more serial port troubles

[Sam Varshavchik]

Mike Wright writes:

> Yesterday I installed putty and selecting "serial" gets me nowhere, so
> for spits and giggles I chose "ssh" and entered the hostname of the
> target machine.  It asked for my login name, presented my key and,
> voilá, I'm logged into the remote machine.
>
> That step alone verifies that com port hardware, flow rates and
> handshakes, and null modem cable all work.

I dont't see how it does. ssh doesn't know anything about serial ports,  
AFAIK. Furthermore, I'm unable to figure out how ssh, or anything else,  
would have the knowledge to map some arbitrary hostname to a specific serial  
port on my machine.

More than likely, ssh connected to your machine via TCP/IP, thus proving  
nothing.

> Does anybody else have any clues here; I obviously don't :/

This should not be a difficult problem to solve.

1) Verify that you have both serial ports wired correctly. Presumably, you  
have a "null modem" adapter wired in, to cross the appropriate pins. I used  
to do serial port hacking, and I used to remember all the pinouts by heart.  
Don't remember them anymore, but I still remember the logical names.

2) Run minicom on both machines, set them to to the same speed/parity/stop  
settings, but turn off flow control, and turn off echo. Use 9600bps, it's a  
reliable failsafe. It's possible that older hardware may not support higher  
speeds. It's possible that your null-modem adaptor does not cross the  
RTS/CTS and DTR/DSR pins, so hardware flow control will not work. If they're  
set to the same speed, anything you type on one machine, in minicom, should  
show up in the other one's screen, and vice versa.

3) If you're not getting any response from minicom, and you have them at the  
same speed/parity/stop setting, and no flow contorl, there could only be two  
reasons for that:

A) Your null-modem adapter is not a null-modem adapter.

B) One or the other minicom opened the wrong serial port device. If your  
machine has two serial ports, it's a trial and error which one is ttyS0, and  
which one is ttyS1. Even if your server has one serial port visible, it's  
entirely possible that your motherboard's chipset actually has two UARTs,  
but only one of them is wired to an actual serial port, so even if you have  
only one serial port sticking out, it may very well be ttyS1, rather than  
ttyS0.

Well, you also mentioned Xen, and if one of your machines is a virtual  
machine, it's possible that Xen is not connecting your VM's logical serial  
port to your actual hardware serial port. I don't know much about Xen, so  
you'll need to try to dig through its documentation to see if it says  
anything about supporting serial ports.

If in doubt, get rid of Xen temporarily, and run minicom natively on both  
hosts, and get them talking to each other. Once you've proven that you have  
the serial ports wired correctly, then bring Xen into the picture, and try  
to get it working with Xen.
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