Re: [PATCH/RFC] 8250: Auto RS485 direction control

[Laurent Pinchart <laurentp@cse-semaphore.com>]

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On Monday 04 August 2008, Grant Edwards wrote:
> On 2008-08-04, Tosoni <jp.tosoni@acksys.fr> wrote:
> 
> > Last remark:
> >
> > Interestingly, the RTS envelope on the Oxford chips is
> > implemented with... the DTR pin. On our cards we have a piece
> > of hardware which redirect the uart DTR pin to the external
> > RTS in this case.
> 
> Well, that's just plain wrong.  The RS-232 standard was quite
> clear that RTS is what's used to enable transmission.  ;)

UARTs are not restricted to RS-232. The pin labelled RTS in the 16850 datasheet is a dual purpose pin that can be used either as RTS in RS-232 mode or as a bus direction signal in RS-485 mode.

Beside, even in RS-232 mode, RTS has at least two widely used modes. The historical usage asserts RTS when the DTE (computer) wants to send data. It then waits for the DCE (modem) to assert CTS and then sends data on the TX line. This was used with half-duplex modems. The newer flow-control usage asserts RTS when the DTE is ready to receive data. The DCE uses CTS for the same purpose in the opposite data flow direction.

This means we already have at least 3 hardware RTS/CTS modes that we need to support in the serial API.

-- 
Laurent Pinchart
CSE Semaphore Belgium

Chaussee de Bruxelles, 732A
B-1410 Waterloo
Belgium

T +32 (2) 387 42 59
F +32 (2) 387 42 75

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