On Sun, Aug 05, 2001 at 10:27:21AM +1000, Peter May wrote:
> Hi all. First off let me say I only tag along on this group as a second
> source of info. I use PicBasic to program and cannot understand assembler at
> all but still the engineering bits and pieces handy and informative. In no
> way am I knocking assembler but am more knocking myself for not taking the
> time to learn it.
>
> Anyway the question.
>
>
> I want to create a network of micro's. So far rs485 seems the go. What my
> problem is how to get them all to talk and not have I suppose PACKET
> colisions.
>
> I have a PC that is connected to the world that I use as a Windows server
> program. It is connected to the serial port and can send and recieve info
> from that port.
>
> I have several pics on the network.
>
> I was going to make a hub or a main pic. One part of the hub connects to the
> pc and another to the rs485.

Bug probably isn't the correct word then, gateway is probably better.

>
> What is a good way of getting these devices to communicate? I was going to
> have each device listen for an incoming ID string but this meant (well in
> pic basic anyway) that part of the time it was simply sitting there
> waitning. If nothing came in it would go off and do the rest of the program
> and then come back and listen again. How would the hub know it was sitting
> there listening and simply the sending of the string and the waiting for the
> incoming could be out of sequence thus missing each other.

Well that's pretty simple to solve. You didn't say which PIC you were using,
but if you have a PIC with a hardware USART, they have a 9 bit mode which
sends 8 data bits along with a 9th address/data bit. The cool thing about it
is that you can program the USART not to interrupt unless the address bit is
set. So it works like this:

- All listeners setup the USART to interrupt on address only.
- Some transmitter sends an address of the target. Note that the listeners
  can be doing something else because it's an interrupt.
- The listers check to see if the address is theirs (or a broadcast presumably).
  If the address matches, that listener turns on receive all data. All the
  others return to normal duty.
- So the target listener gets the data stream and all of the other nodes on
  the network ignores the data packet until the next address is transmitted.

>
> I use a cat 5 cable for the network so I have 8 wires I can use.

Another idea is out of band signalling.

>
> Another idea was I use an additional wire other than what the data goes back
> and foward on to send an id on. In picbasic you can use a command to send a
> PULSE OUT of a certain length. I can then assign a ID to each unit and along
> this cable send a lenght that matches the id. I have set up 2 pics and sent
> a series of lengths to the second from 1000 to 5000 in steps of 20 and it
> picks it up each time. So if we hang each device via a resistor and we have
> the hub send a pulse out for say 500 millisecs and in each unit we have a
> detection command in each cycle.

This is out of band signalling. I'd argue that the pulse width is pretty long.

I've described another way to do this here:

http://www.infosite.com/~jkeyzer/piclist/2001/Jul/1317.html

Note that the login and password are the name of this list.

It involves a real short initial pulse and then a delay to a paricular time
slot. The good things about the scheme is that polling is not required
and that it's fair if everyone needs to use the network.

Hope this helps.


>
> I think you get the idea. How do you guys do it? I am after a reliable way
> to poll each unit. Additioanly if I could......be able to raise an alarm on
> the hub if any of the units want to.


>
> I have the software written to do the internet comms part and can get that
> ifo from remote sites to the hub....
>
> Ideas???????.......Suggestions???????........
>
> Regards, Peter.
>
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