Found something about that
[link|http://www.jagshouse.com/modem.html|Connecting two computers using their modems, without a telephone line]
One necessity for a reliable connection of this type is a "line simulator", a device to provide simulation of a telephone line, supplying the line current that modems are designed to work with. For our purposes, a very simple home-made circuit will do the job (you should be able to find most, if not all, of the parts at a Radio Shack store). This connects between two telephone modular jacks, and the two modems plug into the jacks:
I don't think the parts are at Rat Shack anymore. If the ATO1 doesn't work I'll investigate this info further.
Hadn't thought about lower baud rate being an issue. I did notice that only the 128 was making modem sync noises. Maybe the Mac's modem needs the "line simulator" but the 128's doesn't.
I've found reference to SwiftLink and Turbo232, which add a real RS232 via the cartridge port. That would work well with a null modem cable(I already have a USB serial adapter on the Mac for my [link|http://www.arminvogl.de/KrokodileCartridge/|Krokodile Cart]), plus they go faster than the 128's 1200 baud modem does.
What I'd really like to do is ethernet on the 128, but haven't had any luck in finding the parts they used to make the [link|http://www.dunkels.com/adam/tfe/|The Final Ethernet - C64 Ethernet Cartridge]. I thought I'd found a possible replacement for the Embedded Ethernet board they used, but the [link|http://microcontrollershop.com/product_info.php?cPath=98&products_id=1202|CS8900A Ethernet Controller Header Board] is based on the new low-power 3 volt version of the CS8900A :-/
Darrell Spice, Jr. Trendy yet complex\nPeople seek me out - though they're not sure why\n[link|http://spiceware.org/gallery/ArtisticOverpass|Artistic Overpass] [link|http://www.spiceware.org/|SpiceWare]
