Radio/Modem I/O Connections:

The radio is connected to Connector J1 (9-pin) on the rear panel of the modem. 

PIN FUNCTION                   I/O LIMITS 
1   RX Audio from receiver     +Input 5V p-p
2   RX Audio from receiver     -Input 5V p-p
3   FSK Output                 Output +50V @ 100 ma
4   TX Audio to Transmitter    +Output + 6dBm (1.5 V)
5   TX Audio to Transmitter    -Output + 6dBm (1.5 V)
6   Ground
7   SEL-CAL Output             Output +50V @ 100 ma
8   Push-to-Talk Output        Output +50V @ 100 ma
9   Ground

The computer is connected to Connector J3 (25-pin) on the rear panel of the modem.
 
Pin Function      Pin Function
1 Ground          14 STXD
2 TXD             15 NC
3 RXD             16 SRXD
4 RTS             17 NC
5 CTS             18 NC
6 NC              19 SRTS
7 Ground          20 DTR
8 DCD             21 TNC
9 NC              22 NC
10 NC             23 NC
11 NC             24 NC
12 SDCD           25 Boot
13 SCTS

Technical Specifications:

AUDIO I/O:     TI TLC-320AC01 14 Bit A/D & D/A Converter
               Transformer isolated audio input and output

DSP PROCESSOR: 24-bit DSP-56001A @ 20.48 MHz

CONTROL:       16-bit 68EC000 Control Processor @ 10.24 MHz

MEMORY:        Flash ROM loaded via serial I/O port

DATA PORT:     Serial RS-232 Control/Data Port; to 57,600 bps (DB25)

POWER:         +10 to +18 VDC @ 250 ma power (500 mA to program ROM)

INDICATORS:    STBY, CALL, LINK, ERR, TX, RX

RADIO I/O:     RX Audio, TX Audio, PTT, FSK, SEL-CAL (DE9)

CABINET:       Black, Extruded 5.25" x 1" Aluminum

CONNECTORS:    Radio: DE-9S Connector
               Data:  DB-25S Connector
               Power: 5.5/2.5 mm Coaxial Power Connector

WEIGHT:        2.75 lbs (1.25 kg) net, 5.0 lbs (2.3 kg) shipping


So you need 3 wires hooked up normally.  RX Audio, TX Audio, and PTT.  Set normally comes with a cable with 4 Phono plugs from this with Sel-Cal essentially not used very much.

J2  Pins 1-2 sets AF out ranges.  Normally closed 0 dBm 775 mV rms adjusts -20 to +6 dBM  78 mV to 1.5 V.  Open  -30 dBm adjusts -50 to -20 dBm. Little pot back there does the fine adjustment for ALC.

J2 Pins 3-4 adjust FSK output. Its open collector On is normal 0V but Off pulls it up to 5V. 

Power connecter is center pin positive BTW. J5 is a Test options thing. Normally all are open no jumpers.