[Elecraft] uh, oh -- keying loop again

[Dick Green WC1M]

> Hi Barry,

> 

> We will definitely have TX inhibit as an input on the K3 AUX I/O 

> connector. Its not turned on yet, but should be shortly as we add this 

> feature to the firmware. It certainly helps for some set ups when using 

> the rign in a SO2R arrangement etc.

> 

> 73, Eric

 

Perhaps I was too hasty in registering approval. Another reader of the
reflector has called my attention to this section of the K3 manual:

 

"TX INH (Transmit Inhibit Signal)

Pin 7 of the AUX I/O connector can be configured

as a transmit inhibit input signal by setting

CONFIG:TX INH to ON. Holding pin 7 low will

then prevent the K3 from transmitting. An external

2.2 to 10 K pull-up resistor (to 5 VDC) is required."

 

First, this is backwards. All other transceivers on the market require TX
INH (or TX ENA) to be grounded to enable transmit, not to inhibit it. This
is what the QSK amplifiers with keying loops expect. Second, it is standard
practice for the transceiver to provide the pull-up resistor and voltage.

 

In order to use this implementation with a standard QSK amplifier, the user
will have to modify the K3 or, if +5VDC isn't handy inside the K3, the user
will have to add an outboard interface with a separate power supply.
Needless to say, this will be an undesirable inconvenience for most owners.

 

The problem is that the K3 is sharing this I/O port with another function,
XVRT ON, for which the port is an output, not an input.

 

If there are no other unused ports available for this function, I'd suggest
a hardware revision with an internal slide switch to select between the TX
INH and XVRT ON functions, along with a pull-up resistor to +5VDC.

 

Meanwhile, two things would help users to get around this unfortunate
circumstance. First, Elecraft should determine if there's a handy place to
pick up +5VDC inside the chassis. If there is, they should let us know where
it is. Second, they should change the firmware to reverse the logic so that
raising the input inhibits transmit and grounding it enables transmit. That
should be simple to do. The default should be XVRT ON so that the K3 will
transmit out of the box. These steps will allow the owner to get the desired
functionality simply by connecting a pull-up resistor between the +5VDC
source and Pin 7 of the AUX I/O connected, and will avoid the owner having
to come up with a transistor inverter circuit.

 

73, Dick WC1M