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May 11, 2006
about 8 years ago
The I and O on the TX and RX pins refers to those pins, specifically. The RX-I pin, the one on the four-pin header, is an input, and this is connected to pin 2 (RX) on the DE-9 connector. The TX-O pin, the one on the four-pin header, is an output, and this is connected to pin 3 (TX) on the DE-9.
This makes your microcontroller a DCE (modem), as opposed to a DTE (terminal), and as such, you should connect this to a computer (which is a DTE) using a straight-through DE-9 cable, not a null-modem cable.
With regard to level shifting, this board does not have a MAX232 or equivalent, so it does not comply with the classical RS232 spec (0 = Space = +12V, 1 = Mark = -12V). Instead, its input will accept RS232 voltages on the input and shift them (-12V [mark] -> VCC, +12V [space] -> GND). For its output, it merely inverts the signal (VCC->GND, GND->VCC) so you must be using a newer UART that will tolerate this.
I believe this is accurate - if it’s not, feel free to correct me.
(Edited, accidentally referred to the TX-O pin as the TX-I pin.)
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