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SubjectSerial line, was: Re: 921Kbps on Linux
>>>>> "Theodore" == owner-linux-kernel-digest  <> writes:

Theodore> ...

Theodore> First of all, given the RS-232 specs, there are real distinct
Theodore> limitations as you go to the higher speeds. The original
Theodore> RS-232C specification specified a maximum speed of 20kbps
Theodore> before noise, signal degredation, etc. would become a problem;
Theodore> the original RS-232 specification didn't support anything
Theodore> faster than 20,000 baud. Of course, the original RS-232 spec
Theodore> also said that the maximal cable length was 50ft., and what
Theodore> has happened over the years is that we've used faster baud
Theodore> rates with shorter cables that are very heavily shielded. But
Theodore> the faster you go, the shorter the cable has to be and the
Theodore> better quality it had better be. So at the hardware level,
Theodore> anything above 115200 bps starts requiring very careful
Theodore> attention to cabling issues. Even at speeds as "low" as
Theodore> 230400, I've found that cables can be the reason why things
Theodore> don't work. You can't be using long, cheesy cables! At
Theodore> 921kbps, while I haven't tried it myself, I'd surprised if
Theodore> cables longer than 3ft worked at those speeds without seeing
Theodore> data corruption.

Hallo Theodore,

the signaling protocoll on the serial line is not necessarily RS232
only. RS422 is another option for the serial line. The speed on these
differential lines still has some reserves, as 10 MBit ethernet on Twisted
Pair is standard and uses the same signaling. So 921kbps probably is not the

Implementing serial communication is not too hard with some external
processors, and perhaps for data acquisation having a high speed option is
fine. Using USB or Firewire will sure mean much more effort in that case.

Even with the standard PC, RS 422 is an option in this situation, as
translating from RS232 to RS422 can be done with few chips, and RS422/485
cards are readily available.


Uwe Bonnes

Free Software: If you contribute nothing, expect nothing

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