(me too)! Re: [Soekris] rs485 instead of rs232

[Bernd Walter]

On Thu, Jul 01, 2004 at 11:26:35AM +0200, Poul-Henning Kamp wrote:
> In message <8B966E43-CB1C-11D8-B611-000393DBC6BE at gridworx.ca>, =?ISO-8859-1?Q?S
> =E9bastien_Taylor?= writes:
> >Tip/cu should work fine with rs-485, as would any other serial app.  
> >rs-485 uses the same uarts as rs-232, it's just the physical medium 
> >that is different. rs-485 also supports addressing using switches, not 
> >quite sure how those work, but they're invisible to software.
> 
> No, this is about as incorrect as it could be.

It depends on the bus topology and hardware capabilities.

> RS-485 is RS-422 in multidrop configuration.  The signals are differential
> and can therefore run very long distances if the cable is sensible and
> termination is correct.
> 
> Being multidrop, you need to tell the hardware when to drive the
> line and when to go tri-state.  This happens in all sorts of weird
> ways.  Some cards figure out for themselves, some even disable the
> receiver so you don't hear your own transmissions.  Some cards you
> have to wiggle RTS to enable the transmitter, some you have to wiggle
> other bits.

In full duplex environments you usually have a single master which is
the only transmitting device for that line pair, which allows having it
enabled all the time.
It's also not unusual to have automated direction control in
half-duplex environments.

> Addressing and all that happens higher up, although I don't doubt for
> a second that hardware exists which implement some protocol for that.

As I already wrote - I've put the protcol into an external USB
hardware - this makes many things much easier, but the device is
not generic anymore.

-- 
B.Walter                   BWCT                http://www.bwct.de
bernd at bwct.de                                  info at bwct.de