I found an account of Commodore’s rise to prominence on a vintage computing forum. It’s interesting reading. Then again, I’m partial to anything about the rise of Commodore.
Connecting a single drive to a Commodore C-64, 128, or VIC-20 is pretty easy: Plug a 6-pin serial cable from the port on the back of the computer to one of the two ports on the back of the drive. It doesn’t matter which port you use. The second port is for “daisy chaining” additional peripherals, such as a printer, or multiple drives. And even though a fast load cartridge speeds up the drive, it plugs into the computer.
Older drives like the 1540, 1541, and 1571 are self-contained. Plug a power cable (which, conveniently, is no different from the power cable used on a most desktop PCs) into the back and power it on. Later 1541-IIs and 1581s use an external power brick. The two drives’ power bricks are interchangeable; however, they do differ from the power brick used by the computer itself. Fortunately, the original power bricks are labeled with the compatible devices, either on a silver sticker on top or molded into the underside.
It’s multiple-drive setups that get trickier. Read more
I guess it’s not exactly obvious, to someone looking at a Commodore 64 or 128, how a modem plugs in. Commodore modems plugged into the port on the far right hand side, looking from the back. If the port is labeled at all, it will be labeled “User port.” Although it had other uses, that port was used for modems far more than for any other purpose.