On Sun, Jun 22, 2008 at 11:29:20AM -0400, Olin Lathrop wrote:
> Yair Mahalalel wrote:
> > Regarding the RS4xx family, I find the descriptions in Wikipedia and
> > elsewhere quite complicated, but would I be correct to say that the
> > half-duplex, multi-master nature of RS-485 makes drivers built for it
> > a better match for SPI?
> 
> Not really.  Your signals are all point to point with a fixed direction.

That is only partially correct, as on the SPI bus all the nodes share
the same I/O lines. I want to make sure that the line drivers will not
pull the line when not in use, as might be acceptable in true point to
point communications.

> The only part of RS-485 that is of any use to you is the single to
> diffefential to single line conversion.

True. And I prefer to use an off the shelf solution instead of trying to
build a solution from not gates or something and taking care of timing
the transitions correctly myself.

> What you really want is just some
> differential line drivers and receivers, preferably ones that are matched to
> the impedence of your cable.

Indeed, but as I wasn't formally educated in electrical engineering and
my experience is very limited, I am unaware of the various
considerations that go into the problem, and since it isn't a simple
question with a closed answer, I acknowledged the limits of my
autodidactic abilities and contacted the list.

Nowhere are the gaps in my knowledge more apparent, perhaps, than in
matters related to line impedance, of which I heard only in the context
of sinusoidal waves. You mentioned earlier in the thread adding low pass
filters to the line. Wouldn't adding small capacitors at the ends of the
line serve, together with the source/line DC resistance, as low pass
filters that restrain overly sharp transitions of the driver? If that is
indeed a viable solution, what transition time should I aim at? Are the
pair impedance (~100 Ohm) and capacitance (~50pF/m) the relevant values
to put in the time constant equation?

> It may be that using off the shelf RS-485
> chips is a available way to get that if you hard wire each end to the
> appropriate tranmitter or receiver mode, but other talk of RS-485 is
> misleading.

I guess another of the shortcomings of only learning the narrow range of
subjects applicable to the problem at hand is that talking to people
with wider understanding tends to create confusion as to the context in
which terms are used. I thought it was clear I'm trying to adhere to SPI
as much as possible at the logical level and my interest in RS-485 is
only with regards to the suitability of the driver chips used with it to
my application. If I failed to convey the distinction, I apologize.

Yair.

> 
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