Uart Bus

McColl06 · Joined: 05 Oct 2008 Posts: 1

Is it possible to create a serial bus connecting multiple pics together in a master multi slave config? Connecting the master tx to the slaves rx and the master rx to the slaves tx. Just wondering, I've heard this can't be done, although I had it working on a system with two slaves but adding a third brought the bus down. I'm in col but I'm just messing with this project, any insight or advice is welcomed, thanks.

PCM programmer · Joined: 06 Sep 2003 Posts: 21708

See this recent thread on "What is RS-485 ?
http://www.ccsinfo.com/forum/viewtopic.php?t=35986

Summary:
http://en.wikipedia.org/wiki/EIA-485

jaymiller · Guest

Yes it can and has been done. Did that 'trick' 30 years ago on my Model III as the master to several(8 or 10) slave units.
Today it's just as easy to use RS-485.
The bottom line is that it's just chips and code......
been there..done that.
Jay

SherpaDoug · Joined: 07 Sep 2003 Posts: 1640 Location: Cape Cod Mass USA

If you have several PICs close to each other, sharing a common ground, and your baud rate is not terribly high you can use a "Wired Or" configuration AKA "Open Collector". I believe CCS has an option in the #useRS232 statement to inplement this, but I don't have my manual or the compiler at hand.
_________________
The search for better is endless. Instead simply find very good and get the job done.

FvM · Joined: 27 Aug 2008 Posts: 2337 Location: Germany

The other option is to use tristatable RS232 drivers and create a kind of RS232 bus. Although RS485 is the more reliable technique, multi-drop RS232 is sometimes used in instrumentation. It works well in a small area according to my experience. The main adavantage is, that you can use a PC master without level converter.

John P · Joined: 17 Sep 2003 Posts: 331

First of all, is the "master" always the same processor? If you want to allow more than one PIC to take the master role, it gets much more complicated.

If it's the same master all the time, the major design issue is how to allow the slaves to talk without interfering with each other's data. You can't connect all the outputs together unless you're sure you can tri-state all except one of them, either on a time-slot basis or when they're explicitly commanded by the master. An open-collector scheme solves the electrical problem, but not the issue of multiple slaves talking at once and delivering a gabled message.

But these things have all been solved in many ways, and it's fun to play with, if the pointy-haired boss isn't hassling you to get it working quickly.

red · Guest

Hi, i am newbie, but how can i perform a open collector scheme in ccs? Links are very much appreciated. Tnx

Ttelmah · Guest

The actual 'open collector' bit, is easy (provided you have a reasonably recent compiler). Look at the 'float_high' option in the RS232 setup. This basically tells the code to actually 'switch off' the UART, when not sending, and switch the pin to a 'float' state. You then need to have a pull up resistor on the line.
In terms of reliability, for more than a very few feet, much better, to use RS485.
Code wise, it becomes a little more work. First, you really need to operate the bus 'half duplex', and have all the receive lines monitoring the serial line. Then, before sending, your 'intended sender', needs to check that the line is idle (high). If it is, then it needs to send, _and check that it receives what it sends_. If it doesn't, then the assumption is that something else has tried to send at the same time (a collision). You then need a collision recovery algorithm. If the number of slaves is small, a simple one, is to have each pause for a different time, before retrying. Ater the time, again testing that the bus is idle. In a sense, the devices with the shortest 'retry' times, will be the highest priority devices on the bus. The only downside of this, is that times may become long, if you have a lot of devices. Alternative is some form of random time selection, but you need to be very careful here, that 'random' number geerators ae of course only _psuedo random_, and if multiple chips wake up together at the same time, all could end up with their number generators 'in sync', resulting in a deadlock.....

Best Wishes

frothbeast · Joined: 08 Jul 2010 Posts: 4

PCM programmer · Joined: 06 Sep 2003 Posts: 21708

Humberto · Posted: Mon Jul 12, 2010 6:15 pm

dyeatman · Joined: 06 Sep 2003 Posts: 1924 Location: Norman, OK

FWIW, CCS has an example of a multi-PIC one wire comm setup:

http://www.ccsinfo.com/faq.php?page=connect_pics
_________________
Google and Forum Search are some of your best tools!!!!

frothbeast · Joined: 08 Jul 2010 Posts: 4

I found a post by Bill Boucher

http://www.ccsinfo.com/forum/viewtopic.php?p=39163&highlight=#39163

The part where he mentions the tx pin of the master can go to the rx pins of the slaves and the tx pins of the slaves can be can go to the rx pin of the master. I think this will work for me, I will let you know.

As for re-inventing wheels, I forgot to design my project with wheels. So I am attempting to fix my skis.

Thanks for the help. Next time I will use a max232 chip or go with an equivalent rs485 chip.