Interfacing
For use in Sysop mode, your transceiver connects to a hardware
interface, which in turn connects to your computer's sound card and
serial port.
Audio from the receiver goes to the interface board or your sound
card's line-in jack, and audio from the sound card's speaker out (or
line out) goes through an attenuator to your transmitter's microphone
input. (The attenuator is needed to reduce the sound card output
to mic level). If your rig has a line-in connection, sometimes
available on a back-panel accessory jack, you can usually connect the
sound card output directly to line-in.
Interface Types
Two types of interface boards are supported by EchoLink:
ASCII-controlled: This type of interface,
which was specifically designed for EchoLink and iLINK, accepts ASCII
commands from the computer over its serial port. These commands
key and un-key the transmitter, and request information about incoming
DTMF digits. High-quality boards of this type are available in kit
and assembled form from WB2REM
or VA3TO. Details
of the original WB2REM circuit were also described in QST for March,
2002.
Direct-controlled: This type of interface,
which is commonly used for PSK31 and other digital modes, keys the
transmitter in response to signals over the serial port's RTS or DTR
pins. The interface usually includes an attenuator to simplify the
connection between the computer's sound card and the transceiver's mic
jack. This type of interface may be the best choice if you intend
to use digital modes in addition to EchoLink, or if your set-up requires
additional isolation between the computer and the transceiver. The
interface does not include a DTMF decoder, but EchoLink's internal
decoder may be used instead. The RIGblaster from West
Mountain Radio is an example of this type.
Carrier Detect
In Sysop mode, EchoLink needs some way of knowing when there is
activity on the local receiver. The most common way is to use
EchoLink's built-in VOX (voice-operated switch). The VOX monitors
incoming audio from the receiver. When properly adjusted, the VOX
will trigger only when a station is being heard over the receiver.
One advantage of using VOX is that no other connections between the
receiver and the PC are required.
VOX is particularly useful when EchoLink is connected to a
transceiver which is monitoring the output frequency of a repeater.
The VOX will trigger only when a station is actually speaking through
the repeater, and will not generally respond to the repeater's
"tail", which could be as long as 10 seconds after each
transmission.
However, VOX is not completely reliable, since it is unable to detect
a dead carrier. Also, it introduces an additional delay of a
second or so after each transmission. To help circumvent these
issues, EchoLink also supports a direct connection between the receiver
and the computer's serial port for positive carrier detect.
This type of connection is especially useful for simplex links.
Many transceivers have a back-panel connection (called
"busy" or "carrier detect") which tracks the squelch
circuit directly. This can be wired to the CD, DSR, or CTS pin of
the PC's serial port, and EchoLink can be configured to respond to it
instead of using VOX. Typically, this connection must be made
through a simple interface, such as a one-transistor circuit. The
circuit should drive the CD, DSR, or CTS pin high (+5V or above) when
the squelch is open, and low (0V or below) when it is closed.
Several versions of the interface products mentioned above also support
this feature; check the product literature for details.
To enable this feature, choose Sysop Settings from the Tools menu,
select the RX Control tab, and choose Serial CD, Serial CTS, or Serial
DSR.