uart vs sio differences ?

Marcel Moolenaar xcllnt at
Mon Dec 8 21:02:20 UTC 2008

On Dec 8, 2008, at 12:49 PM, Mike Tancsa wrote:

> At 02:18 PM 12/8/2008, Marcel Moolenaar wrote:
>> I see, so the FreeBSD box with uart(4) is missing data,
>> not the Windows machine, right?
> Hi,
>        Correct.  The FreeBSD box never gets the full data, nor do we  
> see it on the "protocol analyzer".  Our analyzer is just a special  
> serial cable that copies the data from both sides to the monitor  
> program (a dos app) on another machine.

Interesting. If it never shows up on the analyzer, then
doesn't that indicate that it was never sent?
Put differently: doesn't that indicate that the transmitter
stops sending, and not that the receiver stops receiving?

>> Do you know if you get the first 30 bytes or the last 30
>> bytes or some mix?
> Just checked, and we get the first 31 bytes each time.


Could you check if any of RTS/CTS, DTR/DSR or DCD toggles
after about 30 characters?

If the analyzer also just gets the 30 characters, then maybe
the receiver signaled the transmitter to stop (think of the
16950 as having HW flow-control enabled and doing it on its
own, without knowledge of the kernel).

>  Is it possible the larger fifo buffer of the 16950 is holding onto  
> the data too long ? The sio sees it as a plain old 16550, but the  
> uart driver sees it as the 16950 that it is.

The 16950 has a 128-byte FIFO, so even if it holds on to
data too long, I would expect to receive at least 128
bytes of data...

Marcel Moolenaar
xcllnt at

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