Now it would be nice to get the data from the sensors in the Greenhouse into the main house for capture, processing, etc.
To do this, I decided to use Series 2 XBee radios. These powerful little radios are very easy to interface to and have a very nice feature, they support mesh networking. A lot of people look for cheaper radios, XBees are not the cheapest in the world, but these little radios have a whole lot of features and, if you need these features, spending some extra money rather than implementing those features yourself can be the right decision to make.
XBees
Let's start off by talking a little bit about Series 2 XBee radios.
What Is Mesh Networking?
So what is mesh networking? Mesh networking is useful in situations where you have a bunch of radios that need to talk to each other, some are too far away from each other to communicate directly, or perhaps there is a wall in the way that doesn't let the signal thorough. Also, sometimes noise prevents a signal from getting through, or something breaks. So how can we have a reliable network that makes sure that all data is received?
Suppose you have radios A and B, and A wants to talk to B, but B is too far away from A, or there is a wall in the way or something that means that the connection won't happen. One way of handing this is to have a 3rd radio C that is within range of both A and B. A could send its message for B to C and C could then forward the message to B, A -> C -> B.
One issue here is if C stops working for some reason, suddenly A can no longer talk to B.
So suppose we have a radio D that is also reachable by A and by B. It doesn't matter if C can talk to D or not, just that A and B can talk to D. But say C is working. Now there are 2 different routes from A to B, either A -> C -> B, or A -> D -> B. If radio A finds that C isn't working, it can route its message for B through D. Of course, if both C and D are not working for some reason, once again there is no way for A to talk to B.
It is possible for these chains to be longer. Each additional radio in the chain is called a hop. Say radio A can't reach B, but can reach C. C can't reach B, but can reach E. And suppose E can reach B. Now A can send a packet to B via the path A -> C -> E -> B. This path has 3 hops.
So radios don't necessarily have to be able to talk directly to each other to transmit information to each other, they can use intermediate radios. Not only that, but they needn't use the same path each time. Some radios in a path that used to work may not be available at any given time, but if there is a path of radios who can talk to each other, the packet can eventually get from A to B. This is the basis of mesh networking and is similar to how the Internet works.
Other XBee Concepts
There are a few more concepts useful for understanding XBee radios. Every mesh has to have 1 radio that is the Coordinator. A mesh can have only one Coordinator. This radio controls the network. It determines the ultimate local addresses that each radio in the mesh gets. Radios have 2 addresses, one short one local to the mesh the radio is part of, and a much longer one that is unique to the radio no matter what mesh it is in.
Another radio type is a Router. Router radios can act as intermediaries and radios who cannot talk directly to each other must go through one or more Routers.
Finally there can be End Devices. These End Devices can be battery powered and lower powered. They can go to sleep and turn themselves back on. They can talk to Routers or Coordinators, but cannot talk to each other. This limited functionality means they can be lower cost or can work on battery power for very long periods of time because they can sleep between transmissions.
An example mesh network might look like below.
One more important concept is that of the PAN (Personal Area Network) ID. All XBees must have a PAN ID, and all radios that are going to communicate with each other must have the same PAN ID. The PAN ID is determined by the Coordinator, and can be manually set or automatically created by the Coordinator. Finally, you have have multiple independent XBee networks in the same physical location, each with its own PAN ID. These networks will not be able to communicate with each other, only radios with the same PAN ID can communicate with each other.
In this post we will set the PAN ID manually.
The XBee Hardware
You can use the XBee hardware in several ways.
You can talk to it over a serial connection and control it from a host computer or microcontroller. This style of usage gives you the most flexibility, but as it requires some programming, it is also the most complex usage. The radios have standard RX and TX pins for communication.
It is also possible to wire digital and analog sensors directly to an XBee, eliminating the need for a host computer or microcontroller. In this style of usage, circuitry and programming are minimized. It is also possible then to make the radio sleep for the intervals between sensor samples, which means that a battery powering the XBee could last a very long time The number of sensor pins is limited, but if this is all you need, it is a great option for the simplicity and long battery life.
Configuring the XBees
XBee radios are configured with software from Digi, the manufacturer of XBee radios, called XCTU. When I first started using XBees, XCTU only ran on Windows machines, which meant I had to use virtual machines or Wine on my Linux box. But Digi have since rewritten the program in Java and it is now possible to run it on all major OSes.
