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[questions] Re: GPS+PPS vs NTP server, why a huge offset ?



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
jitter...


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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