Macintosh Modem Cable Pinouts

About Mac cables as contrasted with "PC" cables: the RS-232 standard was designed with a mindset that, in retrospect, was pretty myopic. Well before personal computers existed, it was perceived as just so: if you were a piece of equipment having anything to do with data communications, then you must either be a modem or a terminal. Printers were terminals. Personal computers, if they were allowed to exist, were terminals -- since they were not modems. So how do you hook a printer to a PC? You are attaching two terminals to each other, and the way to do that with a standard cable is to attach each terminal to a "null modem" -- just an adapter that swaps the signals. Add to that the "gender" of the cable or connector, and you have way too many possible combinations to deal with and keep your sanity.

Along comes the Mac, and a better way. How about this: either you are a piece of equipment, I'll say "box" henceforth, or you are a cable. Cables have male connectors and boxes have female. Cables have signals going into them on particular pins, and leaving on other particular pins. And so forth. You plug it in, it just works. What a concept!!

The only problem left is connecting to things with the old connectors. Here are a couple of hints.

                Macintosh (DTE)                           Modem (DCE)
                    DIN-8                                    DB-25

                Pin  Signal                               Signal  Pin
                 1    HSKo   ---------------------------   RTS     4
                 2    HSKi   ---------------------------   CTS     5
                 3    TxD-   ---------------------------   TxD     2
                 4    GND    ---------------------------   GND     7
                 5    RxD-   ---------------------------   RxD     3
                 6    TxD+   (nc)                    .--   DSR     6
                 7    GPi    (nc)                    '--   DTR    20 
                 8    RxD+   ---------------------------   GND     7  

                    shield   ---------------------------   shield

                                Figure 1
If your modem cannot be configured to ignore DTR, or if you are using an old 1200 baud or 2400 baud modem, the pinout in Figure 1 is probably best. However, if you are using a high-speed modem that can ignore DTR (and/or Remote AppleTalk) you should instead use the pinout in Figure 2, which is the pinout "recommended" by Apple:
                Macintosh (DTE)                           Modem (DCE)
                    DIN-8                                    DB-25

                Pin  Signal                               Signal  Pin
                 1    HSKo   ----------------------+----   RTS     4
                                                   '----   DTR    20
                 2    HSKi   ---------------------------   CTS     5
                 3    TxD-   ---------------------------   TxD     2
                 4    GND    ----+----------------------   GND     7
                 8    RxD+   ----'
                 5    RxD-   ---------------------------   RxD     3

             

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