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

Using 32K and 64K ROMs and SRAMs in the BBC

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.

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. 

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

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

Instructions

Using a 28-pin IC socket, carefully bend pin 27 to the side and solder a 15cm long 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 long length of wire to it. Connect IC clips on the other end. Alternatively, the other ends can be soldered, as described later.

You will now have an IC socket with wires connected like this: 
                       +----__----+
   64K ROM or  --------+ 1     28 +
  32K SRAM only        + 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 11 or 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 11 or pin 12.

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

    ROM Sockets

    There are four ROM sockets on the BBC motherboard. Depending which socket you plug the carrier in and whcih 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. 
       ROM      32K ROM or RAM                     64K ROM
  Socket   Flying lead to IC76:          Flying leads to IC76:
              p12      p11             p11/p12          p12/p11
  IC101      15  11   15  5          15 11  7  3      15  7 11  3
  IC100      14  10   14  6          14 10  6  2      14  6 10  2
  IC88       13   9   13  7          13  9  5  1      13  5  9  1
  IC52       12   8   12  8          12  8  4  0      12  4  8  0

    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