[BBC-Micro] Econet on BBC issue 3 , having problems
acorn at acford.co.uk
Sun Nov 2 20:14:34 GMT 2008
Pete Turnbull wrote:
> Pins 9 and 10 on IC27 are the two inputs to an open-collector 2-input
> NAND gate, and the output, pin 8, is connected to the NMI line (as is
> pin 3, which is the NMI signal from the Econet). Pin 10 is connected to
> +5V, so it's always in a '1' state, and therefore the output is
> controlled solely by whatever is on pin 9. Making the link S9 should
> ground that pin, thus ensuring the NAND gate is never on, which is how
> you want it when there's no 8271 (or 1770) fitted. Pin 9 is also
> connected to the interrupt output of the 8271 (so don't have the link
> fitted if the 8271 is present or you'll short-circuit it's output) --
> except on Issue 3 boards, the track on the board is wrong.
> One Issue 2 and 3 boards, the West side of S9 is connected to ground (on
> the underside of the board) as it should be. The East side of S9 is
> connected to the INT output of the 8271 (pin 11) as it should be. The
> problem is that the track (on the component side of the PCB) from IC27-9
> goes to the West side of S9 instead of the East side.
> If you want to remove the 8271 to use in another machine, you'll need to
> short pin 9 of IC27 to ground in some way, otherwise the NMI line will
> not work correctly.
All sorted now. I connected a small lead (solder side but using the
already present wire link) from pin 9 of IC27 to 0v. I used the very
nearby 0v terminal.
All tested and it seems fine. :)
Many thanks for your time in helping me solve this, appreciated :)
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