Date   : Sat, 11 Feb 2006 22:52:49 -0000
From   : "neil f" <faz@...>
Subject: Re: B+ 64k won't respond to keyboard, Econet fitted

 

> -----Original Message-----
> From: Majordomo List Manager [mailto:majordomo@...] 
> On Behalf Of David Harper
> Sent: Saturday, February 11, 2006 10:01 PM
> To: bbc-micro@...
> Subject: Re: [BBC-Micro] Re: B+ 64k won't respond to 
> keyboard, Econet fitted
> 
> "neil f" <faz@...> wrote:
> 
> >> Therefore the system 6522 is functioning.  Do you have sound
> >> - does Ctrl-Break give a beep?  If yes, does Ctrl-G give a beep?
> >
> > Ctrl/Break doesn't beep. Ctrl/G doesn't beep. The machine beeps at

> > switch-on but only the lower tone, not the higher of the two tones

> > (this is with no secondary ROMs installed, to ensure Econet 
> is not a 
> > factor). Interestingly, after it has been on for about an 
> hour or so, 
> > the machine beeps once. Does it every time it's left on.
> 
> It all points to what I suggested earlier, a failure of the 
> system 6522 VIA to cause interrupts in the CPU. (The 6522 
> generates the 100-per-second timer
> interrupts.) If this failed then the beeps would not work, 
> and keypresses will not get entered into the keyboard buffer. 
> (The first part of the two-tone switch-on sound is just the 
> default noise issued by the sound chip before the reset 
> sequence sets up the chip properly. The second part is a 
> "proper" beep, the same as VDU7, under timer interrupt control.)
> 
> A dry joint or a broken motherboard track (perhaps affecting 
> pin 21 of the
> 6522 - the IRQ line) looks the most likely cause to me. If 
> you can find the break you could try bridging it.
> 
> David Harper 

Just checked continuity between the keyboard connector pins and the
System VIA (IC20) and everything seems to have the required continuity
pin to pin, so no broken tracks. However, when checking the IRQ pin
(pin 21), it appears to be a dead short to the 5v VCC line (pin 20).
Can this be right? There's no such reading on the neighbouring 6522
(IC 10). Does that mean the System 6522 is kaput?

-Neil.

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