Date   : Sun, 22 Mar 1998 13:55:19 GMT
From   : Theo Markettos <theomarkettos@...>
Subject: Re: AMX Pagemaker Plus

> In message <19980321.200117.17@...>, Theo Markettos
> <theomarkettos@...> writes
>
> 
> >Probably :-)  Think about it - if the chip was getting rubbish, why
> >would produce the same sound every time?  It would be equally likely to
> >go berrrr, beeeee, or even shhhhh (don't you just love email sound
> >effects...) 
> 
> We could test that by removing the system VIA and powering up.  I'm
> positive that will result in a continuous tone, sounding identical to
> the power-up brrrrrrrrr.
> 
> Therefore, with the sys VIA missing, there can be no rubbish on the slow
> data bus...  so the tone must be caused by something else.

It would seem likely that the pullups are within the VIA - I can't find my
6522 datasheet ATM, but it would seem sensible that some pullups were put on
lines used for input.

You could try pulling out the OS Rom - so nothing initialises the VIA, and
measure the voltages on the slow data bus.  I would try it, but my Beeb has
recently killed it's third PSU...

> >Since the VIA powers up with ports as
> >input, 
>
> Ha.  I didn't know that.  Thanks.
> 
> >it would seem likely that either inside the keyboard, 76489 or the
> >6522 there are some pull-ups, which force the data bus all high on a reset.
> 
> Hm.  I have a keyboard cct diagram for the Beeb - it doesn't show any
> pullups on the slow data bus.  There are 8 resistors used to pull up the
> scan lines on the keyboard matrix (on the diagram, they're 2k2, on my
> keyboard they are 10k.)

They could also be within the devices on the keyboard - this is probably less
likely because the data bus isn't routed all to the same device.
                       
> Bear in mind I've never programmed the sound system in the Beeb, so I don't
> know for instance, if the 76489 just interprets what's on the slow data bus
> and outputs that, or whether it is more sophisticated.

According to the AUG, you have to send it bytes in a particular format. 
There doesn't appear to be a duration field, so presumably you set it going
and it never stops unless you tell it to.  You can send it 'commands' using
the top nibble:

%1000ffff:     Tone 3 frequency (low 4 bits of frequency)
%1001aaaa:     Tone 3 volume    (0 = max, 15 = off)
%1010ffff:     Tone 2 frequency
%1011aaaa:     Tone 2 volume
%1100ffff:     Tone 1 frequency
%1101aaaa:     Tone 1 volume
%1110xpff:     Noise control (p 0=periodic noise, 1=white noise)
                             (freq:0=low, 1=medium, 2=high, 3=tone 1 freq)
%1111xxxx:     Noise volume
%0xffffff      Frequency second byte (high 6 bits of frequency)

So sending %11111111 would set the noise volume to off, which wouldn't do
much.

-- 
Theo Markettos         Home:   theomarkettos@...          
Liphook                        Work:   marketto@...                     
Hampshire
UK                     Web site, including Acorn backup software
                       http://www.marketto.demon.co.uk/