[ntp:questions] GPS clock for Linux

Martin Burnicki martin.burnicki at meinberg.de
Mon Sep 15 09:12:24 UTC 2008


Uwe Klein wrote:
> I will have to read up if the usb2serial ics have some kind of fifo
> included ( and enabled or not ). This could be an explanation.

The basic problem with serial to USB converters is that the serial lines
basically use transmission of characters whereas USB transmission is
block-oriented.

If a converter receives one character from the serial side it either can put
that single character into an USB block and send that block immediately,
which generates much overhead but low latency, or it could wait for a
certain interval whether additional characters arrive, which could be
packed together and sent in a single USB block. A new USB block is sent if
either enough serial characters have arrived, or no characters are arriving
anymore and a timeout occurs.

The latter is good for USB throughput but generates a latency up to the
timeout limit.

So the behaviour should depend on the implementation of an individual
converter type. BTW, similar problems arise with serial-to-LAN converters
which redirect a serial data stream using IP packets.

Martin
-- 
Martin Burnicki

Meinberg Funkuhren
Bad Pyrmont
Germany




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