(me too)! Re: [Soekris] rs485 instead of rs232
ticso at cicely12.cicely.de
Wed Jul 14 19:59:55 UTC 2004
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
> 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
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