I agree with Dennis Laczny that a PC system can not provide accurate
timing. It is a combination of the internal clock and the interrupt system.
In the DOS environment, the kernel can be modified to provide nearly 1
microsecond precision. The accuracy over the course of a timed run is
pretty good. The Windows environment is far less timer friendly.
However, it's the relatively low interrupt priority on the input ports that
causes the greatest innacuracy. The PC gives the much higher priority to
the disk and keyboard subsystems. If the PC timing system is using the
timing of the bus interrupts caused by the trip devices, it will either
miss some of them or have occasional inaccuracies depending on the nature
of the input pulse from the trip. This is not reliable enough for a system
which is supposed to have 0.001 sec accuracy.
If the PC system employs an input card that passes a time stamp to the PC
via the system bus when it receives a trip, that IS an accurate way to
provide timing. The content of the time stamp is independent of the timing
of the system bus communication.
Someone posted earlier that the stand alone timers are computer based and
should have this same problem. This is not the case because the stand
alone systems employ "embedded control" microprocessors which are designed
to accommodate accurate timing of external control inputs.
This has been a great opportunity to discuss a topic that seems to be of
broad interest. I too would be glad to provide additional detailed
background on hardware and software timing features.
John Carriere
JACircuits Autocross Timing Systems
(734)665-3322
jacircts@mich.com
http://www.mich.com/~jacircts
Dennis Wrote:
>
>John's point of having to reset the PC time-of-day is right on. A PC is
>driven by an interrupt system and the I/O ports where sensors are connected
>and the internal time-of-day clock are the lowest priorities due to the
>slow data rates (compared to what is going on inside the PC at 200Mhz +)
>and this lower priority causes the time-of-day clock (and the .001 second
>requirement) to vary slightly.
>
>All of the software timing solutions are cost effective (if you don't count
>the cost of the PC) and offer the driver registration and scoring, some
>offer the timing also. Software timing will vary slightly from one PC to
>another based on what's in the PC, so comparing results from one region to
>another would not be accurate to .001 seconds. Comparing results from one
>event to another would require using the same PC hardware for both events,
>but even then, that would not guarantee .001 second accuracy for both
>events.
>
>Hardware timers have accuracy far beyond the .001 seconds. Comparing
>timing results from one event to another or one region to another would be
>completely accurate irregardless of which hardware timer was used (okay
>there are some really old timers that would be excluded).
>
>- --- I meant 'timers that are old' and not 'old-timers', okay ---
>
>Most hardware timers and most software packages interface to large displays
>to display race results. The only difference here is what information can
>be displayed, that varies with the display as well as the hardware/software
>used to score.
>
>Cost is always a factor in timing equipment. Redundancy should be
>considered just as important, after all, when the automated timing goes
>away ... it's back to stopwatches.
>
>I would be glad to provide additional detailed background on hardware and
>software timing features and advantages. If you are interested, give me a
>call at (408) 988-6188 or reply back with a phone number and I will give
>you a call.
>
>
>Dennis Laczny
>RACEAMERICA Timing Systems
>http://www.raceamerica.com
>
JACircuits Autocross Timing Systems
http://www.mich.com/~jacircts
- Timing the SCCA Solo II Nationals since 1985
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