One of the most important considerations for an EchoLink repeater node is the method of detecting the presence of a local RF signal.  Although the best approach is usually to wire a COS signal into the COM port of the PC, it is often necessary (or desirable) to use VOX instead.  Several techniques are described below.

COS from Repeater Receiver: If the node is hard-wired to the repeater controller, the best source of carrier detect is the COS output from the repeater receiver itself -- or an equivalent signal from the repeater controller.  This ensures that EchoLink transmits to the Internet only when a signal is being received on the input.  Also, the audio connection to the sound card should come from the receiver's audio output, rather than the repeater transmitter's audio path.

COS from Link Transceiver: If the node is remotely located, it may be desirable to use the COS signal from the link transceiver -- but only if the repeater's "tail" is extremely short.  Otherwise, EchoLink will keep transmitting to the Internet 5 to 10 seconds after the local user finishes a transmission, severely interrupting the flow of a QSO.  Some repeater-node operators have successfully incorporated DTMF tones in their custom Connect and Disconnect announcements to automatically shorten the repeater's "tail" while an EchoLink station is connected, on repeaters which support this type of remote command.

VOX: If the node is remotely located, but the repeater's "tail" cannot be shortened, VOX can be used.  When properly adjusted, EchoLink will detect voice signals coming through the repeater, but ignore other incidentals such as the "tail", the courtesy tone, and the squelch crash at the end.  This is very important when two repeaters are linked to each other, to prevent endless ping-ponging of one repeater bringing up the other.  Here are some tips for adjusting the VOX for use with a repeater:

1. Set the VOX threshold carefully.  (This is the horizontal slider below the audio-level indicator.)  The VOX threshold should be set just above the audio level of the repeater's dead carrier, so that it "trips" on voices, but not on the repeater's tail.  Watch the purple SIG annunciator while adjusting the VOX threshold.
2. If necessary, adjust the VOX delay.  The default value of 1200 ms is appropriate in most situations.  (If you decide to change the value, type it in directly, rather than using the up-down buttons.)
3. Use the "Smart VOX" feature.  This is enabled by checking the "Squelch Crash Anti-Trip" box on the RX Ctrl tab.  When this feature is enabled, EchoLink's VOX will ignore short noise bursts, such as the repeater's courtesy tone and the squelch crash when the repeater's carrier drops.  Set the time constant to a value slightly higher than the longer of these two signals.  Typical settings are 250 ms for a repeater with a (short) courtesy tone, or 80 ms for a repeater with no courtesy tone.  Note that the Smart VOX feature does not necessarily suppress these signals in the audio path, it merely prevents them from triggering (or holding open) the VOX.
4. Anti-Thump:  Use the Anti-Thump feature if the squelch crash, as heard over the repeater when your transceiver stops transmitting, seems to be triggering the VOX.  Start with a low value and move it gradually higher until the SIG indicator no longer appears when your link stops transmitting.

When the VOX is properly adjusted, the "acid test" is to connect the link to the *ECHOTEST* conference server.  After hearing the initial welcome message from the test server, the repeater should drop normally and then remain idle.  If the repeater continues to be keyed up by responses from the test server, re-check the above settings.  

When the settings are correct at both ends of a repeater-to-repeater link, both repeaters should remain idle except during an actual QSO, or while either repeater sends its ID.

CTCSS Control: If the node is remotely located, this may be the best technique of all -- but it requires cooperation from the repeater.  In this set-up, the repeater transmits a CTCSS (a.k.a. PL) tone only while its receiver's COS is active; that is, only while a station is transmitting.  The EchoLink transceiver is configured to open up only when this tone is received.  The advantage of this system is that EchoLink triggers only on a true signal, and ignores incidentals such as courtesy tones and CW IDs.  The disadvantage is that most PL-guarded repeaters transmit a continuous tone, even when no signal is present on the input, so it may require configuration changes to the repeater itself.  (Note that this technique can be used whether or a not a PL is required to activate the repeater.)

73,
K1RFD