Serial Port Programming

My Powerplant application doesn't receive characters from an external device. What have I done wrong (see below)?

I have a PowerMac 7600 that communicates with an RS-232C device through the serial port. When a certain string is sent to the device, a LED blinks and confirms that the device received something. The Mac doesn't get anything back
from the RS-232 device.

To verify that the string is received correctly I've used another 7600 to simulate the device. Yes, it receives the string correctly.

The correct parameters (speed, parity, etc.) are set. This has been checked with a PC-to-the-external-device test. The communication with a PC works well.

There doesn't seem to be anything wrong except maybe (?) two things: problems with Mac-to-device (RS-422 to RS-232) or
bad cable design.

The cable used is a Mini DIN-8 (Mac side) to 9-Pin (device side). According to the spec of the device (talked with manufacturer too) the RS-232 side of the cable looks like:
  Pin No.  Signal
  -------  -------
    1        FG
    2        SD
    3        RD
    4        RS
    5        CS
    6        --
    7        --
    8        --
    9        SG

The RS-232 9-pin is connected to the Mini DIN-8 according to:
  RS-232   Mini DIN-8
    1        4 (SG)
    2        8 (RxD+)
    3        6 (TxD+)
    9        4 (SG)
 
No handshaking is used. RS-232 pins #4 and #5 are connected to each other. Mini DIN-8 pins #1 and #2 are connected to each other.
ovjAsked:
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PADConnect With a Mentor Commented:
To connect a RS232 to a mac RS422 you just have to connect Ground to Ground.
Then connect the RS232 "SEND" to the RS422 RECEIVE+, ground to RS422 RECEIVE-.
Connect the RS232 "RECEIVE" to RS422 "SEND+", and do not connect RS422 "SEND-"

In fact Mac's RS422 to RS232 compatible, but NEVER forget in that case to Ground "RECEIVE-" and that what is missing in you cable i think.

And to make sure all is ok if you don't use RTS/CTS you just have to connect them together on each side.

Good Luck
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ovjAuthor Commented:
I think that it'll work with this connection. I got tired and changed the device to an RS-422 interface, so I never could test it.
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