Genrad Megahertz Digibridge

Unofficial supplement to service manual.

Fix your Digibridge or back up your ROMs so you can keep it running forever! ROMs have often been known to fail after a few decades. Note that the sensing probe contains active circuitry. Feel free to share this info or to post it on your own web site.

1687 Megahertz Digibridge ROM information - Older revision hardware

      1687 Megahertz Digibridge Mask ROM information:
The 3 ROM sockets in the 1687 Digibridge are wired in parallel,
so it makes no difference which ROM you put into which socket.
You can tell the ROMs apart by the part numbers stamped on top.
These MK28XXX Mask ROMs have a 2Kx8 data format. The pinout is:
   A7 |1      24| Vcc
   A6 |2      23| A8
   A5 |3      22| A9
   A4 |4      21| CS3 (or /CS3)
   A3 |5      20| CS1 (or /CS1)
   A2 |6      19| A10
   A1 |7      18| CS2 (or /CS2)
   A0 |8      17| D7
   D0 |9      16| D6
   D1 |10     15| D5
   D2 |11     14| D4
  GND |12     13| D3
The logic sense of CS is a mask option and it is configured
differently for each of the three Mask ROMs as shown here:
   |   ROM   | CS3 | CS2 | CS1 |
   | C28145M    L     L     H  |
   | C28186M    H     L     H  |
   | C28187M    H     H     H  |
The ROMS can easily be read on an EPROM programmer as a 2716
if you jumper the ROM chip-selects to VCC and GND as per the
Chip Select table shown above. The correct checksums are:
   | C28186M = 35DC9 |
   | C28187M = 37FB0 |
   | C28145M = 31968 |
There might be other revisions of these ROMs out there,
but so far these are the only valid checksums I have seen.

Use a Hex Editor utility to stuff the data from the three
2Kx8 Mask ROMS into a single 2764 EPROM as follows:
   | C28145M -> 0000-07FF |   You should fill the unused
   | FF fill -> 0800-0FFF |   region 0800-0FFF with FF.
   | C28186M -> 1000-17FF |   The checksum for this new
   | C28187M -> 1800-1FFF |   all-in-one EPROM is 11EEE1.
Finally hook up the 2764 up via a socket adapter as shown
below. Notice that most of the pins correspond one to one:
   Socket         2764          Socket
    Pin#          EPROM          Pin#
    +--+       +----o----+       +--+
    |24|------1|Vpp   Vcc|28-----|24|----+
    |21|------2|A12  /pgm|27-----|24|    |
    | 1|------3|A7     nc|26-----|24|    \ 1K ohm
    | 2|------4|A6     A8|25-----|23|    / resistor
    | 3|------5|A5     A9|24-----|22|    \             Socket
    | 4|------6|A4    A11|23-----|18|    |    D         Pin#
    | 5|------7|A3    /OE|22-------------+-----|| G     +--+
    | 6|------8|A2    A10|21-----|19|         ->|-------|22|
    | 7|------9|A1    /CS|20-----|12|----------||       +--+
    | 8|-----10|A0     D7|19-----|17|         S
    | 9|-----11|D0     D6|18-----|16|
    |10|-----12|D1     D5|17-----|15|       VN10KM FET
    |11|-----13|D2     D4|16-----|14|       transistor
    |12|-----14|Vss    D3|15-----|13|
    +--+       +---------+       +--+
The VN10KM FET and 1K resistor inverts the OE logic state.
The above procedure has been tested and worked perfectly.

A 2716 EPROM has the same pinout except for the chip selects,
so you need to build a socket adapter and some extra CS logic:
   Socket         2716          Socket
    Pin#          EPROM          Pin#
    +--+       +----o----+       +--+
    | 1|-----A7|1      24|Vcc----|24|
    | 2|-----A6|2      23|A8-----|23|
    | 3|-----A5|3      22|A9-----|22|
    | 4|-----A4|4      21|Vpp----|24|
    | 5|-----A3|5      20|/OE----| from Decoder chip below
    | 6|-----A2|6      19|A10----|19|
    | 7|-----A1|7      18|/CS----|12|
    | 8|-----A0|8      17|D7-----|17|
    | 9|-----D0|9      16|D6-----|16|
    |10|-----D1|10     15|D5-----|15|
    |11|-----D2|11     14|D4-----|14|
    |12|----Vss|12     13|D3-----|13|
    +--+       +---------+       +--+
You can make up the address decoder CS logic from a 74LS138:
   Socket        74LS138        Socket
    Pin#         Decoder         Pin#
    +--+       +----o----+       +--+
    |18|------1|A     Vcc|16-----|24|
    |21|------2|B     /Y0|15-----| to pin-18 of ROM C28145M
    |12|------3|C     /Y1|14     |  |
    |12|------4|/G2A  /Y2|13-----| to pin-18 of ROM C28186M
    |12|------5|/G2B  /Y3|12-----| to pin-18 of ROM C28187M
    |22|------6|G1    /Y4|11     |  |
    |  |      7|/Y7   /Y5|10     |  |
    |12|------8|GND   /Y6|9      |  |
    +--+       +---------+       +--+
The above procedure has not been tested but it should work.

1687B Megahertz Digibridge EPROM Information - Newer revision hardware

Send me ROM information for this one if you have it. I think it uses an EPROM.

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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