You need a specific cable to work with the printer. Also, there might be a switch box on the printer itself where you set baud rate, parity, etc., which (of course) has to match the settings on the computer. I don't have the printer anymore, though.
Oh awesome - thank you, @oldappleguy! I found a PDF of that manual early yesterday but the copy I found was not scanned especially well, and the details were a lot harder to make out - yours is much clearer!