Through this I can see that it is the ttyS0 that has an electrical connection.
I'm sorry, but you're mistaken here.
The RTS and DTR signals only indicate that something is activating the port on the local side, i.e. that there is
agetty or a similar program running on the port on your computer. They tell nothing at all about the remote side, since both are outgoing signals. Use
fuser -u /dev/ttyS0 as root to get the PID of the local process that is activating the port.
If you don't believe me, run
screen /dev/ttyS1 and while it's running, run
cat /proc/tty/driver/serial again in another window. You should now see the ttyS1 RTS and DTR signals active too.
The CTS and DSR signals, on the other hand, would indicate two things:
- the cable actually has those lines connected (some serial cables only have three wires: incoming data, outgoing data and ground) in some way
- if the CTS and DSR signals appear alone, without RTS and DTR, then and only then it would indicate there is something on the other end for sure. Some null modem cables connect local DTR to local DSR and local RTS to local CTS in each connector, as a workaround for the situation where the device at one end must use hardware handshaking, and the device at the other end cannot do that. In that case, the incoming CTS and DSR signals would always appear as soon as the port is powered up and the outgoing RTS and DTR signals are asserted, even if the other end of the cable was not connected at all.
Frequently, you must also have the parameters of the serial connection right before it works at all. The most common variable is the baud rate (speed) of the connection: on a console port of a T5140 I'd first try 9600 bps. Other devices might also use 38400 or even 115200 bps; those are the most common values in my experience.
The other parameters are the number of data bits (8 is the most common value today), parity ("none" is the most common), and the number of stop bits (1 is the most common).
Even if you have all the parameters correct, the other end might not send anything until it gets some valid characters as input. So, once you've started your
minicom, press Enter once or twice.
Most serial console connections work on the "remote echo" principle: when you type something, the characters typed are just sent out the serial port without displaying them on the screen. Only when the remote end echoes the characters back will they be displayed. This allows for things like entering passwords without them being visible on the screen (as the remote end just disables the echo function for password input), and tells you that the characters you typed are correctly received.
On serial connections that are not intended for human use, this remote echo function may be omitted: if you are troubleshooting such a connection, you might want to activate the "local echo" function in your terminal emulator program (Q in the "screen and keyboard" settings section in
minicom, for example). If you have both local and remote echo in effect simultaneously, anything you type appears ddoouubblleedd on your screen.