Elecraft KAT500 Serial Command Reference - Manual (Page 10)

SET/RESPONSE format: F nnnnn; where nnnnn is the approximate frequency of the last transmitted
signal, in kHz.
F causes the ATU to change bands (if needed) and recall prior settings for the specified frequency,
without transmitting. See also the BNnn; command and MT fffff;
F SET is ignored while tuning.
FA (Memory Recall Tune, SET only)
SET format: FA000fffffiii; where fffff is the frequency in kHz, and iii must be digits. The command
requires exactly 11 digits of frequency to the Hz. The leading three digits must be 0. The final three digits
are unused.
FA000fffffiii; is functionally equivalent to MT fffff;
FA was introduced in firmware version 01.34.
FLT (Fault query, GET only)
GET format: FLT;
RESPONSE format: FLTc; where c is a fault code:
Fault code Name Description
0 No Fault No error
1 No Match The ATU tune algorithm was unable to find a satisfactory
match
2 Power Above
Design Limit for
Antenna SWR Transmitter power exceeds the design limit for the
unmatched antenna SWR. This power limit varies with the
SWR of the antenna: 600 watts at 10:1 SWR, 1000 watts at
3:1 SWR.
3 Power Above Safe
Relay Switch Limit Transmitter power, with the amplifier key line interrupted,
exceeds the ATU’s safe relay switching limit of 100 watts.
A FAULT may be cleared by tapping the TUNE button or turning the KAT500 off (Hold the MODE button)
and back on.
FLTC (Clear Fault, SET only)
SET format: FLTC;
Turns off the FAULT LED and clears the current fault.