EFI Bung in head update

On a nine pin DB9 pin 2 is rx, pin 3 tx, pin 5 signal ground. On DB25, pin 2 tx, pin 3, rx, pin 7 ground. Note pins 2 and 3 change. Then things get more complicated, since it depends on DTE or DCE. DTE is typically the PC, and DCE the MS, but DTE and DCE can sometimes be swapped in settings with old hardware ports.

Rx connects to tx at other end. Tx is transmitter, rx is receiver. When working in the field, with bear skins and stone knives, swap pins 2 and 3. One way will work, one not, no damage if wrong. There are breakout boxes with jumpers and leds that some use. I use scope and terminal emulator to send characters and see what is transmitting. Once you get it right all is well. So if loop back works, it is only a matter of getting 2 and 3 correct, and ground connection.

There is no power per RS232 standard, unless stealing only in mA from a handshake signal.