This article is from the Mac communications FAQ, by Bruce L Grubb BruceG6069@aol.com with numerous contributions by others.
There are currently two type of modems out there; the older modems
which have DB-25 (25 pin) connector labelled RS-232C on their
backsides and the newer modems which have a USB port.
Connecting one of the older modems to a Mac with a serial port
(Mac Plus to the first G3s) involves getting a DIN-8 to DB-25
hardware handshaking cable which typically sale $10.00 (10 ft)
via mail order. Connecting one of these older modems to a USB
port involves getting a USB to DIN-8 adapter in addition to the
cable which given the cost makes it more practical to go with a
There is no way to connect a USB modem to a serial port though a
USB card can be put into a PCI baced Mac.