[sdiy] Another SID-related question

Hugh Blemings hugh at blemings.org
Wed Jul 30 02:21:08 CEST 2025 

Hi Antti, All,

I'll offer some thoughts on this, but with the important caveats that:

a) I'm working from first principles only - I've not done such mods myself
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... :)
c) there are -many- more experienced folk than I on this list who's 
advice I'd heed ahead of my own :)

That said then...

  * 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
  * 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
      o 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.
  * 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
  * 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.
      o 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
      o 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 ?
      o I found this photo and associated discussion useful
        https://retrorepairsandrefurbs.com/2021/05/26/commodore-c128dcr-restoration/#jp-carousel-657
  * 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
      o 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
  * 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 ?

Hope that helps and/or inspires others more experienced than I to comment :)

Cheers,
Hugh



On 29/7/2025 21:40, Antti Pitkämäki via Synth-diy wrote:
> Hi,
>
> 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.
>
> 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 :)
>
> 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.
>
> 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.
>
> 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...
>
> 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.
>
> 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 :)
>
> Regards,
> Antti
>
> Commodore-128D-006-Motherboard.jpg
>
>
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