Interfacing Ameritron RCS-4 with SainSmart 8 USB Relay Board

This is simple to connect, how ever if your asking if it will work using the FLEX USB control panel your on your own.

Simply the RCS-4 has a 4 position switch, a 12VAC transformer so you will need 3 of the relay contacts with the AC transformer tied commonly to the three relays.
It requires 12 Volt AC to one relay, another relay requires +12V DC, and the 3rd Relay requires -12DC, and the 4th position is ALL voltage off so the easiest way is to connect into the RCS-4 control box then the output of each SainSmart8 relay will connect inside the RCS-4 control box to the appropriate points. 

Hope this helps, I run mine from a Control by WEB 4 Relay controller. 
OH VERY important make sure you only have ONE relay on at a time, this is why I use the "Control by WEB" box as it uses Scripts and I built a script so only one relay can be on at a time.
AND don't have your Control Box switched to anything but the OFF position when using it remotely. 

73's
Bret
WX7Y

So Bret is 100% correct as i have an RCS-4. I can't speak to the SainSmart or Flex interconnect but you will replace the control box using the Sainsmart to send 0V, +12V (rectified AC), -12DC 9V (rectified AC) or 12V AC injected into the coax much like the control boc does. In effect, you replace the rotary switch with relays. You'll also have to figure out how to deal with the LEDs if you want visual indication as the control box has a 2 pole rotary switch, one for control and one for LEDs.