RS485 Design - Master/Slave

[Heath]

Howdy folks,

I'm going through a couple designs and was wondering what you guys thought or if you had any suggestions.

My current setup is one controller that will act as a Master and will communicate to all of its possible slaves and the slaves must respond with a temperature status. The slaves address can be configured by hand (something I would like to change). All of these nodes have the same PIC on them: PIC18F6527. The Master will always be talking to all slaves. There should have to be a scenario where a master needs to talk to a particular slave.

Currently, I have two designs:

First, the master will transmit a message and every slave node acts on it and sends a response back. A slave will answer when it is his turn. Slave with an address 1 answers 20ms after the message from the master. Slave 2 answers 40 ms after the message from the master. etc.

Secondly, the master will talk to each node individually. The slave will respond when it is spoken to and only then.

Obviously, there are pros and cons for each. For now, I favor the second approach because it seems very flexible. I am not constrained by that message window. Albeit, the first approach seems to be more suited to meet my needs because I will always have to talk to every node.

The biggest problem I see with the second approach is the slave addressing. There will have to be some extra overhead on the second protocol for the master to figure out what slaves are connected to him and they their addresses are. Then you run into of a slave responding to an equery. When should I respond? This would almost require a protocol like DHCP looking at the MAC address but AFAIK the serial number of the board or PIC is not stored anywhere in the firmware so there isn't a way for me to uniquely identify a board.

Thanks for the comments.

[yerpa]

I think your second approach is better, and it is commonly used in industry, for example: Johnson Controls N2 bus. The master polls the entire range of slave addresses with a simple "Are you there address n?" command. The slave has a short time (in milliseconds) to reply, or else that address will be treated as failed. The master can periodically loop through all available addresses (256 max in N2 bus) to discover new devices. A slave never initiates communication in this scheme.

It all depends how many devices on your bus, the amount of data per device, the polling rate required, and the available baud rate. Good luck, there are a lot of examples out there - google for "rs485 protocol"

[Ken Johnson]

First or Second - why not both?

I often implement a protocol this way, using slave address 0 as a "broadcast" message. Address 1 or more is always directed to a particular slave.

A given message may or may not require a response. If it does, broadcast message responses are sent in a time slot, as you described. Directed message responses occur right away, after a short delay for line turnaround if 2-wire half-duplex.

We use 1 or 2 bcd switches to set the slave id (1-9 or 1-99). Customers are often not comfortabe with hex switches which read 0-F.

Hope this is helpful.

Ken

[Heath]

I favor the second design but it appears wasteful in the current scenario since the master says the same thing to every slave. If I have 8 slaves the master needs to repeat himself 8 times. (1 to each slave)

I guess I could work out a protocol where the master finds out who is on his bus and keeps a copy that he updates every X seconds. When he sends a message he will attach a delivery order in that message like "Slave A -> Slave C -> Slave Z". Slave A will get the message and pop his name off the list, send a response to the master and send it on to Slave C. Slave C will act accordingly.

[Heath]

Ken,

I believe my scenario would be where the master is effectively always broadcasting. (i.e. when he has something to say he says the same thing to every node) Given that to be true, it would appear that the first one is the best option even though it has greater limitations and needs to be considering the message size when the windows are framed.

One thing that bugs me about the first design is that I can have one slave and configure it as slave 8 and it will wait a set time before it responds even if it is the only slave. It just appears to me to have design limitation written all over it. :/

[gjs_rsdi]

the safest and simplest way is to talk to each slave by addressing it.
simple and no bus colissions.
joseph

[Neutone]

The MODBUS protocol supports would support your needs. Network broadcast are supported by sending commands to address 0. All traffic that requires a response is addressed to individual slaves. The protocol is well documented and you should be able to find code examples to work from.

If each device attached to the network has a default address of 1 that is read from EEPROM, you can attach devices one at a time and reassign them to use a different address you don't even need DIP switches or such.

At any rate read the MODBUS specification. Its well thought out and from there you will be better able to figure out what will work for you.

www.modbus.org/docs/PI_MBUS_300.pdf

[asmallri]

[Heath]

I've been looking at the modbus driver provided by CCS but I have a few questions on it.

It looks like it will wait 10 MS after it sends a message (at 9600) and check for a response. Am I correct in saying that, according to the MODBUS spec, this isn't correct?

I also don't see a check for the 1.5 delay between characters.

Thanks for the assistance.

[Heath]

Hrm. the 10ms wait is just a matter of opinion for a response. that question is answered. Still trying to find the 1.5 delay check.

[Storic]

Maybe off the subject a little, have a look at BACnet as a RS485 (MSTP) communication protocol. here is a couple of links

http://www.answers.com/topic/bacnet

source code
http://sourceforge.net/project/showfiles.php?group_id=117598
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[Neutone]

[SET]