XBee newbie, needs some answers

Wireless & IoT
1

Hi all

Whilst not new to wireless comms I am new to the whole Zigbee approach. If I could recieve some advice as to whether they are suited to my requirements I’d be grateful.

I propose to use them as an and point to a telemetry system whereby each group of XBees consists a Master back to back with a long haul radio.

The master is to broadcast over the XBee network and addressing is to be handled by the end application (Modbus) so essentially the XBees are replacing a multidrop cable network.

Each XBee network (PAN) would consist only 15 - 30 XBees, and I would prefer operate as a mesh, although likely that only a few will be out of radio range of the master, this would add redundancy and simplify configuration.

I would like to use the high power modules- 60mW.

My application involves solar power, hence low power consumption when idle is required.

I don’t entirely understand the operation of the cyclic sleep modes, ZigBee or DigiMesh, or which one would be best for my application, or indeed how the cyclic sleep works in conjunction with broadcasting.

Any help would be appreciated.

Regards

James

2

It sounds like either the DigiMesh or the ZigBee protocols should work for you.

You may want to call Digi Int and talk to an app engineer. He/she would make the correct suggestions for your application.

3

Topology: Mesh vs. Star or Cluster-tree - largely depends on RF propagation impairments: Path lengths vs. transmitter power vs. extend of non-line-of-sight, plus antenna gain.

It’s best to avoid meshing if at all possible: 100 times more complex that star topology. DigiMesh (Digi Series 1) is arguably leaner/meaner than bloated ZigBee (Digi Series 2). For star, just use 802.15.4 mode, peer to peer, no PAN coordinator needed.

You need mesh if neither the 2 or 60mW XBees can provide the necessary range, INCLUDING use of a star topology where the main node has a high gain antenna, say, a 8-12dBi omni.

Do some RF link budgets.

Glad to help.

4

Thankyou for the replies

I understand that star topology is inherently more simple in terms of only having one central “hub” if you like, however whilst this is likely to work in the majority of cases, and certinally would if using the higher powered radios with some antenna gain, I am trying to simplifiy and reduce costs in enclosure design by utilising internal (wire) antennas. This being the case I’d like to have the option to use as a mesh network so that in field setup is nil, just plug in battery and walk away (PAN would be preprogrammed prior to install, and coordinator would be in the unit with the long haul radio)

Am I expecting too much? :slight_smile:

And I will definately consult an app engineer at Digi, but sometimes it’s best to get the bigger picture :slight_smile:

Regards

James

5

I suggested talking to Digi since there are many possibilities.

I do have a ZigBee network and have found that once the initial learning of what the XBee parameters mean and how to get them to do what I needed it works well. ZigBee requires one unit to be the Coordinator that creates the network, PAN etc, and is the central master for issuing commands to remotes and as a data collection point. I have a couple of units running as Routers that do relay to/from units that are too far from the Coordinator to link directly.

The network part just takes care of itself.

In addition I do have a couple of units that are part of the network but pass data between themselves and not through the Coordinator. These two are set-up in Transparent mode (AT command mode) and act as a wireless RS232 link. When one gets out of range of the other the data gets Routed through another unit and communications continues after a delay.

All this is done with the low power 2mW units. A couple are battery powered but I an still not totally happy with the battery life even though I believe they sleep for a large percentage of time. I don’t imagine ever needing the high power units.

6

Project I’m on now… star topology. 802.15.4 only.

Central node has a 12dBi omni antenna, elevated about 15 ft.

remote nodes have a 5 inch rubber-ducky antenna. About 2dBi gain. )The XBee with the little wire antenna on the module has about 2dB less gain; the chip PC board antennas have about -5dBi loss.)

XBee Pro modules.

Signal strength is about -78dBm at central from remote 1/4 mile away, 8 ft. elevation, fairly clear line of sight, but not totally clear.

This provides 90-78 dB or so of fade margin (where 90 is what I use for minimum desired received signal strength for 802.15.4@2.4GHz), or better if your path length is shorter. So no mesh (repeaters) needed.

Or you could spend some of that margin on lower antenna gain at either end, or maybe use an XBee instead of XBeePro. The XBee is about 3dBm transmitter power and the Pro is about 18dB, or 15 dB better. Lots of margin in these link budgets makes for a robust system.

Another node is about 300 ft. from the central node and has good margin using an XBee Pro with an on-module wire antenna.

(The received signal strength reported by the XBee (or any other brand) is not true dBm, but an estimate based on the measured noise).

In line of sight, you can calculate the signal strengths to about 5dB of accuracy by simple math. In non-line-of-sight, it’s all empirical and experimentation, unless you have a lot of practical experience and can closely eyeball it.

In other projects, without resorting to directional antennas, I’ve gotten 1/2 to 3/4 mi. range with the 60mW radios. 802.15.4’s use of 2MHz DSSS and QPSK makes it work well with weak signals. And of course, the inverse square law in RF propagation is your friend, as path length increases. Which is why NASA can talk to the Voyager deep space probe!

7

Er…thanks Zenkarman

That helps me in the same way that the following may help someone that uses hot water…

See features of Saucepan…

  • Has handle

  • Can heat slow or fast

  • On gas or electric stovetop

  • Half fill it or fill it to the top

  • Works with or without lid

  • Still heats water regardless of low or high heat from stovetop

  • Lid comes off for flow control

  • Made is the same way as every other saucepan

  • Orifice is located at the top for holding water rather than falling out