I set up my Raspberry Pi and ran some Cat-5 to it from the router. Using a level shifter and a variety of old adapters, I managed to get a serial cable working between the Pi and the Mac. That took care of the hardware.
On the software side, I scrounged around and after several failed attempts found a PPP client that would run on the Plus and a super-simple PPP server called SLiRP for the Pi. Documentation for the combination of MacTCP, MacPPP, and SLiRP was, surprisingly, still available. After a bit of tinkering with the configuration, I was able to get the MacTCP to talk to MacPPP, MacPPP to talk to SLiRP, SLiRP to use the Ethernet connection, and so on through my router and out to the Internet. Since serial I/O on the Mac Plus was processor-intensive, throughput was limited to about 19 kbits/s, but 19 was a lot higher than 0.
Jeff published an excellent explanation of the project, read it here!
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