Mark,
While I don't have a need for your mod, I wanted
to congratulate you on some "First Class" thinking.
Had
K7MLR
At 09:50 AM 7/24/2009, you wrote:
I recently got in an HP5370A that had been
robbed of the ROM card and the oven oscillator
power supply card... ah, crap... Swapping the
missing cards from a known good unit showed the
counter was in otherwise excellent operating condition.
The HP5370A firmware is stored on eight 2708
1Kx8 EPROM chips on the ROM card. The firmware
is compiled to execute at addresses
7800-7FFF. The system normally does not use
address line A15 and the upper and lower halves
of the address space are mirrored. (BTW, the
system address bus is inverted
polarity... important to remember if you are looking at the code).
It turns out that the 5370A CPU card has an
empty socket for a firmware ROM. This socket
supports a 4Kx8 ROM that will appear in the
address space at C000-FFFF if switch 3 on the
CPU card is turned on. Hmmm, with a little
modification it should be possible to install an
8Kx8 EPROM there with a copy of the firmware.
Motorola did make the MCM68764 and MCM68766 8kx8
EPROMS in a 24 pin package. They are hard to
find (but are on Ebay for 4 bucks each) and most
programmers do not support them. The EPROM A12
address line is on a pin tied to +5V on the CPU
board socket (pin 21). You could easily patch
the board and use the Motorola chip.
I have a pile of 27C512 (64Kx8) EPROMS in a 28
pin DIP package. Most of the pins match the
CPU board ROM socket connections so I chose to
use them. To use the 27C512 chip, first I read
in each of the eight 2708 chips from a good
5370A ROM board using a Data I/O Unisite
programmer (note that the DIO 2900 and 3900
series programmers don't support
2708's!!!). Next I used the DOS COPY command to
concatenate the 8 1Kx8 ROM images into a single
8Kx8 ROM image file (remember to use the /B binary file parameter!):
COPY ROM1/B+ROM2/B+ROM3/B+ROM4/B+ROM5/B+ROM6/B+ROM7/B+ROM8/B IMAGE.ROM/B
Finally, I concatenated 8 copies of the 8Kx8
image into a 64Kx8 ROM image which I then
programmed into the 27C512. You could just put
one copy of the 8K ROM image into the upper 8K
of the EPROM, but this was easier to do.
Lift pins 23 (A11) and 20 (/OE) on the 27C512 so
that they won't go into the EPROM
socket. Solder a jumper wire between EPROM pins
20 (/OE) and 22 (/CS). Be careful not to short
to pin 21. This connects the /CS and /OE pins
of the EPROM. Solder a jumper wire between
EPROM pins 1,28,27,26. This puts +5V on these
pins (A13,A14,A15,VCC). Install the EPROM in
the CPU board socket. Pins 1,2,27, and 28 of
the EPROM chip should be hanging in the air.
Solder a jumper wire between pin 23 of the EPROM
chip (A11) and U14-7 on the CPU board (on the
photo, I ran this wire to the back of the board
and picked up the signal on pin 18 of the 24 pin
the EPROM socket) Solder another jumper wire
between pin 2 of the EPROM chip (A12) and U16-9
on the CPU board. This connects the CPU address
bus A11 and A12 signals to the EPROM. (you
should first verify that the CPU board pin
numbers mentioned match the revision/layout of your CPU board)
Next, you have to modify the CPU board to
properly address the new EPROM chip. Cut pin 3
of U6 on the CPU board flush with the
board, lift the nub of the pin up so it no
longer touches the board (this pin normally
connects to A15). Solder a jumper to the lifted
pin and pin U16-11 (A13) on the CPU board. This
modifies the CPU board so that the CPU external
data bus buffer will be disabled and the EPROM
chip will be enabled when the external ROM board
address space is addressed (i.e. A13 and A14
high instead of A15 and A14 high).
Turn on DIP switch 3 on the CPU board to enable
the EPROM socket. If all goes well, you should
have a 5370A working without the ROM board (just like the 5370B does).
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