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SubjectRE: [PATCH] serial: Revert RS485 polarity change on UART open
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On Tue, 2022-03-29 at 12:55 +0000, David Laight wrote:
> 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.
>
> David
>

Indeed. As far as I can tell, this property of UARTs is what got us
into this mess: Some people interpreted SER_RS485_RTS_ON_SEND as "set
the RTS flag in the MCR register on send", while other thought it
should mean "set the RTS pin to high on send", leading to opposite
behaviours in different UART drivers (and even different UART variants
in the same driver, in the case of the 8250 family).

Regards,
Matthias

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