuration; if it thinks the current drawn by the device is
too low or too high, it aborts with a device insertion error.  The
MK36000 draws significantly less current than the MCM68764.  I tried
putting an appropriately valued resistor in parallel, but still got
the error.

I ended up kludging the masked ROM to the expansion bus interface of a
TI Launchpad board with a Tiva TM4C1294 microcontroller (ARM Cortex-M4
based), using an SN74LVC245A buffer on the data bus because the Tiva
is not 5V-tolerant. I wrote C code to read the ROM and send a hex dump
out the USB-serial interface.


As I've needed to use the expansion bus interface of the Tiva for
other projects, even though this was a lot of work to read one ROM,
the experience and maybe even the code may be useful in the future.

The next issue is that it appears that the 9895 may permute the
address and/or data busses, as the contents of the ROM don't actually
look like reasonable Z80 code.

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