All Mac serial ports can be set to run at up to 57600 bit/s. They can all also run at 230400 bit/s when running a LocalTalk connection. In the case of 68000 Macs running a normal TCP/IP PPP connection, the fastest a serial port (only one at a time!) can run at without some regular dropped packets is 19200 bit/s (based on tests using a GV 33.6 modem connected to my Plus running System 7.0.1, MacTCP 2.0.6, MacPPP 2.0.1, and Fetch v3.something). Having said that, 28800 bit/s actually increased the average data rate despite the increase in dropped packets (needing re-transmission). 38400 bit/s was a disaster.

You can use any speed modem you like with a 68000 Mac. The only point to remember is that the serial port speed is (effectively) limited to 28800 bit/s for TCP/IP PPP internet connections, so a 56K modem won't help much.

This applies to all 68000 Macs, with the exception of the 128K original which doesn't have enough RAM to be an Appleshare workstation or run a TCP/IP PPP internet connection. The later 68000 Macs are less inefficient in some ways than the Plus, so I wouldn't be surprised if (say) a Classic turned out to be able to run a TCP/IP PPP internet connection at 38400 bit/s. But then again, I'd not be surprised if it was as hopeless with a serial port speed of 38400 bit/s connected to a modem as my Plus was.

Rowland.