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

John Ackermann N8UR jra at febo.com
Sun Oct 15 17:28:02 UTC 2006

Thanks, Jason.  Interestingly, it's really hard to find a clear answer
to the definition of "assert" and "clear" on the web.

One reference I'm looking at now says:

A low level (–3 to –15 volts) is defined as a logic 1. asserting a logic
1 or turning on a signal is historically referred to as “marking”.

A high level (+3 to +15 volts) is defined as a logic 0. Releaseing to
logic 0 or turning off the signal is referred to as “spacing”.

That would indicate that "assert" is going positive-to-negative voltage
or going from logic 0 to logic 1.  By implication "clear" would be the
opposite, or a negative-to-positive transition.  That would indicate the
opposite of what you said.

But another reference,
http://www.camiresearch.com/Data_Com_Basics/RS232_standard.html, says

Pin 8 - Received Line Signal Detector (CD) (also called carrier detect)
This signal is relevant when the DCE device is a modem. It is asserted
(logic '0', positive voltage) by the modem when the telephone line is
"off-hook", a connection has been established, and an answer tone is
being received from the remote modem. The signal is deasserted when no
answer tone is being received, or when the answer tone is of inadequate
quality to meet the local modem's requirements (perhaps due to a noisy
More generally, that reference consistently uses "assert" to mean a
positive voltage, logic 0.  But it doesn't use the term "clear" in that
context but says "deasserted".  I can't vouch for how definitive this
reference is, but it seems to be thorough and internally consistent, so
I tend to believe it.

Based on that definition, your statement is correct.  It also seems
clear that there's plenty of room to get very confused about all this.

We really need to document this.  When I get the strength, I'll try to
add something to the wiki.



jason at extremeoverclocking.com said the following on 10/15/2006 12:34 PM:
> John,
> 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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