Adaptor to enable 32-pin EPROM to be used in BBC Master

This page describes how to build an adaptor, so that the 1 megabit ROM may be replaced with an EPROM device. This enables a fix to be made for the Y2000 problems with the BBC Master computer, or to alter the set of ROM images.

The 1 megabit ROM used in the BBC Master computer is a 28-pin device, but 128K × 8 EPROMs (e.g. 27C010) are 32-pin devices and so can't be plugged directly into the BBC Master:

1 megabit ROM27C010 EPROM
[Pin-out of 1 megabit ROM] [Pin-out of 27C010 EPROM]

An adaptor is needed, let's make one.

Wiring Diagram

Thanks to Jonathan Graham Harston, there's now a wiring diagram for the adaptor. This shows the 32 pin EPROM in the middle, with the two halves of the 28 pin ROM socket either side:
28 to 32 pin adaptor wiring diagram

Requirements

The 28-pin Turned Pin IC socket will plug in to the existing socket in the BBC Master Computer. A Turned Pin socket is chosen to cause little deformation of the contacts in the original socket.
The 32-pin wirewrap socket provides physical support to the adaptor.

Construction

  1. Align the 32-pin socket to that the lower 28 pins align with the buckets on the top faces of the 28-pin socket.
  2. Solder all the left hand wirewrap pins into the corresponding buckets on the 28-pin socket (i.e. pins 3 to 16 on the 32 pin socket are connected to pins 1 to 14 on the 28-pin socket).
  3. Cut pins 1,2,24,30,31,and 32 of the 32-pin socket to about half length
  4. Solder all the full length pins on the right hand side of the wirewrap socket to the corresponding buckets on the 28 pin socket (i.e. pins 17 to 23 on the 32-pin socket are connected to pins 15 to 21 on the 28-pin socket, and pins 25 to 29 on the 32-pin socket are connected to pins 23 to 27 on the 28-pin socket).
  5. Connect pin 2 of the 32-pin socket to pin 22 of the 28-pin socket
  6. Connect pin 32 of the 32-pin socket to pin 28 of the 28-pin socket
  7. Connect pins 1 and 24 of the 32-pin socket to pin 16 of the 32-pin socket
  8. Connect pin 31 of the 32-pin socket to pin 32 of the 32-pin socket
Now remove the MOS ROM from the socket in the BBC Master Computer, and plug the adaptor in its place. An appropriately programmed EPROM, when fitted into the 32-pin socket, will provide the Operating System firmware for the computer.

A guide to the order of the ROM images in the 1 megabit ROM, and how to capture the images whilst the 1 megabit ROM is still in the BBC Master, forms part of my BBC Master 128 and Y2000 page.

Information supplied by
Andrew Benham (G8FSL), Southgate, London N14 4XD, United Kingdom