Hi Holger
Thanks for getting back to me. After analyzing the issue, I believe the issue is the overrun error the USART reports. The issue appear to stem from the fact that each byte received by the USART is handled in the interrupt handler, but the USB sending code locks out the interrupt for the entirety of its call. So I change the code in the following manner and it appears to solve the issue for me:
1) Increase the small buffer to be large enough to hold 1 typical apdu: 270 bytes: 4 byte SIMTrace header, 4 byte command, 2 byte status, 1 byte instruction that appears to be repeated?? 256 bytes payload
2) Disable USB interrupt before deciding to Continue to send stuff to USB until returning from function.