RS485 Communications
(Under Construction)

After searching high and low for a decent, open RS485 protocol, I finally gave up and wrote my own. Here are my goals:
I wanted each sensor to be able to output data if it had it, and every display to be able to collect that data and display it. In a 'normal' RS485 network, there is a single PC whose job it is to poll each sensor or actuator, and perform all data collection and control. I wanted no PC to be required,  with it's complexity and power use. To coordinate the traffic on the bus, any one node can be designated the master. All it needs to know is how many nodes are on the network, and how often to poll the network. and starting at node 0, it asks: "Hey node N, do you have any data?" If there is a node N, it replies with its data. Then it goes on to the next node, until they're done. It times out quickly if a node doesn't respond, so bus bandwidth isn't wasted waiting.  It's an easy job and any one of the nodes on the network can do it.

To easily support multiple displays, I eliminated the concept of a single node doing all the data collection and dissemination. Using the PC paradigm, each master would need to poll every slave it wants data from. Then there is the issue from a node: well do you want my most recent data or some timed sample? It is redndant and uses extra network bandwidth. Instead, each sensor simply outputs data if it has any, and each display can listen to any sensors. If it is a weather display, it listens to weather sensor data. I use a simple data dictionary for this. Each variable in the network is defined so every display understands what thedata type means, and whetehr it needs it or not.

Each node has a node address. In addition, each data variable has an identifier that is transmitted transmits along with it's data. A common data dictionary is used to define what the variables are.

There are three types of nodes: Master, Slave and Receiver and I/O. The master simply polls each slave in sequence. Slaves simply output data if they have it. Receivers look at the data and collect the data that they want.

So the master needs only to know how many nodes are (or might be) on the network. And how often to poll the network. Say there are 5 nodes and it polls the network once per second. It starts with 0 and then goes on to the next node. After it polls the fifth node, node 4, it waits until the poll time is up and repeats the process. Forever.

Most nodes are slaves also. When the master polls them, they output their data. That's it. The receivers always look at the network and collect data if they need it.

So each slave must know which data they are to transmit. They maintain a list. For example, weathernode outputs the wind speed, wind direction, outdoor temperature, and barometric pressure. This is a simple example where there is typically only one sensor for each type of data on the network. In the case where there are multiples, each generated by a different slave, the slave must include it's slave address along with the data identifier. For example if there were individual slaves in each room of a house and each could output room temperature. So there would be multiple temperatures, each identified by its room. Instead of having the code for each node generate a random, unique identifier for its temperature, it would need to assemble an identifier from it's node ID plus a variable identifier. I haven't worked out all these details yet.

There are a few node configuration values that each node needs. They currently live in mynode.h

#define NODEADDR     2        /* My node address */
#define MASTERNODE   1        /* 1 if I'm the master, 0 for all slaves */
#define NUMSEND      3        /* Size of send data struct */
#define NUMREC       8        /* Size of receive data struct */

Ultimately there shold probably be a way to specify a node's configuration at runtime. This would require some simple user interface, and storing the values in EEPROM. For now I just recompile the code with these values set for that specific node. It takes about the same time as it would to run some configuration program, and this doesn't require writing the program.

Here is a diagram of the communication responsibilities of each type of node.

If you are interested in the code for this project, please contact me.

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LCDs are a pain. but fun!

AVR Processors
Communications via RS485
How I prototype, and the Lab
Last Updated: Apr 20, 2008