[ntp:questions] Re: Meaning of PPS flag2 -- assert/clear

jason at extremeoverclocking.com jason at extremeoverclocking.com
Sun Oct 15 16:34:05 UTC 2006


In RS-232 terms:

Assert = Positive Voltage = logic 0
Clear = Negative Voltage = logic 1

The only reason I mention the logic 0 & 1 is because in TTL terms it's
backwards. A logic 0 for a TTL signal is 0 volts, and a logic 1 is ~5
volts. Many GPS devices that have a PPS output at TTL voltage levels (0
- 5v) but are RS232 biased. So if you tried to stick like a MAX232 chip
on there to bring the voltages into the full RS232 voltage levels it
would end up being upside down. You would need a voltage inverter to
flip it back. It's best just to leave it as-is if the GPS device says
they are already RS-232 biased.

Back to your question though...

flag2 0   = Voltage goes from low to high at the timemark (default)
flag2 1   = Voltage goes from high to low at the timemark

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