Date   : Sun, 02 Nov 2008 20:14:34 +0000
From   : acorn@... (Andy Ford)
Subject: Econet on BBC issue 3 , having problems

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 :)


Andy.