How to interface Yaesu FAS 1-4 antenna swich with 6400 for auto swithing

I had a quick look at the manual for the FAS, and it 'can' be done, but it will not be a plug and play solution.

You will have to break out the **** driver and interface a Bit mapped relay board that will essentially work in place of the remote switchbox.  This is described in the USB document on the download area.

This is the board https://www.amazon.com/SainSmart-Eight-Channel-Relay-Automation/dp/B0093Y89DE

BIT Cables

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

• ●  Pin – Enables/disables each pin/bit individually

• ●  Polarity - Controls whether the pin/bit is high or low when triggered

• ●  Source - Determines which source is used as the frequency check for this bit/pin. The sources

  are the same as for the CAT cables, listed above.

• ●  Trigger - Determines the band/frequency range that the source must be within for this bit/pin to

  be active.

• ●  PTT - When set, the pin/bit will become active only when the radio is in transmit and the source

  frequency is within the output band/frequency range. Individual pin/bit PTT and TX Delays canbe set and will be used by the radio when keyed.