I must be missing something very simple. I have hooked an Arduino UNO to a MAX232N. It is from the sample and does nothing fancy. Takes input and echos back. Connected with ZTerm on OSX it works as expected.
if (Serial.available() > 0 )
incomingByte = Serial.read();
What I am confused about is that when I power via USB, the LED hooked to PIN 11 of the UART flashes on TX. However, when I am not hooked via USB but to an external power supply it doe not. TX (Pin 1) on the UNO goes to RX on the MAX232. If I look at the UNO schematic it looks like only the USB goes through the UART and in other situations the UART is bypassed. So is it correct to assume that on the TX pin (#1) on the UNO board I am simply getting a HIGH and LOW signal which the MAX picks up and converts to appropriate RS-2323 levels? No going through the UART and hence no LED activation. If that is the case then I would need to add an LED and resistor from TX to GND?