The program is really nice now. Not only does it work on all OSes, but it also has a much improved user interface. It also has new functionality that is very useful for debugging, you can visualize your entire XBee network and see which radios can talk to which other radios, transmit data to any radio by typing the data into the UI. All in all, XCTU has become a very nice tool.
So jump into your favorite search engine and search for "digi xctu download" and pick the version appropriate for your computer and follow the directions for installation.
My Initial Network
The wireless network I build will eventually cover both the greenhouse and the house itself, but this post is about getting data from the greenhouse into the house. To do this I will use 3 XBees. Two will be XBee Pros. These radios have a lot more output power and I figured they would be good for getting through the outside walls of the greenhouse and the house and cover the distance between the 2 buildings. They might also be overkill, but I thought I would give them a try. The Coordinator Radio will be a standard XBee in the house and doesn't have to be as high powered.
Hardware for Configuring
It is necessary to have a serial connection to the radios to program them. To help with this, I bought several Sparkfun XBee Explorers. These little boards are awesome. You plug the radio into the board, hook up a USB cable and you are working with them in no time. I often hook the Coordinator Radio up to a host computer using an Explorer in production.
The Radios are blue and the Explorers are red in the picture below. You may also notice tape on the antenna for the radios. I usually use tape to mark the PAN ID and whether the radio is a Coordinator, Router, or End Device when building a network so I know which is which.
Running XCTU
Running XCTU is easy. If you follow the directions on the Digi site on Linux, it will install the software in /opt/Digi/XCTU-NG. I have not yet puzzled through how to set permissions so that I can use XCTU without running as root, for now I use the command
sudo /opt/Digi/XCTU-NG/app &
This will run the program as root and run it in the background so that I can do other things in the same terminal window.
The initial window you should see is the following.
Now we need to scan for the radios. First I want to program the Coordinator radio, so I labeled the Coordinator Radio with tape that had C written on it, plugged it carefully into the Sparkfun USB Explorer, and then plugged the Explorer into my computer.
If you look at the initial XCTU window, you will see an image kinda shaped like an XBee with a magnifying glass next to it. It is the second large icon from the left just under the menu bar. Click this and you will see the following dialog box.
On my machine the Explorer got assigned /dev/ttyUSB0, so I click the checkbox and hit Next >.
The next dialog box will ask me how I want to talk to the XBee, and will let me pick serial properties like baud rate, number of stop bits, etc.
The default baud rate for XBee radios is 9600 baud, in fact all the values seen are the defaults, so just clock Finish.
Another dialog box will pop up and give messages about its search for radios on the port given. If everything is wired property and you left the connection values alone you should eventually see the following dialog box.
Select the checkbox next to the radio and click Add selected devices.
You should then see a window like below. To be honest, this window is from after I made the radio a Coordinator, so your picture will be slightly different. But that won't matter at all.
Now double click on the radio entry in the left hand pane. This will bring up data in the right hand pane about the radio.
Configuring the Coordinator
The radio plugged in now is to become our Coordinator. If you look over the above window, you will see that the radio is currently a Router. I have found that most brand new radios I buy have the Router AP firmware in them. That's fine, XCTU makes it pretty easy to change. There are a series of 5 large icons in the right pane labeled things like Read, Write, Update and others, and we will use Update.
Before we continue, let's discuss what is going on. XBee Series 2 radios are basically small computers and each radio can either be a Coordinator, a Router, or an End Device. What determines the type of radio is the firmware uploaded to it. Right now the radio seen in that last window shown has the Router AT firmware and so is a Router, and we want it to contain the Coordinator API firmware so that it will be a Coordinator. We won't worry about the different between AT and API in this post, we need API. So click the Update button to see the following dialog.
Make sure the product family is selected as XB24-ZB. If this isn't chosen, you have the wrong kind of radio and should go make sure you order a Series 2 radio. Scroll through the middle list until you find ZigBee Coordinator API. As for the Firmware version, I always make sure I chose the latest firmware. I have never tried to find out if my Coordinator, Router, and End Device radios are running the latest firmware, but it probably is safer to make sure they are always running the latest at the same time.
Then click Update and wait.
Once the update is done, XCTU will read the values from the radio again and you should see the following.
Notice the function set for the radio is now labeled as Coordinator, though if your window is like the picture I have, you will see a ... and some of the letters of Coordinator. Rolling the mouse over this area will give a tooltip giving the function set name without the ... in the middle of the text. If it is not ZigBee Coordinator API, trying doing the update again, you didn't select the correct firmware.
