This is an RS-422 interface not RS-232. RS-422 is balanced (paired IO pins, + and -), while RS-232 is unbalanced. The pins can't just be connected and it work.
For a quick and dirty solution, attach the - pins to the RS-232 data pins, and RD+ to the RS-232 ground (not the apple's ground). The handshake in and out are the hardware flow control, GPinput is carrier detect. This may dammage the apple though, insert standard disclaimer etc.
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