BBC model A or B

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BBC micro
Card - early
Card - later

Misc h/w
Misc info


The BBC micro has a fiddly Econet interface. It is supplied as a collection of components which are supposed to be soldered onto the main board.
This information is given in good faith. Liability for damages is hereby disclaimed.


BBC micro - American version

The American version of the BBC micro is slightly different inside. This means that while the requirements and components are largely the same, the component numbers and placing differ (quite dramatically).
I have, somewhere, a diagram showing how to upgrade a US model B. If you require the addition of Econet to this rarity, please get in touch!


What you'll need

To upgrade a BBC model B (and model A?), you will need:
  1 x 2764 EPROM with NFS, DNFS, ANFS or 'some other kind
                      of FS with network capabilities'.

  5 x 14-pin DIL IC sockets

  1 x 20-pin DIL IC sockets

  1 x 28-pin DIL IC sockets

  1 x 74LS163                 for IC76
                (this is already fitted on model B)

  1 x 74LS123                 for IC87
                (not required if disc is fitted)

  1 x 68B54                   for IC89

  1 x 74LS132                 for IC91

  1 x 75159                   for IC93

  2 x LM319                   for ICs 94, 95

  1 x 74LS244 (or 74LS240?)   for IC96

  1 x 74LS74                  for IC97

  1 x 10F Tantalum Capacitor for C18

  1 x 10F Ceramic Capacitor  for C23

  1 x 2.2nF capacitor         for C17
                (only required on issue 1, 2, 3 PCBs)

  1 x 5-pin 180 DIN socket   for SK7

  1 x 8x22K SIL resistor pack for RP2

  2 x Rows of 8 MOLEX pins    for S11

 20 x 2% tolerance ¼W resistors as follows:-

      R46 is 1K     R48 is 1K     R52 is 1K     R61 is 1K

      R47 is 1K5

      R34 is 10K    R35 is 10K    R45 is 10K    R51 is 10K

      R59 is 56K    R60 is 56K    R62 is 56K    R63 is 56K

      R38 is 100K   R39 is 100K   R40 is 100K   R41 is 100K

      R36 is 1M5    R44 is 1M5    R64 is 1M5


Performing the upgrade

You'll need the single ROM OS v1.2. If you don't have this, please upgrade from earlier MOS versions first.

This upgrade can operate in tandem with a 5¼" floppy disc drive, in which case you will require a DNFS EPROM to provide disc and network filing systems.
Alternatively your computer may work as a discless workstation using only the Econet for filing (nobody considers tape to be useful these days, it is way too slow), in which case you will require an NFS EPROM.
If you have an EPROM programmer, or know a geek with a programmer, you can find the latest known versions of (D)NFS on the Econet Area index.


This information from the BBC Micro Service Manual:

Solder all of the above passive components onto the main PCB.

Solder IC sockets into each IC position.

Insert all of the above ICs into their sockets.

Cut the wire links at link positions S2 (Econet NMI), S12 and S13 (S12 and S13 should already have been cut on the model B - they are ROM selects).

Set/ensure the following link positions using MOLEX jumpers:

  S18 - North [IC100 fast access]
  S19 - East  [IC52/88/101 fast access]
  S20 - North [high ROM select is ROMSEL1]
  S21 - 2 x East/West [MOS is one ROM, not 4 EPROMs]
  S22 - North [low ROM select is ROMSEL0]
  S32 - West  [A13 to pin 26 of ROMs IC52/IC88]
  S33 - West  [A13 to pin 26 of ROMs IC100/IC101]
On issue 1, 2 or 3 boards only, the following modifications are required:
  • Remove Capacitor C17 and a replace it with the 2.2nF capacitor.
  • Cut the PCB track from IC26 pin 6 to IC96 pins 1 and 19 leaving the track from IC26 pin 6 to IC97 pin 2 intact.
  • Cut the track from IC89 pin 26 to IC97 pin 4.
  • Link IC26 pin 9 to IC96 pins 1 and 19 and also to IC97 pin 4.

As you can see, it's a doddle that can be performed with a metal spatula and an iron set to "Cotton". ☺ Just to make life more fun, it appears as if those line driver ICs have been discontinued; so you'll need to try to track down an Econet upgrade kit.



From the Advanced User Guide:

BBC micro econet upgrade schematic

Copyright © 2008 Rick Murray