linux-kernel - RE: [PATCH] serial: Revert RS485 polarity change on UART open

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Message-ID: <749eee7dd2c7464a8c4d9ea5972205fa@AcuMS.aculab.com>
Date:   Tue, 29 Mar 2022 12:55:42 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Matthias Schiffer' <matthias.schiffer@...tq-group.com>,
        Lukas Wunner <lukas@...ner.de>
CC:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Jiri Slaby <jirislaby@...nel.org>,
        Russell King <linux@...linux.org.uk>,
        "linux-serial@...r.kernel.org" <linux-serial@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Ilpo Järvinen <ilpo.jarvinen@...ux.intel.com>,
        "Lino Sanfilippo" <LinoSanfilippo@....de>
Subject: RE: [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.

	David

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