Multiple PICs on usart network

[Demon]

Nope, not my video. That's Jose's and he uses transistors. I preferred a software solution, no extra parts involved.

All Slaves keep their TX line disabled until they are spoken to by the Master and have to send back a reply.

This post is for the next guy that tries to do this. A few things I learned:

- you can put a basic MAX232 on the TX and RX lines and "watch" what is being sent around your network.
- 4K7 pull-up resistors are good for master and slave RX pins (supposedly only necessary in case PIC is disconnected).
- a structured approach to communication helps, ie: a PIC address, a command, data and checksum.
- slaves then look at the PIC address and decide if it applies to them, a "calling all slaves" address is also useful.
- a local command can help support various processing.
- data can be as big as you need it.
- and checksum can be as simple as addition of every byte sent, a word variable has more room.
- RS232 is great for Master to multiple Slaves, as long as only 1 slave replies back UNLESS:

...
TXSTA.5 = 1                                 ' Enable USART TX function

HSEROUT (slave sends reply only when spoken to by master)

TXSTA.5 = 0                                 ' Disable USART TX function
...

There's more than one way to skin a cat, but this was by far the easiest to have more than 1 slave on the MASTER RX line. There may still be an issue with this solution, but I have 6 slaves sharing a master RX line right now and they're all purring nicely.
...

The only addition I would add to my list is using a MAX232 on each PICs for stability when using very long cables. It's not known at this point how "long" a network has to be for USART to become "less reliable".

My project will communicate underground between the house, garage and shed (at least 100' end to end). I plan on using MAX232 chips; it's not like they are expensive.

Robert


EDIT: The existing "fried" commercial alarm system has a MAX232, so it's a known solution.