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[questions] Re: GPS+PPS vs NTP server, why a huge offset ?
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- Subject: [questions] Re: GPS+PPS vs NTP server, why a huge offset ?
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- Date: Fri, 17 Jun 2022 12:32:59 +0100
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On 17/06/2022 00:55, chris wrote:
No argument with that, but some have tried to bypass a converter,
feeding the ttl pps into the rs232 port, which may work in some
cases. TLL pps low level, in particular, won't guarantee the rs232
input line to switch, whereas, of course, the ttl high will switch.
The rs232 ip needs zero or a minus level to properly work and avoid
In practice, RS232 line receivers will emulate the characteristics of
the 1489 or 1489A ICs (which are still available). In the simplest
configuration for those, the input low threshold voltage is no less than
+0.75 volts and the input high threshold voltage is no more than +2.25
volts, which makes nearly all real life true RS232 line receivers TTL
compatible. (The A variant has a larger hysteresis, and results in a
higher high threshold.)
The main reason for not using TTL are that is isn't designed for driving
more than a few inches of wire, and it doesn't have the huge noise
margins of true RS232 drivers.
In terms of level converting to TTL, the decades old 1488 line drivers
are also still available, although they need +/- 12V supplies, as are
the newer, but still decades old, MAX232 devices, that have charge pumps
to derive the 12 volts from a 5 volt supply. As such, there is no
really sensible reason to re-purpose device intended for shifting
between 3.3 an 5 volt CMOS levels.
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