[time-nuts] RS 232

[briana]

Ever since WINxp arrived on the scene hams who send code  via computer 
to radios via parallel, serial or usb ports (with serial port converters 
following) have seen the latency issue in spades.  We're talking about 
effective baud rates less that 50.   3-4 milliseoond variable latency 
changes making the code nearly unreadable.   The killer is that the 
latency changes randomly.

Previous to WINXP one could do direct writes to the ports under software 
controlled timing.  All was good.

The solution for WINXP  was to bypass WINDOWS handling of port data via 
a DLL called DLPORTIO
There is a similar one for WIN7.   I haven't timed how accurate it is.  
However 65 words per min (6 characters/second) code can be sent with no 
detectable timing problems.

The simple act of open and closing a set of contacts at precise times 
now requires a huge, faxt machine, tons of software and software to work 
around the normal software.   That's progress?

Brian


On 7/25/2013 10:40 PM, John Miles wrote:
>> john at miles.io said:
>>> Agreed, nobody should be using RS232 for anything nowadays.
>> RS232 works much better for capturing PPS timing.
> Unless you are watching it with a ring-0 (kernel) driver, and/or using a
> hard realtime OS to run the client software, it really won't matter that
> much.  Anyone running Windows or most flavors of Linux has more to worry
> about than the distinction between USB and RS-232, when it comes to latency.
>
> For truly critical applications it's best if the counter itself does the
> timestamping.   For ordinary NTP use on Linux or Windows the distinction
> between RS232 and USB is pretty questionable.  Submillisecond jitter has
> been documented in USB PPS applications (e.g.,
> https://lists.bufferbloat.net/pipermail/thumbgps-devel/2012-March/000109.htm
> l ), albeit with unspecified latency.  If that's not good enough, you need
> to tackle the issue somewhere besides the physical layer.
>
>> Another advantage of RS232 over USB is that the configuration is stable
> when
>> things get unplugged and replugged, or powered off, or ...  Of course,
> that's
>> a disadvantage if your program wants to know when the gizmo got unplugged.
> USB devices have gotten a bad reputation in this regard because of
> developers' failure to understand the idea behind serial numbers.  As with
> noise immunity, it's possible to do it right, it's just that too many people
> don't bother.
>
> -- john, KE5FX
> Miles Design LLC
>
>
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