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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

I think I understand your situation and, as far as I can tell, you could
try using PICs with a 9-bit usart, such as the 18f4620. These PICs use
the 9th bit in the serial word to indicate whether an address or data is
being sent out to the 'networked' PICs. This allows up to 255 PICs to
have their TX & RX pins tied together with all the others on the network,
which will then only respond when their corresponding address has been
'called'.

 --------
|        |
|     RX |---+
| PIC 1  |   |
|     TX |-+ |
|        | | |
 --------  | |
           | |
 --------  | |
|        | | |
|     RX |---+
| PIC 2  |   |
|     TX |-+ |
|        | | |
 --------  | |
           | |
 --------  | |
|        | | |
|     RX |---+
| PIC 3  |   |
|     TX |-+ |
|        | | |
 --------  | |
           | |
      ( COMPUTER )

The only problem with this setup is connecting the 9 bit network to a PC.
You'll probably have to use another PIC with a 9 bit uart to translate
two 8 bit words from the computer into the single 9 bit word for the network.

- --
Brendan Gillatt | GPG Key: 0xBF6A0D94
brendan {a} brendangillatt (dot) co (dot) uk
http://www.brendangillatt.co.uk
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