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SubjectRE: [PATCH] serial: Revert RS485 polarity change on UART open
From: Matthias Schiffer
> Sent: 29 March 2022 11:39
> I guess that would work. The fact that even the different
> variants of the 8250 are implemented inconsistently makes this
> especially ugly... It certainly puts a damper on the efforts to make
> the handling of RS485 in serial drivers more generic.

One thing to remember is that RS232 (IIRC really V.38) line driver
chips are typically inverting.

So the modem signals on a TTL level output will have the
opposite polarity to that required on the actual connector.

Normally a UART will have an 'active high' register bit for
a modem signal that drives and 'active low' pin so you get
the correct polarity with an inverting line driver.


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