[time-nuts] Thunderbolt/Arduino RS232 Interface Issue

[Dan Quigley]

Hello,

I'm integrating an Arduino (Due/Mega have tried both) with a surplus Thunderbolt and need some advice.  

The Thunderbolt is a known working unit (supplies 10Mhz to my HPSDR rig) and the serial interface works fine between it and a PC.  I've read the coverage about this kind of integration (and some of the archived discussions on this board) and am employing a commercially available MAX232-based "shield" (RS232 V2) to handle the level translations.  Just three lines (RX/TX/GND) are  used.  The MAX232 circuit is working, reliably passing data between the Arduino and a PC.  

With the Thunderbolt is connected to the MAX232 no data passes.  Activity LEDs on the non-RS232 of the MAX232 circuit show no activity when the Thunderbolt is providing data, indicating the MAX232 is not performing the intended level translation.  I've scoped the RX/TX lines coming from the Thunderbolt both when connected to the Arduino and not.  There is a distinct difference. When disconnected, the pulses are about 10.3v and nice, crisp and square.  Connected, the pulses are ~50% lower in voltage (4.1v) showing an exponential rise in voltage and trace noise. 

I'm pretty much stumped at the moment and wonder if anyone can offer a suggestion or thoughts on a direction to take.

Thanks in advance,
Dan Quigley (N7HQ)