Date   : Sun, 02 Nov 2008 17:09:44 +0000
From   : pete@... (Pete Turnbull)
Subject: Econet on BBC issue 3 , having problems

On 02/11/2008 14:26, Andy Ford wrote:
> Pete Turnbull wrote:
>> If the 8271 isn't fitted, has link S9 been re-made?  It controls the NMI 
>> from the disk controller.

> S2 is already open (I would guess this was done when the disk interface 
> was originally fitted)

So the NMI from the Econet circuitry is enabled.

> To recap: After making S5 south the No Clock message disappeared, but I 
> was unable to get a response. I then fitted an 8271 and the machine 
> (econet-wise) functioned perfectly :)

Leave it that way, then :-)

> I am a bit unsure about S9, it is currently open so I re-connected it as 
> advised, but this resulted in a repeat of the problem.

S9 controls the NMI from the disk interface in the same way that S2 
controls the NMI from the Econet interface.  So if the disk interface is 
fitted, or more exactly if the 8271 and IC27 (a 7438) are fitted, then 
S9 should be open.  But see below...

> Interestingly, someone has already connected a small wire from S9 (the 
> right / east-most) pad) to IC27 (a 14 pin sn7438 device) pin 9. I am not 
> sure if this is now causing the problem and whether I should remove this 
> "extra" wire that someone had previously fitted (I do not know the 
> reason for this extra wire)

No, don't remove it.  There's a track error on Issue 1, 2, and 3 circuit 
boards, which means that the connection from IC27-9 is wrong; the fix is 
to disconnect it from the circuit trace and wire it to S9 East.  One way 
(in the original service manual) is to cut the IC leg right at the 
board, bend it up, and solder a wire between the stub of pin 9 and S9-E. 
  Another way is to cut the trace to pin 9 on the component side of the 
board, and solder a wire from the pin to S9-E -- the cut is usually made 
about a centimeter east of IC27, under the socket for the 68B54.

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.

-- 
Pete                                           Peter Turnbull
                                               Network Manager
                                               University of York