Date   : Sun, 12 Feb 2006 01:20:01 -0000
From   : "Mike" <profpep@...>
Subject: Re: B+ 64k won't respond to keyboard, Econet fitted

>> > > >> 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 ;-(
>

Just done the check on an old ACW motherboard I have here. Pin21 on the
VIA's gives 3K to the 5 Volt rail.

I can get some pins of the right hand one connected to IC5, which is the
printer output buffer chip, so my money is on the left hand one of the pair,
IC20 being the system VIA. The diagram I've got is very poor quality, and I
can't read the IC numbers!

I haven't got a MOS for the ACW board, or I'd run it up. The only B+ I've
got has (guess), soldered in chips or I'd have tried pulling the printer VIA
here.Uou could simply try cutting the pin with a view to resoldering if it's
the wrong one. No interrupts could be either one being faulty and pulling
the other down.

Best of luck.

If you need a couple of 6522's - just ask - they're yours for the postage.

||\/||ike

Hope this helps