Hi Vic,
Thanks, that is a very helpful and informative reply! Yes, I will have the DUE be the master and the AC board just sending time info every minute, say, or upon request by the DUE.
Certainly I want to keep those panel meters, the coolest thing about the Angular Clock kit!
I have already used your ICSP header to power your board from my DUE's 5V supply from its SPI header.
I am not a DUE expert, so a lot of the specs on their page are greek to me, maybe not to you:
https://www.arduino.cc/en/Main/ArduinoBoardDue
It does have two USB ports, both with micro connectors.
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"The Native USB port is connected to the SAM3X. It allows for serial (CDC) communication over USB. This provides a serial connection to the Serial Monitor or other applications on your computer. It also enables the Due to emulate a USB mouse or keyboard to an attached computer. To use these features, see the Mouse and Keyboard library reference pages.
The Native USB port can also act as a USB host for connected peripherals such as mice, keyboards, and smartphones. To use these features, see the USBHost reference pages."
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I can certainly handle the communication programming once I get the two boards talking to each other. I think that "Software Serial" is a library that does not work on the DUE, only Arduinos with ATMEL processors. But the DUE does have a ton of dedicated digital lines and ports that can be use for serial comm.
Here is a fantastic diagram of all the DUE's pinouts, by Rob Gray:
http://forum.arduino.cc/index.php?topic=132130.0
Maybe if you look at it, the best approach will be obvious to you.
Any extra insights you or other Forum readers may have are appreciated!
-Steve