uart vs sio differences ?

Mike Tancsa mike at
Mon Dec 8 19:06:53 UTC 2008

At 01:49 PM 12/8/2008, Marcel Moolenaar wrote:

>On Dec 8, 2008, at 8:21 AM, Mike Tancsa wrote:
>>Unfortunately, we only control the FreeBSD side of things and the
>>other end of the serial connection is a windows app we dont
>>control.  Everything seems to work ok from our side, but the other
>>side which we dont control seems to be missing some things we are
>>sending it and vice versa.
>It looks to me that flow-control is disabled, is that right?
>Not only does uart(4) make use of the larger buffer of the
>hardware, it's also more efficient under puc(4) than sio(4)
>is because of the use of the serdev I/F. It's possible that
>the receiver can not keep up when uart(4) is used. A serial
>line analyzer should tell you more...

         Yes, flow control is supposed to be disabled.  When we hook 
up a serial line analyzer, the behaviour is rather odd.  We only use 
1200bps, so I dont think its a speed issue.  Also, as part of the 
protocol, we poll the other side.  We send a 3 byte poll, which the 
Windows side always sees, and it sends us back a 1 byte answer, which 
we see fine.  However, when the Windows side has "something to say", 
it will send a different 1 byte response (which we get) and then the 
data, which is approximately 100 to 200 bytes which we only get about 
30 bytes of.


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