HP 3456A Multimeter

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Unofficial supplements to the Service Manual.

Fix your Multimeter easier or back up your ROMS and MCU, so you can keep it running forever!
EPROMs have often been known to fail after a few decades.
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GENERAL FIRMWARE INFORMATION

There are only two known firmware versions for the 3456A, and the newer version firmware works in all hardware revisions of the meter even though you may have to split or combine the ROM data in order
to distribute it among the ROM sockets available on your A4 board.
Note that the MC68A00P or HD468A00P CPU is the fast 1.5MHz version.

ROM INSIDE NEWER 3456A METERS

Newer meters contain a single 28 pin 32Kx8 TMS27PC256NL EPROM on a new revision A4 board which can also be used in older meters.
The new rom part number and checksum is: U7=03456-65501(469BE9)
You can simply burn this image into an industry standard 27256.
This single ROM actually contains the same data as the three individual ROMS found in older meters and it packs as follows:
   0000-1FFF <-- fill with binary FF since this is not used.
   2000-3FFF <-- same data as 8Kx8 ROM U5=1818-1629(C9CF5)
   4000-5FFF <-- same data as 8Kx8 ROM U7=1818-1630(CD212)
   6000-7FFF <-- same data as 8Kx8 ROM U8=1818-1631(D4CE2)
The procedure above is verified to work without any problems.

ROMS INSIDE OLDER 3456A METERS

Older meters contain three Mostek 24 pin 8Kx8 MK36XXX mask ROMS.
They have been known to die or become corrupted due to old age.
This used to be a common pinout.
Equivalents are Signetics 2664, AMI S68364, Harris HM6388, NS DM87S228, NS DM77S228.
These ROMS can be read on an EPROM programmer as the pin compatible Motorola MCM68766C35 aka MC68766 prototyping EPROM, as long as the reader is smart enough to strobes CS or OE when reading each address.
Meters with serial numbers HIGHER-THAN 2015A03070 contain a NEWER ROM revision having the following part numbers and checksums:
U5=1818-1629(C9CF5)   U7=1818-1630(CD212)   U8=1818-1631(D4CE2)
Notice that the newer ROMS are certified to work in older meters.

The ROMS can be directly replaced with pin-compatible EPROM MC68766 or MCM68766 but these are obsolete though easily available as surplus parts for about 10 dollars each. The
SCM90448C might also be a direct replacement.
Your EPROMS should have the following checksums: U5=(C9CF5) U7=(CD212) U8=(D4CE2)
The procedure above is verified to work without any problems.
Modern EPROMs type 27HC641 aka M27HC641 should also work and are pin compatible, as long as you cook the data a before burning, because A10 and A12 pins (if I remember correctly) are swapped.

The data from each mask ROM can be split into two ordinary 2532 type EPROMs which are easily available. You just need to solder in the 3 missing ROM sockets and change the jumpers at the chip select logic on PCB A4 as follows. Remove jumpers: 2-3, 4-5, 7-8 and install the following new jumpers instead: 1-2, 5-6,8-9, 4-7.
Split the original ROM from socket U5 across sockets U1 and U2.
Split the original ROM from socket U7 across sockets U4 and U5.
Split the original ROM from socket U8 across sockets U7 and U8.
The checksums for the six split ROMS will turn out as follows:
U1=(65C3C) U2=(640B9) U4=(6581A) U5=(679F8) U7=(6DBED) U8=(670F5)
The procedure above is verified to work without any problems.

To replace these ROMS with ordinary 2764 EPROMs you will need to make a socket adapter with the following pin connection changes.
This is very easy if you use a wire-wrap socket having long pins.
   EPROM-Vpp-pin1 connects to +5V
   EPROM-PGM-pin27 connects to +5V
   EPROM-VCC-pin28 connects to +5V
   EPROM-/CE-pin20 connects to GND
   EPROM-/OE-pin22 connects to ROM-/CE-pin20
   EPROM-A11-pin23 connects to ROM-A11-pin18
   EPROM-A12-pin2 connects to ROM-A12-pin21
To replace these ROMS with ordinary 2564 EPROMs you will need to make a socket adapter with the following pin connection changes.
This is very easy if you use a wire-wrap socket having long pins.
   EPROM-Vpp-pin1 connects to +5V
   EPROM-/CS1-pin2 connects to GND
   EPROM-/CS2-pin27 connects to GND
   EPROM-VCC-pin28 connects to +5V

      2764                                       2564
   +----o----+             MK36xxx            +----o----+
 1 |Vpp   Vcc| 28          2364           Vpp | 1     28| Vcc
 2 |A12  /pgm| 27       +----o----+      /CS1 | 2     27| /CS2
 3 |A7     nc| 26    A7 | 1     24| Vcc    A7 | 3     26| Vcc
 4 |A6     A8| 25    A6 | 2     23| A8     A6 | 4     25| A8
 5 |A5     A9| 24    A5 | 3     22| A9     A5 | 5     24| A9
 6 |A4    A11| 23    A4 | 4     21| A12    A4 | 6     23| A12
 7 |A3    /OE| 22    A3 | 5     20| /CS    A3 | 7     22| PD/PGM
 8 |A2    A10| 21    A2 | 6     19| A10    A2 | 8     21| A10
 9 |A1    /CE| 20    A1 | 7     18| A11    A1 | 9     20| A11
10 |A0     D7| 19    A0 | 8     17| D7     A0 |10     19| D7
11 |D0     D6| 18    D0 | 9     16| D6     D0 |11     18| D6
12 |D1     D5| 17    D1 |10     15| D5     D1 |12     17| D5
13 |D2     D4| 16    D2 |11     14| D4     D2 |13     16| D4
14 |Vss    D3| 15   Vss |12     13| D3    Vss |14     15| D3
   +---------+          +---------+           +---------+
Your EPROMS should have the following checksums: U5=(C9CF5) U7=(CD212) U8=(D4CE2).
The procedure above is verified to work without any problems but is a bit messy and time consuming.

The data from the three original ROMS can probably also be packed into a single 27256 EPROM and still be used on an old revision A4 board, but a socket adapter must be made, and a couple of address lines from the chip select logic must be tied to the EPROM.

ROM IMAGE FOR THE INTEL 8048 MCU

The 8048 Microcontroller on PCB A30 in the meter is a mask ROM part which can be read on many EPROM programmers as an Intel 8748 MCU.
You can then burn the image into an Intel C8748 EPROM MCU.
The correct HP part number and checksum is: U13=1820-2539(1C936).
The procedure above is verified to work without any problems.

Safety, accuracy and completeness of information provided herein is not guaranteed,
so be inspired by it but do not use it as a basis for experimentation or other actions.

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