Re: capturing Serial Port with pcap

[Phil Vandry <vandry () TZoNE ORG>]

On 2009-10-28, at 12:06 , Paul B. wrote:
> I need to simultaneously observe the timing of ethernet traffic and  
> a discrete
> control signal. For the discrete signal, I was thinking that I could  
> tie the
> signal to a control signal such as CTS or RTS on the serial port and  
> modify
> the serial port driver to send a character or maybe a known PPP  
> packet when
> the discrete signal changes. Then, pcap would timestamp and capture  
> both the
> real ethernet packets and the PPP packet. Does this make sense?  Has  
> anyone
> done anything like this? Assuming RedHat linux on a relatively new  
> PC what
> kind of timestamp resolution might one expect?

Take a look at the source for gpsd and ntpd to see how they does it.  
When using a serial GPS as a stratum 0 source, a modem control signal  
on the serial port is used to mark the exact moment that the second  
changes. Because it's NTP, they have presumably gone to some length to  
make sure they get a good, precise timestamp.

-Phil
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