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McColl06
Joined: 05 Oct 2008 Posts: 1
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Uart Bus |
Posted: Sun Oct 05, 2008 10:48 am |
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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. |
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PCM programmer
Joined: 06 Sep 2003 Posts: 21708
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jaymiller Guest
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Posted: Sun Oct 05, 2008 4:03 pm |
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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 |
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SherpaDoug
Joined: 07 Sep 2003 Posts: 1640 Location: Cape Cod Mass USA
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Posted: Sun Oct 05, 2008 9:18 pm |
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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. |
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FvM
Joined: 27 Aug 2008 Posts: 2337 Location: Germany
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Posted: Sun Oct 05, 2008 11:40 pm |
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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. |
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John P
Joined: 17 Sep 2003 Posts: 331
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Posted: Mon Oct 06, 2008 8:39 am |
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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. |
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red Guest
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help: same problem. |
Posted: Tue Mar 17, 2009 1:42 am |
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Hi, i am newbie, but how can i perform a open collector scheme in ccs? Links are very much appreciated. Tnx |
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Ttelmah Guest
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Posted: Tue Mar 17, 2009 3:28 am |
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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 |
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frothbeast
Joined: 08 Jul 2010 Posts: 4
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Posted: Mon Jul 12, 2010 1:07 pm |
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Ttelmah wrote: | 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 |
Is it possible to have 2 pics 'float_high' and one normal so that the 2 pics can pull the one pic's line down? Or would that be like shorting 5V to ground?
I have done something similar to the original post. I have 3 pics listening to a master(c7RX slaves wired to c6tx master) and each of the three will transmit back to the master when addressed(c7rx master wired to c6tx slaves). I plan to use a pull-up on the slave's tx and float them all high, unless I can leave 1 slave normal and float the other 2.
Does anyone know?
PS sorry for not jumping to RS 485. Next time...
I forgot to mention that it works with 2 slaves and 1 master, but 3 slaves can receive from master but none can transmit. |
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PCM programmer
Joined: 06 Sep 2003 Posts: 21708
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Posted: Mon Jul 12, 2010 4:37 pm |
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Quote: |
Is it possible to have 2 pics 'float_high' and one normal so that
the 2 pics can pull the one pic's line down?
Or would that be like shorting 5V to ground?
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You would be stressing the pin drivers. It's not recommended design practice. |
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Humberto
Joined: 08 Sep 2003 Posts: 1215 Location: Buenos Aires, La Reina del Plata
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Posted: Mon Jul 12, 2010 6:15 pm |
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Quote: |
Is it possible to have 2 pics 'float_high' and one normal so that the 2 pics can pull the one pic's line down? Or would that be like shorting 5V to ground?
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Quote: |
Is it possible to create a serial bus connecting multiple pics together in a master multi slave config?
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Quote: |
Hi, i am newbie, but how can i perform a open collector scheme in ccs? Links are very much appreciated. Tnx
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I know that all these "fresh ideas" are looking for the simplest/easiest way to get something running,
but there is not any reason to re-invent the wheel.
A regular microcontroller pin out was not designed to be wired away to perform a data transmition bus line,
there are more than 25 arguments for a NO.
To overcome the arising problems of this, over the years had been created hundreds of hardware
interfaces and procedures to handling communications (protocols), these rules are know as standards.
Every standard used widely (RS232, RS422, RS485, LIN, CAN, etc) has been impossed for many good
reasons, there are one that is optimum for each application and you should know them at least before
trying for yourself something that was already done and is warrantied that it must run.
All this bla-bla-bla has the only purpose to suggest you newbie guys to read and investigate a little
regarding this, and finaly for your application, I suggest to use LIN (Local Interconnect Network) is an
easy standard to implement and can be do it in a cheaper way.
Humberto
Edited: to remark add bold letter
Last edited by Humberto on Mon Jul 12, 2010 8:53 pm; edited 1 time in total |
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dyeatman
Joined: 06 Sep 2003 Posts: 1924 Location: Norman, OK
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frothbeast
Joined: 08 Jul 2010 Posts: 4
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Posted: Mon Jul 12, 2010 9:45 pm |
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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. |
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