Elecraft KAT500 Serial Command Reference - Manual (Page 6)

15-pin ACC cables used between the K3, KAT500, and KPA500 provide this amplifier key line interrupt
capability. Alternatively, two-wire cables with RCA connectors may be used between transceiver and
KAT500, and KAT500 and amplifier. See the KAT500 Owner’s Manual for connection diagrams.
It may take a few tens of milliseconds after issuing an AMPI SET before the relay is changed.
The SET form of this command may be ignored if the current transmit power exceeds the AKIP power
threshold.
AN (Select Antenna, GET and SET)
GET format: AN;
SET/RESPONSE format: AN1; AN2; or AN3;
Antenna selection is deferred if a tune is in progress. Antenna selection is ignored if the antenna is
disabled on the current band (see the AE command).
It may take a few tens of milliseconds after issuing an ANx setter before the relays are changed.
The SET form of this command is ignored while transmitting or tuning.
AP (Antenna Preference, GET and SET)
GET format: APbb; where bb is a Band Number, see the BN command.
SET/RESPONSE format: APbba; where bb is a Band Number and a is an antenna number, 0, 1, 2, or 3. 0
is interpreted as “the last used antenna on this band”.
The KAT500 normally returns to the last-used antenna on band change. However a “preferred” antenna
may be configured so that the ATU always starts at a fixed antenna for a given band.
For example: AP103; causes ANT3 to be selected whenever you band switch to 6 meters, regardless of
the last-used antenna on 6 meters.
AP SET is ignored if the antenna is disabled (see AE)
AP is used by the KAT500 Utility Edit Configuration dialog Antenna tab.
ATTN (Enable Attenuator, GET and SET)
GET format: ATTN;
SET/RESPONSE format: ATTN0; (disabled) or ATTN1; (enabled).
Selects relay K22, which switches in an attenuator resistor.
It may take a few tens of milliseconds after issuing an ATTNx setter before the relay is changed.
Firmware releases the attenuator if it detects power high enough to damage to the attenuator.