[sdiy] Another SID-related question

[Ben Stuyts]

Hi Antti,

It’s hard to tell how much influence a long ribbon cable would have. If I remember correctly, their capacitance is around 50-100 pF per meter, between adjacent wires. (That is why it is a good idea to have a ground wire between each data wire, as Hugh mentioned.)

I found the 6510 datasheet, and it specifies max 15 pF for the data lines, and 12 pF for others. So even with just the cable you would be over the spec. So buffers directly on a piggy-back board would definitely be needed.

However, chances are good that it will work in some cases, and not in other cases. If it’s a private project, who cares. If you want to sell it, or put it out there for people to build themselves, I would not do it.

Do you have a scope? It would come in handy for checking rise/fall times and over/undershoot of all these signals.

Ben

> On 29 Jul 2025, at 13:40, Antti Pitkämäki via Synth-diy <synth-diy at synth-diy.org> 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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