On Sat, Aug 23, 2008 at 08:13:38AM -0400, Jinx wrote:
> Progressing with the network sensor project of a couple of weeks ago
> 
> The set-up settled on is a group of up to 5 PIC-based analogue and
> digital sensor units per network node. Each unit will have a unique ID
> and they will respond to network requests directly when called or
> separated in time according to their ID if called as a group. Either way
> there would/should be no collisions
> 
> A prototype with just one PIC unit has been installed remotely and tests
> OK, reporting various voltages, switch states etc. Kind of cool reading
> a battery voltage and temperature several miles away. I'm sure the novelty
> will wear off
> 
> Two questions -
> 
> 1) how could MAX232's be paralleled ?
> 2) I'd rather not use a transceiver IC if the job could be done with a few
> transistors. I realise this will mean making a -ve voltage for data '1',
> which
> may have to be distributed too
> 
> The background ...... and attached graphic
> 
> Network interface to that PIC's Rx and Tx is via a MAX232. The next
> step is to build a few more of these units and stack them up in parallel,
> connected by a 4-way cable of 0V, 5V, Rx, and Tx, to a distance of a
> few metres
> 
> So all the MAX232 Rx are tied together, ie all the pin 8s or 13s. For
> the Tx's I think I'd have to use at least one transistor to isolate the
> MAX232 driver from the Tx line going back to the network. That's
> assuming I stick with the MAX232, but I'd rather not if it can be done
> a better way (cheaper, no unused transceivers) if possible

Your project just screams for a RS-485 network. Take a read of this Maxim
AppNote:

http://www.maxim-ic.com/appnotes.cfm/appnote_number/1833

You'll be able to hook up to 32 nodes in a shared half-duplex network and
be able to transmit at speeds upwards of 5 Mbps. Of course node selection,
collision resolution, and further interfaces to RS232 are left up to the
reader. ;-)

RS-232 was designed as point to point. Think about using the right tool for
the job.

BAJ
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