BBC Microcomputer Service Manual Oct 1985 Section 1 BBC Micro ...

19. WEST forces slow access to ROMs IC52, IC88 and IC101.
EAST allows fast access to ROMs IC52, IC88 and IC101.
- Diodes D10, D11 and D12 may be selectively added to slow down ROMs
IC101, IC88 and IC52 respectively when link 19 is in WEST position, but
for any ICs to have slow access, R55 must be added.
D10, 11 & 12 and R55 are not fitted from issue 7 onwards.
20. SOUTH connects high ROM select bit to IC20 decoder from A 13.
NORTH connects high ROM select bit to IC20 decoder from ROMSEL 1.
21. 2 x NORTH/SOUTH selects blocks 8 to B in IC51 and blocks C to F in
ICs 52, 88, 100, and 101. (4 EPROMs for OS)
2 x EAST/WEST selects blocks C to F in IC51 and blocks 8 to B in ICs
52, 88, 100 and 101. (OS in IC51)
22. SOUTH connects low ROM select bit to IC20 decoder from A 12 .
NORTH connects low ROM select bit to IC20 decoder from ROMSEL 0.
23. OPEN RS 423 receiver not terminated (DATA).
CLOSED RS 423 receiver terminated (DATA).
24. OPEN RS 423 receiver not terminated (CTS).
CLOSED RS 423 receiver terminated (CTS).
25. SOUTH selects CAS 1 only, for 16K RAM configuration.
NORTH selects CAS 0 and 1 for 32K RAM configuration.
- If removed altogether, this selects CAS 0 only, but this should only
be used for testing purposes on a Model B.
26. WEST selects normal video output.
EAST selects inverted video output.
27. WEST selects 8 MHz clock for 5 1/4" disc.
EAST selects 16 MHz clock for 8" disc.
28. WEST selects base baud rate. (1200 baud)
EAST selects 1300 baud cassette rate.
- If link 28 EAST position RS 423 baud rate is also changed by the
same factor:- 13/12.
29. EAST selects base baud rate. (1200 baud)
WEST selects 1300 baud cassette rate.
- If link 28 is in the WEST position, RS 423 baud rates are also
affected.
30o increase the flexibility used for the addition of extra sideways
ROM sockets. This would be in connecton with other links (S20,21,22)
to enable a total of 16 sideways ROMS to be selected.
31. WEST selects +ve CSYNC to RGB video output.
EAST selects -ve CSYNC to RGB video output.
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