[lug] 16550 Serial port buffering?

Scott A. Herod herod at interact-tv.com
Fri May 25 09:56:35 MDT 2001

"D. Stimits" wrote:
> "Scott A. Herod" wrote:
> >
> > After 3 days of trying to get it to work....
> >
> >   "man setserial"  search for low_latency.
> Makes me wonder if this could have a beneficial effect on the serial
> port of a 56k modem while working with games that require low latency.
> When I get around to it, I think I'll give it a try (not exactly serious
> work, but hey, someone needs to be less serious).
> D. Stimits, stimits at idcomm.com

I think that it would depend on the rate at which you are receiving
data.  From watching a scope and reading the man page, it looks like
the serial port ( with default latency ) buffers for about 10 msec 
if it gets a small ( 4 byte in my case ) burst.  I imagine this 
happens to wait for more data before triggering an interrupt.  

I was needing to get 60 hz clock ticks ( 59.97 hz actually I think )
and start sending a response in about 11 msec.  Now, setting the
low_latency flag, I can get data in and out in 4 msec consistantly.

I'm not sure why 4 msec seems to be the minimum.  I was watching
on a scope a pulse coming in followed by my response to a read 
followed immediately by a write.  It's consistantly 4.3 msec.
I've tried using select, poll, and just looping on a read and
it doesn't seem to change how fast I can turn around.  


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