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[questions] Re: NTP community feels broken



On 19/06/2022 16:10, William Unruh wrote:
So teminate it to get rid of the refections.

It's no longer RS232 if you terminate it anything less than, ISTR, 4k, or reverse terminate in anything other than 300 ohms.

I think 100 ohm is typical for the characteristic impedance of a wire pair, so given the open source voltage of the driver is 12 volts, by terminating it you have removed all your noise margin, if you accept the transition region is the full +/-3V.

If you want to use an EIA standard that uses terminated lines, you should use RS 422, which also uses differential pairs, which also improves noise immunity. Or RS 423, which, although it uses a differential line receiver, is unbalanced, with one of the pair tied to local functional ground at the transmitter.
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