Re: Using (various stuff) to expand PIC outputs



"Peter S. May" wrote:

Eeyore wrote:
"Peter S. May" wrote:
Peter S. May wrote:

I'm going to investigate RS-485 as far as possible with my current
intellectual means, now that you're not the only one to have suggested
it. But, in the meantime...

I'm so far under the impression that it's similar to what RS-232 would
be if it used differential signaling. Is it possible to keep the TX
lines of RS-485 and discard the rest; i.e., can/would I use RS-485
drivers without implementing the whole of RS-485? Would I have one TX
line each for signal, shift clock, and latch clock?
Follow-up on this: Would it be reasonable to use RS-422 instead? The
data sheets seem to be easier to understand and the receivers and
drivers appear to be quite a bit less expensive. Thoughts?

How far and how fast does your expansion need to be ? Does it need to interface
to any 'foreign' equipment ?

The only 'foreign' interface is an RS-232 connection to a computer,
maintained separately.

Right, so we can ignore that (put it to one side) from the perspective of this
specific requirement.


The distance could be anywhere from 1m to 3m.

That's 'no distance at all' to my mind. Obviously not suitable for the 'CPU bus'
itself but not a mega-issue.


The output refresh rate,
in toto, needs to be 60Hz or faster (making the maximum clock around
100us, not much of a requirement) but a potential later expansion for
input needs to be much faster than that (my ballpark figure is 20kHz
refresh rate, but I'm sure it could be much slower and still work, and
there will be far fewer inputs than outputs).

And how much data needs to be refreshed during that period ?

60Hz is VERY different to 20kHz (as in 300 times different) so you ought to define
your spec NOW or you'll likely get bitten in the ass.


So far I only see ppl suggesting what may be excessively complex and expensive
solutions.

I have to agree, at least concerning RS-485. But the RS-422 devices
seem to be less imposing in terms of complexity, being nominally
TTL-to-differential converters, and are quite a bit cheaper.

Yes..... but RS232 has all that voltage translation if done properly. Arguably RS485
is simpler.

I'm sorry, but it's effing *INSANE* to use RS-232 or RS-485 to go 1 metre ! or even 2
metres. What is everyone thinking of ?

You need to stop and think hard and *define* your required maximum data rate. Trying
to design kit without a defined spec is a mug's game. Potentially a soon to be
unemployed mug's game.

Graham

.

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