Apple Macintosh LC 630 DOS - Manual (Page 70)

Using PC peripherals
Because of the RS-422 implementation of the Macintosh, the following
RS-232 signals are not available to the PC:
m Carrier Detect (CD)
m Data Set Ready (DSR)
m Request to Send (RTS)
m Ring Indicator (RI)
Applications or serial devices requiring these signals won’t work.
While the DOS-compatible Macintosh does not support Carrier Detect (CD),
you can set most communications application programs to respond to the
CARRIER string sent back by most modems.
For more information about accessing serial ports from DOS, see the
documentation that came with your DOS software.
IMPORTANT The following chart lists the various signals present on the
Macintosh Mini DIN-8 connector, and on what pins these signals are present
on PC-style DB-9 and DB-25 connectors. This information may be of use if
you are planning to have a custom cable built. Refer to the manual for the
serial device that you plan to attach to the Macintosh for the recommended
serial connection, and use the chart below to find out what the proper
Macintosh wiring should be.
Macintosh signal Macintosh Mini DIN-8 DB-9 DB-25 RS-232 name
HSK0 1 4 20 DTR
HSK1 2 8 5, 8 CTS, DCD
TXD- 3 3 2 TXD
GND 4 5 7 GND
RXDA- 5 2 3 RXD
TXDA+ 6 N/C N/C
GP1 7 N/C N/C
RXD+ 8 5 7 GND
59 Operating in the PC Environment