Packet & APRS

The RS-UV3A supports packet radio and APRS (Automatic Packet Reporting System) operations when connected to a TNC (Terminal Node Controller) or software modem.

Overview

The RS-UV3A provides:

TX audio input for packet tones from the TNC
RX audio output for received signals to the TNC
PTT control for keying the transmitter
COR output for carrier-operated relay

Hardware Connections

Using the DE-9 I/O Connector

Connect your TNC or sound card modem to the DE-9:

DE-9 Pin	Signal	TNC Connection
1	TX Audio In	TNC TX audio out
2	PTT	TNC PTT output
3	GND	TNC ground
4	RX Audio Out	TNC RX audio in
5	COR	TNC COR/DCD input (optional)

Audio Level Adjustment

TX Audio: Adjust VR3 on the board for proper deviation (typically 3–3.5 kHz for packet)
RX Audio: Use the VU command to set receiver audio level

VU15            # Good starting point for DTMF/packet decode

Configuration for Packet

Basic Setup for 1200 Baud

Standard 2m packet uses 144.39 MHz (APRS) or 145.01 MHz (general packet):

FS144390        # APRS frequency
AF0             # Wide audio filter for data
DP0             # Disable pre/de-emphasis for flat response
SQ3             # Moderate squelch
ST0             # Save as defaults

APRS Digipeater/iGate Configuration

For unattended operation:

FS144390        # 144.39 MHz
TM0             # No tone squelch
SQ3             # Open on moderately weak signals
AF0             # Wide audio bandwidth
DP0             # Flat audio response
TO000           # Disable timeout (TNC controls TX)
ST0             # Save settings

COR (Carrier Operated Relay)

The DE-9 pin 5 provides a COR output:

Low (ground) when squelch is open
Open circuit when squelch is closed

This can signal the TNC that a carrier is present, useful for collision avoidance in packet operations.

COR Inhibit

After transmitting, there’s often a burst of noise as the receiver recovers. Use COR inhibit to prevent false DCD triggers:

CO0100          # 100 ms COR inhibit after TX
CO0200          # 200 ms inhibit
CO?             # Query current setting

External Serial for TNC Control

If your TNC needs to send commands to the RS-UV3A, configure the DE-9 serial:

EX1             # Enable serial on E_TX/E_RX pins
B23             # Set external serial to 19200 baud

Now pins 7 (E_TX) and 8 (E_RX) function as a TTL serial port.

Using with Software TNCs

Direwolf on Linux

Connect the RS-UV3A to a USB sound card:

ADEVICE plughw:1,0
CHANNEL 0
MYCALL N0CALL-1
MODEM 1200
PTT /dev/ttyUSB0 RTS    # If using RS-UV3A serial for PTT

SoundModem on Windows

Connect RS-UV3A audio to sound card line in/out
Configure SoundModem with your sound card
Use a serial port (or VOX) for PTT

Channel Bandwidth

Standard packet uses 25 kHz channel spacing:

BW1             # 25 kHz bandwidth (default)

For narrow-band packet environments:

BW0             # 12.5 kHz bandwidth

Multiple Frequency Operation

Store different packet frequencies in memory channels:

FS144390        # APRS
ST1             # Save to channel 1

FS145010        # General packet
ST2             # Save to channel 2

FS144990        # Alternate
ST3             # Save to channel 3

Switch channels via serial:

RC1             # Recall APRS frequency
RC2             # Recall general packet

Troubleshooting Packet

Not Decoding Packets

Check audio level: VU15 is a good starting point
Disable filters: AF0 and DP0 for flat response
Check squelch: SQ2 or SQ3 for reliable opening
Verify frequency: F? to confirm

TX Not Working

Verify PTT: Ground pin 2 should key TX
Check audio: Use a speaker to verify TNC is sending tones
Adjust deviation: VR3 controls TX audio level

Audio Distortion

TX too hot: Turn down TNC output or VR3
RX overdriven: Reduce VU setting
Check filters: Try AF1/AF0, DP1/DP0 combinations

9600 Baud Considerations

The RS-UV3A Rev A can support 9600 baud packet, but requires careful setup:

Use wide bandwidth mode (BW1)
Disable all audio filtering (AF0, HP0, DP0)
Adjust audio levels carefully—9600 baud is more sensitive to deviation errors
VR3 adjustment is critical for proper deviation

Complete APRS Setup Example

For a typical APRS tracker/digipeater:

# Set frequency and save
FS144390        # APRS frequency

# Optimize for data
AF0             # Wide audio
DP0             # Flat response

# Squelch and volume
SQ3             # Moderate squelch
VU15            # Good decode level

# TX settings
PW1             # High power
TO060           # 60 second timeout (safety)
CO0150          # 150 ms COR inhibit

# Save as defaults
ST0

Then connect your TNC, configure it with your callsign and path, and you’re ready for APRS!