[sdiy] Another SID-related question

[Antti Pitkämäki]

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

[image: Commodore-128D-006-Motherboard.jpg]
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