Date   : Sun, 12 Feb 2006 01:01:44 -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.

Woohooo! Fixed it.

I took a chance and snipped pin 21 on the System VIA. The stub was no
longer shorting to Vcc, but pin 21 on the User VIA was still shorted
by a few ohms. So I snipped pin 21 on the Userport and the short on
the CPU /IRQ line disappeared. I solder linked the System VIA's pin 21
back up again and powered up. Bingo. Double beep and keyboard action
is back to normal. 'Course I won't have any Userport or Printer port
working until I snip out and replace the User VIA, but that's a job
for tomorrow.

I'm off to bed now - thanks to everyone for herding me into the
correct solution.

Cheers,
-Neil F.

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