The PLL is a TB31202 with a serial interface so I used an Arduino Nano module to watch the PLL data and work out how the native processor was communicating with the PLL. After that I intercepted the PLL data to determine what native processor was doing and rewrite the data I wanted to the PLL. From this I was able to remap the USA channels to the New Zealand channels and that my project was viable.
I then planed my objectives:
- Allow selection of any US or NZ channel from the existing CB front panel.
- Add a scan function.
- Add remote control via the accessory port on the stereo system head unit.
For dual band operation I simply swap bands at wrap around between channels 1 an 40. So for example if you are on NZ channel 40 and you use the channel knob to go up you will be on US channel 1. Stepping down from channel 1 will also swap the band and take you to channel 40. To provide feedback about which band you are on I replaced the yellow backlight with a RGB LED. Green indicates NZ channel when receiving with the squelch closed, blue indicates the same for US channels. Other colours are used when the squelch is open and when transmitting.
For scanning I redirected the CH9/19 button as a scan start stop.
For remote control I based it on the CAT interface used on my Yaesu FT-857. I added audio to that connector to make for a tidier cabling solution. I started using the Yaesu CAT protocol but didn't find it very flexible so switched to the protocol used on the head unit and created a new message type specifically for the task at hand.
The project is almost complete with the remaining work being head unit features.