Date   : Sat, 11 Feb 2006 23:50:15 -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.

Nope, spoke to soon. Both 6522s have the short to Vcc of course, as
they're both connected to the same line.

Problem is, assuming that the short shouldn't be there, which chip is
providing the short? Is it the case that only the two VIAs and the CPU
are connected to the /IRQ line or are there other potential culprits?
I know it's not the CPU as it meters fine when out of its socket.

All this assumes that /IRQ is active low, so would normally be pulled
high internally in the two VIAs by at least 1k to Vcc. So a dead short
can't be right can it? Would holding the /IRQ line high by connecting
it to 5v put keyboard access on permanent hold?

Hope I don't have to swap out both VIAs ;-(

-Neil.

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