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[questions] Re: NTP community feels broken

On 19/06/2022 11:30, chris wrote:

Yes, it does have a spec for the slew rate, but i'm not sure that would
always be met for modern devices, as such uarts can often be run at data
rates of up to 230 Kbytes / second, or about a 4uS bit time. That
implies much faster rise and fall times, which relates to slew rate.

But those aren't RS232 compliant devices, although the slew rate limit is still low enough to support those data rates, but not with the full cable length.

We can theorise about that, but iirc, I bought two ttl to rs232
converters at the time. If I can find the second one, i'll set it up
on the bench and measure the propagation delay on a scope from input to
output. Also, see what effect the 2.5nF cap has on the waveform and timing...

The 2.5nF is mainly intended to represent the capacitance of the cable, which is deliberately operated with both source and load termination impedances well above the characteristic impedances, so behaves like capacitor. With the full length cable, rather than the lumped capacitance, you would expect the voltage to staircase upwards, each time the leading edge reflection returns.

The 2.5nF basically allows for the 50 foot maximum cable at 50pF/ft.
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