connecting an external amp to 6400

Hi Art, do you know which of the pins control which bands? Do you know if the bands are selected high or low? It looks like certain bands are paired in the amp, so you will likely need to use some diodes (diode OR) to logically OR them together.

The Flex USB BIT cable will probably do what you want, but you will need to wire the lines to the connector correctly. You will need to be aware of voltages on the pins (Flex expects 3.3 V), what pins control which bands and their polarity.

Take a look at the SmartSDR documentation, page 125:

BIT cables provide a means to map the tuned frequency of the radio to a set of individual signals/wires at the end of the cable. These cables are designed to control external devices when the radio is tuned to specific bands or frequency ranges, and for external keying or sequencing that is triggered when the transmitter becomes active. The signal that appears on each bit/pin of a BIT cable is configured by use of the controls in the USB Cables menu: 

Hi Art, yep, you will need a transistor to switch 12 V that can be triggered using 3.3 V. It is very important that the circuit never allows anything over 3.3 V to get to the Flex cable.

The other thing that you need to design is a way to trigger one line on the amp from two lines on the Flex (e.g. 30/20). A diode OR should do the trick.