Multiple PICs on usart network

[HenrikOlsson]

Andy,
Just like Richard says the key is to make sure that the slaves never ever starts to speak without being spoken to. If you have one master and multiple slaves you need to come up with a scheme where the master interrogates the slaves, one by one, and the slaves only speak when asked to speak.

Most of my experience with this type of thing comes from writing code for MODBUS. With MODBUS there's one master and one or more slaves (or servers as they're sometimes called). The master sends out messages on the bus containing a slave adress, a command and some data like "Slave 2, give me the value of your register number 5 and a checksum). All the slaves gets the message, verifies the chcksum and then determines if the message was for them or not. Only the slave with the adress matching that of the message is allowes to speak and must do so within a certain amount of time or a timeout error occurs. A slave can never start talking unless asked to by the master.

There are of course other alternatives, like each slave has a "slave select line". The master asserts the slave select line of one slave at the time and the slave sends whatever it needs to send.

If you need to have multimaster capabillity then perhaps CAN is an alternative.

/Henrik.

[ardhuru]

http://www.sampson-jeff.com/article/.../int.htm#async

Please scroll down to figure 3b;

Looks interesting; Tx, Rx, and perhaps power too, all on 1 line.

Comments, folks?

[ardhuru]

Also, from the forum, http://www.picbasic.co.uk/forum/showthread.php?t=4292

Bob talks of a separate line for flow control, but that could be avoided by querying each slave, I guess.

[ardhuru]

Recently, I too needed to connect a couple of nodes to a common controller. I wanted a bus on which I could go on adding slaves "in parallel".

Also, half duplex was acceptable in my application. This is the outcome: works like a charm along a 20 meter cable run at 9600 baud. I'm sure will run much longer at slower speeds..

And, comms happen over a single line, so you also save a pin on the pic.

Hope somebody finds it useful!

[andymuller]

Hope somebody finds it useful!

Attachment 7684

Hello Ardhuru

It is a wonderful idea. I have not already thought about it.

Great work!

Would you please share some routine (source code) about your work?

Andy

[ardhuru]

Thanks, Andy.

Actually, there's nothing to it, really.

Just set up the Rx and Tx routines as shown in the graphic (substitute the pin you're actually using in place of the C.6 I've shown) and call Tx before each transmission, and Rx before receiving (or more accurately, after each transmission).

So, at any time, only one guy is transmitting.

And, make sure the pin you're using is set for digital I/O, of course.

Anand

[andymuller]

Thanks, Andy.

Actually, there's nothing to it, really.

Hello Anand,

Thank you very much buddy in advance for your replying me.

In brief, I am beginner at PIC world, and absolutely not expert such as you bro.

Would you mind if you share an applicable sample here (for Master and slaves), please?

I would deeply appreciate for your understanding and most valuable support.

Regards,

Andy

[Demon]

test post to open a closed thread (I have a pending post)

can be deleted once another post is added (thread auto-closes if I remove this one)

[Ioannis]

I can post here just fine. I see it opened.

Ioannis