Serial ports

Serial port pinouts

Fig's 1&2 show the more common 9 pin serial pinouts, on both the computer side and also the device side respectively. Fig.3 gives a more visual representation of the male variant, most commonly found non the PC side. I think this was historically due to a male being more prone to damage, hence the female end being more commonly used on cables, but I may be wrong.

Fig.5 is particularly useful when testing cables, although I have only really connected pints 2&3 for this purpose. The idea here is that you make a plug that connects the pins together and then you can open the port and whatever you type should be echoed back exactly. 

Fig's 6-10 provide details about Null Modem connectivity.

Fig.11 provides more pinout details for the DB25 connectors

Printing control characters

We can echo a control char as per the below;

echo -e '\x07'

The above will give us a BEL

if we want to add control characters to a file, we can do;

printf 'hello there \x07 my name is Snake' > test.txt

if we then cat test.txt, well see the text and a BEL

We can of course send this to a serial port with something like cat test.txt > /dev/ttyS0