Sid-20 mod for the Vic

[shem]

Ok what I wish to construct is a small pcb which will either fit inplace of the 6502 or the vic chip and host the removed ic along with 2 sids

The voltage for the sids will be selectable using a jumper as I intend to include 2 dc-dc converters one for 12 volts and one for 9 volts that way it will be possible to mix and match sids

I wish to use i/o2 and i/o3 to control the sids unless anyone can suggest a different method for audio out I wish to use the LF347 just like in the amiga's

BTW does the megacart use i/o2 and i/o3

Please feel free to make some suggestions

[6502dude]

BTW does the megacart use i/o2 and i/o3

Yes, IO2 and IO3 are both used by MegaCart.

[shem]

Damn since I/O2 and I/O3 are both used that doesnt leave me much to play with

[amramsey]

Damn since I/O2 and I/O3 are both used that doesnt leave me much to play with

Those are pretty big memory areas.... can't say that I know the implementation of the Megacart, but hopefully it does some address decoding and doesn't just use the entire space?

[shem]

ok assuming i build a prototype of the above
how do i address it what are the poke addresses and do they change depending on how much ram is attached to the vic20?

[carlsson]

The sid sits at $9800 (38912) so just change the s=54272 line to s=38912 and my programs worked right away.

There is your base address. It won't depend on memory expansion, just like the addresses to the VIC-I chip (36864..36879) don't. Of couse if you map the SID to I/O block 3 instead, the base address is $9C00 = 39936.

Then the matter is if you can set the base address with even higher precision, i.e. $9800, $9820, $9840. You (collective) know better about that.

[shem]

@ carlsson

great info you cheers for that info

thanks for those addresses

I wish to actually build a twin sid and use io2 and io3

I will use 2 dc to dc converters to generate either 9 or 12 volts and make the voltage jumper selectable so that both 6581 and 8580 can be used and mixed in any combination eg 2 x 6581 or 2 x 8580 or one off each

the filter caps will be on a socket so that they can be changed depending on the sid type

over the next few weeks I will have a go at designing the pcb i might even make it an internal module so that it plugs into the cpu or socket and leaves the expansion port free for the all important ram

[carlsson]

If you can have more precise address decoding, wouldn't it be better to have both chips in the same I/O block? Or well, I dunno. The most important thing is that everyone who make a SID cartridge etc use the same base address so us software developers have a slight chance of supporting it.

There is a Battleships game for +24K VIC-20 that supports an additional SID chip. According to its docs, the chip should be located at $9140 which suggests it would be an internal modification rather than a cartridge?

http://www.cbm.sfks.se/files/battleships.doc (PETSCII format)
http://www.cbm.sfks.se/files/battleships.bin

Those files were once found on Funet FTP but have since then disappared before it got transferred to Zimmers.

[rga24]

If you can have more precise address decoding, wouldn't it be better to have both chips in the same I/O block? Or well, I dunno. The most important thing is that everyone who make a SID cartridge etc use the same base address so us software developers have a slight chance of supporting it.

There is a Battleships game for +24K VIC-20 that supports an additional SID chip. According to its docs, the chip should be located at $9140 which suggests it would be an internal modification rather than a cartridge?

http://www.cbm.sfks.se/files/battleships.doc (PETSCII format)
http://www.cbm.sfks.se/files/battleships.bin

Those files were once found on Funet FTP but have since then disappared before it got transferred to Zimmers.

That is very interesting! Normally the 6560/1 VIC chip occupies 9000 - 90FF (16 registers mirrored 16 times) and the VIAs occupy 9000 - 93FF addresses where A4 is high (VIA1) and A5 is high (VIA2). The standard locations for addressing these chips are 900x for the VIC, 911x for VIA1 and 912x for VIA2.

Whoever performed the Battleships SID expansion found a way to add a SID in the same address space. If it is to occupy 9140-915F, not only must the SID chip select be decoded for that range, but the VIAs must be deselected, otherwise writing to certain SID chip registers would also write to the VIA(s). In that range, A4 is high in 9150 - 915F, so writing to the second half of the SID chip registers would also write to VIA1.

To deal with this problem, the chip select signals for the VIAs must be further decoded. I/O0# is the signal for the 9000-93FF address range. I would suggest further decoding this so that the VIAs are only selected when A6 is low, leaving 9140 - 917F open for two SID chips. The first one at 9140 - 915F and the second one at 9160 - 917F.

This is the simplest addressing scheme preserving compatibility with the Battleships game as described in the manual. If more SIDs are required, the VIAs could be selected when both A7 and A6 are low, leaving 9140 - 91FF open for six SID chips.

[rga24]

For Pin 27, could you audio out with a female RCA jack and use a Y cable and your video cable to run the sound off the speaker in the monitor?

I'm confused. In the n schematic it shows an audio in on Y, just next to CA13. Is it not really there?

On the old board, Y is not connected. On the new board, Y is indeed the AUDIO IN signal. It is named as such on the schematic:

http://www.zimmers.net/anonftp/pub/cbm/ ... 1027r2.gif

It is connected to the VIC chip audio output amplifier Q1 via a 1K resistor R40. Q1 has the VIC chip audio signal connected via another 1K resistor R8.

http://www.zimmers.net/anonftp/pub/cbm/ ... 1027r1.gif

Unfortunately this is the same audio output amplifer which suffers 1.6kHz low pass filtering, caused by 100nF capacitor C10, so any signal fed in to the AUDIO IN line would also be very heavily filtered. The effect would be very noticeable on the output of a SID chip.

I have suggested replacing this capacitor with a 10nF one, raising the low pass filter cutoff frequency to 16kHz.

[dragos]

Did this ever get to the prototyping stage? anyone actually build one?

[ral-clan]

This is something I'd buy if it was well made so as not to damage the SID chip, and I could use it in music making (i.e. if it had a line-level audio output on the cart).
I have a couple of Spare SID chips for a project like this.

[cobracon]

Has anyone else tried this yet? Was thinking about attempting it. Is it worth it?