K3/K3S Programming Guide

This guide walks through the Elecraft K3/K3S CAT (Computer Aided Transceiver) serial protocol from a programmer’s perspective. It is organized by task rather than by command name, so you can find the commands you need for each aspect of radio control without reading the entire protocol specification.

For a complete alphabetical listing of every command, see the K3/K3S/KX3/KX2 CAT Command Reference.

Serial Connection Setup

The K3/K3S accepts commands over a standard serial connection. Two physical interfaces are available:

• USB via the KIO3 or KUSB board, which provides a virtual COM port on the host computer
• RS-232 via the DB-9 connector on the rear panel

Default serial parameters:

Parameter	Value
Baud rate	38400
Data bits	8
Parity	None
Stop bits	1
Flow control	None

Tip

The BR command can query or change the baud rate. The parameter values are: 0=4800, 1=9600, 2=19200, 3=38400, 4=57600, 5=115200. Send BR; to read the current rate, or BR4; to switch to 57600 baud.

All commands are plain ASCII text terminated with a semicolon (;). Responses from the radio also end with ;. There is no line-feed or carriage-return delimiter — the semicolon is the only framing character.

FA;                  → query VFO A frequency
FA00014200000;       ← radio responds with the current frequency

GET / SET / RSP Model

Every interaction with the K3/K3S follows a simple three-part model:

SET — Send a command with data to change a radio parameter.

FA00014200000;       Set VFO A to 14.200 MHz
MD1;                 Set mode to LSB
KS020;               Set keyer speed to 20 WPM

GET — Send the command letters with no data (just the terminating ;) to read the current value.

FA;                  Query VFO A frequency
MD;                  Query current mode
KS;                  Query keyer speed

RSP — The radio responds with the command letters followed by the current data, in the same format as a SET.

FA00014200000;       VFO A is at 14.200 MHz
MD1;                 Mode is LSB
KS020;               Keyer speed is 20 WPM

Note

Characters sent to the K3 can be upper or lower case. The radio always responds in upper case.

VFO B / Sub Receiver Commands

Many commands accept a $ suffix to target VFO B or the sub receiver instead of VFO A / main receiver. For example:

AG;                  GET main receiver AF gain
AG$;                 GET sub receiver AF gain
AG120;               SET main receiver AF gain to 120
AG$120;              SET sub receiver AF gain to 120

Commands that support the $ suffix are marked with $ in the command reference.

Extended Command Mode

Before issuing most commands, enable K3 extended mode with the K3 command. This unlocks the full command set and extended response formats.

K31;                 Enable extended K3 command mode
K30;                 Return to basic (K2-compatible) mode

Caution

Some commands and response fields are only available in extended mode (K31). Always send K31; at the start of your session to ensure consistent behavior.

Auto-Information Modes

By default, the K3 only sends data in response to a GET command. The AI command enables automatic notifications when the radio state changes — either from front-panel controls or from other software.

Mode	Behavior
AI0;	Polling only (default). The radio never sends unsolicited data.
AI1;	The radio sends an IF response whenever any front-panel change occurs.
AI2;	The radio sends the specific command response matching the changed parameter.

Tip

AI2 is recommended for most applications. It gives you fine-grained, targeted notifications without the overhead of constantly polling. For example, if the operator turns the VFO knob, you receive FA responses with the new frequency rather than a generic IF block that must be parsed.

See the Event Handling page for details on processing auto-information responses.

Guide Organization

Each page in this guide focuses on a specific area of radio control. They are meant to be read in order the first time, then used as a reference afterward.

Page	Covers
Connection & Discovery	Finding the serial port, verifying the radio identity, and establishing a session
Frequency & Modes	Setting and reading VFO frequencies, operating modes, and band selection
Receiver Control	AF/RF gain, filters, bandwidth, AGC, noise blanker, and DSP settings
Transmitter Control	TX/RX switching, power output, SWR metering, and mic gain
Voice, CW & Data	CW keying, stored messages, data sub-modes, and voice operations
Advanced Features	Split operation, sub receiver, diversity reception, and memory channels
Event Handling	Auto-information modes, unsolicited responses, and state synchronization
Error Handling	Timeout handling, error recovery, and retry patterns
KPA500 Integration	Amplifier power, band tracking, fault monitoring, and standby control
KAT500 Integration	Antenna tuner control, tune initiation, and antenna selection
Station Integration	Coordinating K3 + KPA500 + KAT500 as a unified station