Now we need to set the PAN ID. Remember every radio that needs to talk to each other needs to have the same Pan ID. For this post, we will make the Pan ID have the value 2000, though you can pick any value you want as long as all the radios get the same value. Scroll through the radio parameters until you see PAN ID. You probably won't need to scroll at all, it is the top value. Type 2000 into the text box.
Next, the software libraries that will be talking with the XBee are using Escaped Mode. Without going into too much detail, this mode makes it easier to detect XBee radio frames, so many software libraries use it. To use Escaped Mode, we need to tell the radio to set its configuration parameter AP to the value 2.
Now we need to write these values into the permanent memory in the radio so that they are kept when the radio is turned off and turned back on. If you look at those large icons again in the right hand pane, you will see Write. Click that button.
If you want to confirm that your values are set correctly, click the Read button and scroll through the window to make sure PAN ID and AP have the proper values.
Cool, now we have a Coordinator Radio to rule our network!
Configuring the Greenhouse Radio
Now it is time to configure the XBee that will go in the greenhouse. I decided to make this radio a Router for no particularly good reason. My general rule of thumb is that if the radio is not going to go to sleep between sensor measurements and will not be battery powered, I usually make it a Router to have the most flexibility.
Since my greenhouse is across the lawn, I chose an XBee Series 2 Pro, which has a higher power output than a standard XBee Series 2.
I labeled the greenhouse XBee with a piece of tape with the letter R to label it as a Router, carefully seated it on a second Sparkfun USB Explorer, and plugged it into my computer. I then clicked the Search Radio icon on the left side of XCTU and got the following dialog.
Notice the additional serial port /dev/ttyUSB1. Click Next >. You will again see the Port Values dialog where you can chose baud rate, stop bits, etc. Just click Finish. After a little while you will see the following dialog.
Make sure both radios are checked and click the Add selected devices button. You will now see two radios in the left panel of the XCTU window.
If you are looking at these images carefully, the above picture is not exact. This is actually from after the radios were configured, so the radio already has the Router API firmware. But ignore this for now, and imagine it claims the radio is Router AT, which is how the radios are usually set up when bought brand new.
Now double click on the new radio, which is below the Coordinator radio in the above picture, and get to its main screen.
First off is to update its firmware to be XBee Router API by clicking the Update button.
After the firmware update finishes, set its PAN ID to 2000.
Also set its API Mode to Escaped Mode, which is done the same way as the Coordinator Radio, setting the AP configuration parameter to 2. Then click Write.
Confirm the values by clicking Read and scrolling to check PAN ID and AP.
Are The Radios Communicating?
One of the features in XCTU that I really like that the previous version didn't have is the ability to look at the network of radios and see who talks to whom and exactly what packets they are sending and the contents of those packets. You can also construct packets and have the radio send them. All of this really helps with debugging.
To get into this mode, look at the toolbar immediately under the menu bar and click the rightmost icon. As you mouse over it the tooltip will read Switch To Networking Mode. This will show the following window.
Now click on the leftmost icon in the righthand pane, it is labeled Scan. If the radios are properly configured and powered up, you will see a window like the following.
In this window, the Coordinator radio is on the right and the Router is on the left. You can see that they talk directly to each other by the line between them.
If your network were more complicated, you would see more radios and who is talking to whom by the line between the various pairs of radios.
The mode you are in now shows the network as a visual graph. You can also see a table of radios by clicking the Mode button next to the Scan button and selecting Table. You will then see the following.
By clicking on the Connections dropdown on the right hand side of each radio row you can see a list of the radios that radio is connected to.
Consoles Mode
As stated earlier, you can also connect to a radio and tell it packets to send. Though we won't go into much detail here, I will at least show the window. The Consoles Mode is entered by clicking the button just to the left of the Networking Mode button. If you mouse over the button it will show Switch to Consoles working mode. Clicking on this will show the following.
Some day perhaps I will talk about how to use this window.
Conclusion
So now we have a couple of XBee radios configured to get data from the greenhouse into the house network for storage and processing. We have discussed some of the basics of XBee Series 2 radios and mesh networking. You have seen how to use Digi's XCTU tool for configuring radios and for looking at who is talking to whom. I also set up another Pro radio to sit between the greenhouse and the Coordinator, but have not shown that radio as it is configured the same way as the greenhouse Router.
In the next installment of this series we will wire the Greenhouse XBee Router to the sensor platform built before and write the software to capture the sensor data, get it into the house, and then upload it to a time series database in the cloud.
Be seeing you!
