<div dir="ltr"><div dir="ltr">Thanks, Hugh! I might need to get myself the C128DCR finally to be able to proceed with any mounting plans.<div><br></div><div>BTW I found an old bit of information on sid noise-reduction by utilizing 74LS245 buffers between the buses and the SID. So somebody else also had the idea of buffering the address/data lines for the SID. Let's see whether I need to do that for my mod - although I wouldn't do it for noise-reduction but for reliable operation. My noise-reduction technique has always been to short the SID-input to ground by using a shorting plug at the audio input connector. Especially with the later 8580 SID chip there is very little background noise with this trick, it's really not a problem, not at least with a single SID chip. Of course 4 SIDs is four times the noise, but I'm not terribly worried about this.</div><div><br></div><div>Some people seem to have a serious problem with the SID background noise. Perhaps some C64s are noisier or they have connected their C64 in some less-than-optimal way or they have poor equipment, dunno... I've read about so many noise-reduction solutions while all I've ever needed is that input-grounding trick.<br><div><br></div><div>Cheers,</div><div>Antti</div></div></div></div><br><div class="gmail_quote gmail_quote_container"><div dir="ltr" class="gmail_attr">On Wed, Jul 30, 2025 at 3:21 AM Hugh Blemings <<a href="mailto:hugh@blemings.org">hugh@blemings.org</a>> wrote:<br></div><blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex"><u></u>
<div>
<p>Hi Antti, All,</p>
<p>I'll offer some thoughts on this, but with the important caveats
that: </p>
<p>a) I'm working from first principles only - I've not done such
mods myself<br>
b) it's been a while since I've done hardware - but when I did it
as a younger person it was very much in the C64/C128 era... :) <br>
c) there are -many- more experienced folk than I on this list
who's advice I'd heed ahead of my own :)</p>
<p>That said then...</p>
<ul>
<li>Yes, as you allude to, I'd be trying to keep the interposer
cable from the socket to your board as short as reasonably
possible - more like 10-15cm max if you can</li>
<li>Using a 40 way IDE cable seems fair enough, but probably only
gains you the advantage of being able to buy them pre-made
and/or finding 40w IDC crimp plugs/sockets will be easy</li>
<ul>
<li>Extra wires on the 40W could allow you to put some GND
signals between active signals or at least keep the analog
signals a bit further from data ones.</li>
</ul>
<li>A small PCB that slots into the SID socket and is a breakout
to the 40W connector - most of it would be to the right and
forward of the SID chip socket (as viewed in the photo) to clear
the PSU</li>
<li>It looks like there's room behind the front panel - either
flat/horizontally above the PCB or vertically behind the panel -
perhaps put the board there instead - shorter cable runs. </li>
<ul>
<li>With some care I suspect you could use double sided adhesive
PCB mounts to stick to either the PCB or behind the front
panel. Probably want a couple cm of clearance for airflow for
chips underneat</li>
<li>You could then lay out the board so it was landscape
orientation - SID chips in a row so the signals that are
duplicated can just flow from one to the next ? Think ala
EPROM layouts that kind of thing ?</li>
<li>I found this photo and associated discussion useful
<a href="https://retrorepairsandrefurbs.com/2021/05/26/commodore-c128dcr-restoration/#jp-carousel-657" target="_blank">https://retrorepairsandrefurbs.com/2021/05/26/commodore-c128dcr-restoration/#jp-carousel-657</a></li>
</ul>
<li>I don't know if it would be prudent to add some buffering to
the Clock, R/W, Address and Data lines off the main PCB given
you're adding three more devices. I may be overthinking it
(shock!) but be worth looking at the fan out on the CPU etc. for
those signals</li>
<ul>
<li>If you did decide to buffer, perhaps this could be done with
SMT parts on the interposer PCB that goes on the socket.
Would have the helpful side effect of reducing the impact of
the cable length</li>
</ul>
<li>On a quick bit of research I note there are a few 4 and 8
channel SID based designs over the years, I wonder if anything
can be learnt from them layout wise ?</li>
</ul>
<p>Hope that helps and/or inspires others more experienced than I to
comment :)</p>
<p>Cheers,<br>
Hugh</p>
<p><br>
</p>
<p><br>
</p>
<div>On 29/7/2025 21:40, Antti Pitkämäki via
Synth-diy wrote:<br>
</div>
<blockquote type="cite">
<div dir="ltr">
<div dir="ltr">
<div>Hi,</div>
<div><br>
</div>
<div>I have another question regarding my project of adding 3
additional SID sound chips to a Commodore 128DCR computer.
It concerns mounting the extra SID board inside the
computer.</div>
<div><br>
</div>
<div>The basic idea for the mod is to connect an extra circuit
board to the original SID chip socket, which provides most
of the required connections. The SID only "listens" to the
five least significant address lines. Hence, in a stock C64
separate glue logic is utilized to provide the chip select
signal for the SID when memory locations allocated to it are
accessed. In my mod I'll use a 74LS138 decoder, enabled by
the original chip select signal. Two additional address
lines will be connected to its inputs and its outputs will
be connected to the SIDs' chip selects, placing each SID in
its own dedicated memory space. With this system it would be
very easy to add up to 8 SIDs if I wanted to go totally
crazy. But four SIDs for now :)</div>
<div><br>
</div>
<div>Anyways, it would be awesome if I could mount the extra
SID board to the back wall of the C128DCR's case and connect
the board to the SID socket from there. However, I've been
advised to mount the board either directly on the SID socket
or to use as short cables as possible. This is due to
potential "parasitic effects" caused by long cables. If I
mounted the extra board to the back of the case, I would
need to use cables of considerable length to connect it to
the SID socket - I don't have a C128DCR yet, so I don't know
the exact length, but perhaps 30-40cm.</div>
<div><br>
</div>
<div>What do you think, am I asking for trouble with my
mounting approach? How would you mount the extra SID board?
Somebody recommended that I should use an IDE-cable for
connecting to mitigate any problems with my approach, but
I'm not sure if that is enough.</div>
<div><br>
</div>
<div>It would be a bit inconvenient to mount the extra board
directly to the SID socket. The power supply is right next
to it, and its frame could be used to install some
L-brackets for mounting, but it seems a bit fiddly and I'd
like to avoid it if it is possible. Unfortunately my vintage
computer modding skills are limited, hence I'm asking for
guidance...</div>
<div><br>
</div>
<div>Below is a picture of an opened C128DCR for reference. It
has the original PSU. The copyright holder of the image
forbids altering the picture, so I can't add any markings,
but the SID is the chip to the right of the power supply
frame near the lower right corner of the PSU, under the
bundle of cables. My planned mounting spot for the extra SID
board would be on the back wall, above the module (?) port
on the right.</div>
<div><br>
</div>
<div>I admit that my question concerns less synthesizer DIY
and more vintage computer hacking, but I wouldn't be
surprised if I got valuable information here even on this
topic :)</div>
<div><br>
</div>
<div>Regards,</div>
<div>Antti</div>
<div><br>
</div>
<div><img src="cid:ii_1985ccff51e67787deb1" alt="Commodore-128D-006-Motherboard.jpg" width="452" height="339"><br>
</div>
<div><br>
</div>
</div>
</div>
<br>
<fieldset></fieldset>
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