32K and 64K ROMs and SRAMs - MDFS::Info.Comp.BBC.SROMs.32K

Using 32K and 64K ROMs and SRAMs

MDFS::Info.Comp.BBC.SROMs.32K

The BBC computer has four sideways ROM sockets on the motherboard. 8K or 16K ROMs can be plugged straight into them. Additionally, you can plug in 16K sideways RAM modules with a flying lead to collect the WRITE signal. With care you can also plug in 32K EEPROMs.

You can also plug in larger devices such as 32K or 64K ROMs or 32K SRAMs. You need to build a small carrier to collect the following additional signals. You can also modify the PCB to achieve the same result.

                                 16K   32K  64K  32K
                                 ROM   ROM  ROM  RAM

  +----------------------------  Vpp   Vpp  A15  A14
  |
  |        +----__----+
  +--------+ 1     28 +
           + 2     27 +--------  PGRM  A14  A14  WE
           + 3     26 +

Plug-in module

Using a 28-pin IC socket, carefully bend pin 27 to the side and solder a 15cm length of wire to it. For a 64K ROM or a 32K SRAM, also carefully bend pin 1 to the side and solder a 15cm length of wire to it. Connect IC clips on the other end. Alternatively, they can be carefully soldered directly to the IC pins.

You will now have an IC socket with wires connected like this:

+----__----+ 64K ROM or --------+ 1 28 + 32K SRAM + 2 27 +-------- + 3 26 + + 4 25 + + 5 24 + + 6 23 + + 7 22 + + 8 21 + + 9 20 + + 10 19 + + 11 18 + + 12 17 + + 13 16 + + 14 15 + +----------+

32K ROM with flying lead for A14

32K SRAM with flying leads for A14 and WE

Plug the ROM or SRAM into the IC socket and carefully plug the IC socket into a ROM socket in the BBC.
For a 32K ROM carrier, clip or solder the flying lead from pin 27 to IC76 pin 12.
For a 64K ROM carrier, clip or solder one of the flying leads to IC76 pin 11 and the other flying lead to IC76 and pin 12.
For a 32K SRAM carrier, clip or solder the flying lead from pin 27 to IC77 pin 8 and clip or solder the flying lead from pin 1 to IC76 pin 12.

PC modification

Alternatively, you can modify the BBC PCB.

Remove the PCB from the computer and carefully turn it upside down. Identify pin 1 and pin 27 of the IC socket you want to modify. Remember that one the underside of the PCB the pins are numbered in reverse.
With a sharp craft knife cut the track from pin 27 connecting it to pin 28. Normally pin 1 is not connected to anything, but if it is then carefully cut the track connecting it to the rest of the circuitry.
For a 32K ROM solder a wire from pin 27 to IC76 pin 12.
For a 64K ROM solder a wire from pin 27 to IC76 pin 12 and another wire from pin 1 to IC76 and pin 11.
For a 32K SRAM solder a wire from pin 27 to IC77 pin 8 and solder a wire from pin 1 to IC76 pin 12.

You can modify all four sockets to extend all four of them in any combination. For instance, you could wire some sockets to take 32K SRAM and some sockets to take 64K ROMs.

A socket wired for a 32K SRAM will only see 16K of a 32K ROM or 32K of a 64K ROM.
A socket wired for a 32K or 64K ROM will need a flying lead to be used with SRAM.

ROM Sockets

If you now use a *ROMS command you will find additional ROM sockets have become active:

*ROMS list showing 32K
ROM in slot A and E

There are four ROM sockets on the BBC motherboard. Depending which socket you plug the carrier in and which pin you connect the flying lead to, each half or quarter of the ROM or RAM will appear in different logical sockets, according to the following table.

             32K ROM or RAM                     32K ROM or RAM
         IC52 IC88 IC100 IC101              IC52 IC88 IC100 IC101
lead to   12   13    14    15      lead to   12   13    14    15
IC76p12    8    9    10    11      IC76p11    4    5     6     7


               64K ROM                            64K ROM
         IC52 IC88 IC100 IC101              IC52 IC88 IC100 IC101
lead to   12   13    14    15      lead to   12   13    14    15
IC76       8    9    10    11      IC76       4    5     6     7
p11/12     4    5     6     7      p12/11     8    9    10    11
           0    1     2     3                 0    1     2     3

Using 64K SRAMs

You can use a 64K SRAM to give a full four banks of sideways RAM in one socket. The main problem is that 64K SRAMs come in 32-pin packages because there aren't enough pins left on a 28-pin package, so you have to make a carrier board that overhangs the top of the BBC's 28-pin socket. You need to build a small carrier to collect the following additional signals.

         BBC ROM socket                       64K SRAM
                                             +----_----+
                                         NC -+ 1    32 + Vcc
         +----__----+                    NC  + 2    31 + A15 
     Vpp + 1     28 + Vcc                A14 + 3    30 + CE
     A12 + 2     27 + PGRM               A12 + 4    29 + WE
     A7  + 3     26 + A13                A7  + 5    28 + A13

You need to connect to pins 3, 29, 30, 31 and 32 on the 64K SRAM and feed them to the following signals:

pin 3: pick up A14 from IC76 pin 11 or pin 12 (the opposite to pin 31)
pin 29: pick up WE from IC77 pin 8
pin 30: pick up CE from IC25 pin 6
pin 31: pick up A15 from IC76 pin 12 or pin 11 (the opposite to pin 3)
pin 32: pick up Vcc from IC76 pin 16

You will now have four banks of sideways RAM in banks offset in jumps of four in the same manner as the 64K ROM described above. For example, if plugged into IC52 the four banks will be 0,4,8,12.

You can change the ordering of the banks within a device to be, for example, 4,5,6,7 by removing the ROM selection links, wiring the links to IC76 pin 11 and 12 and wiring IC76 pin 13 and 14 to the device pin 1 and pin 27.

Using the BBC B+

You can also use 32K devices with the BBC B+. The B+ already has 32K ROM sockets so, a 32K ROM will plug straight in, as long as the socket's 16K/32K link is set to 32K.

To use a 32K SRAM, the flying leads need to be connected as follows. Note that I have not yet tested this myself.

The flying lead from pin 27 connects to IC27 pin 6.
Remove the jumper from the 16K/32K selection link for this socket, and connect the flying lead from pin 1 to the east (32K) selection pin.

32K ROM plugged into
a BBC Close-up of 32K ROM
plugged into a BBC *ROMS list showing 32K
ROM in slot A and E

Authored by J.G.Harston
Last update: 12-Dec-2015