On Thu, 16 Apr 2020, Hal Murray via devel wrote:
Because RS232 signaling is negative logic.
That's what I used to think, but somebody corrected me many years ago.
The data is upside down but the control signals are not.
From https://en.wikipedia.org/wiki/RS-232
under Voltage levels
For data transmission lines (TxD, RxD, and their secondary channel
equivalents), logic one is represented as a negative voltage and the signal
condition is called "mark". Logic zero is signaled with a positive voltage and
the signal condition is termed "space". Control signals have the opposite
polarity: the asserted or active state is positive voltage and the de-asserted
or inactive state is negative voltage. Examples of control lines include
request to send (RTS), clear to send (CTS), data terminal ready (DTR), and
data set ready (DSR).
But the polarity difference between data and control lines is precisely
the reason for the inversion.
TTL RxD/TxD signals are active-high, while RS-232 RxD/TxD signals are
active-low. The common level converters take this into account, and hence
are inverting. This means that active-high RS-232 control lines become
active-low TTL control lines when the same level converters are used.
That being said, there's nothing in the RS-232 standard about repurposing
a line meant for modem control for a PPS signal, so there's no "official
standard" for the polarity, and it's up to the receiver to decide that.
Unless it clearly states the polarity in the spec, you should check.
Fred Wright
_______________________________________________
devel mailing list
devel@ntpsec.org
http://lists.ntpsec.org/mailman/listinfo/devel
