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All things pertaining to wireless and RF links
By Integrate
#80872
Hello. I have never used Xbee, but I plan to get one for a pet project I'm working on. However, I require a high baud rate. Although the Xbee's 250k baud isn't high enough for me, I can make workarounds to reduce the amount of data I'm transfering. However, I've been reading some tutorials like this one, and I only see the baud rate going up to 115200bps. Is the 250k baud achievable, and is there a tutorial that shows how it's done?

In addition, would this operate the same as this rf reciever/transmitter from sparkfun? What I mean is does the tx see exactly what is sent into the rx pin?

Also, if there is a similar chip that does higher baud rates, please tell!

Thanks!
By stevech
#80874
in ANY wireless device, don't confuse air link speed with the net yield. For example, in 802.11 WiFi, the net IP layer yield is about 60% of the air link rate, and this is half-duplex (one-way at a time).

In 802.15.4 (as XBee), in 2.4GHz, the air link bit rate is 250Kbps (240,000 bits/sec). The net yield is up to about 120Kbps, with ideal interference and signal strength conditions.

Do you actually need such a high rate? Can you encode or compress data, or eliminate duplicate data?

The URL to the SFE product you mention is 2,400 bit/sec. raw bit rate, quite low, but it is due to the 315MHz channel and other factors. This is a very crude radio whereas the XBee and other 802.15.4 are more like a plug and play modem. The 5 to 1 price difference suggests such.

to get much above 100,000 bps you will need something like 802.11.
By Integrate
#80885
Thanks for the quick response :) . I'm new to wireless, what you told me was really helpful.

My goal is to build some sort of rover/boat combination that would run around a small marsh near my home. I wanted to have audio and video capabilities on it.

If I were to send a 128x128 size 8 bit picture, I could only do less than one picture a second with 115200bps. For audio, I can only sample around 5760hz x2.5 for 8 bits. I can cut down data further by eliminating color and quality, but it's already looking pretty bad. I can also switch between audio and video only, or use analogue for the audio (although I really wanted it to be digital for some reason).

I looked up the 802.11 modems, but the ones on sale are pretty expensive and have a small range. An option might be to get a bunch of Xbees to do the job, and give them different ATIDs (assuming that they do not interfere with each other), but that would also be pretty expensive.

Oh, and I brought up that 2400bps product because I'm currently using it for testing purposes. It works very well, but I was wondering if the Xbee worked the same simple way where if I set the Rx pin to high or low, the Tx pin would see the output. If it does, then I wouldn't need to write much new code and the Xbee can directly replace the 2400bps product (other than adding a few voltage dividers and changing the baud).

I'm assuming that making one of these modems myself would be next to impossible. I'm probably going to have to brood over this problem, maybe I can cut some stuff out.

Thanks!
By ttabbal
#80896
Xbee series 1 modules come with a firmware that is set up as a serial modem. It's not so much that you toggle the pin and it toggles a pin on the other module, though they can do that. It's like the old computer modems, you send "A" and "A" comes through on the other side.

Don't get the series 2 modules, they are designed for mesh networks and don't really do what you want them to do.

Bluetooth sucks for range, 802.11 is pretty bad as well. The high power XBee modules might work well. The real question is how much range do you need? The more range you need, the more it will cost. Once you get much more powerful than the XBee high power modules you need to go licensed. Get a ham license and you can use them with power amplifiers to get more range.
By Integrate
#80983
Thanks for the response. I think I can get by with only pictures, not full video, so I guess I'll be getting an Xbee. I'm thinking of allowing selection between taking a maximum quality picture (which will take some time), to a small, compressed, grayscale video, to audio. That I can do with some programming.

waltr - I kinda wanted to build it myself :lol:

ttabbal - I was reading the section on UART in the manual, and it seems to be able to do what I want. However, I have a few questions:

I plan to get this xbee, but it is a series 2. How would the mesh network design interfere with my application?

I'm also still wondering if multiple Xbees with different ATIDs would interfere and slow the baud rate or not. The tutorial says that the Xbees do not "see" each other, but are they just filtering out collisions or do they actually operate at different frequencies? I probably wont be able to get more Xbees anytime soon, but it might be something I want to pursue if it would work.

Thanks!
By waltr
#81014
I like building it myself also. Great way to really learn how things do work.

The series 2 XBee modules can run various protocols by changing their firmware. So they could run basic 802.15.4, ZNet, DigiMesh or ZigBee. I have been using ZigBee since my application requires many sensor locations. There are many ways these modules/protcols can be set-up and used used (too many to go into here and now).
Download the documents on the different XBee modules/firmware and read carefully.

You linked the the high power XBee module. What distance do you need to transmit? How much battery capacity will be available? If it will be run from a battery and the distance is short then the low power XBee may be better.

Running two (parallel) transmitters is good in theory but can be problematic in practice. It can work but be prepared for some experimentation.

Compression of the video stream is what I believe you need. This can greatly reduce the number of bits per second and thus the bandwidth required.
Another option is to send the video as an analog (NTSC composite) signal. What is your requirement at the receiver end? Just a video display for you to look at or is the video being examined by a processor to exact information and/or make discussions?
By stevech
#81035
waltr wrote:I like building it myself also. Great way to really learn how things do work.

The series 2 XBee modules can run various protocols by changing their firmware. So they could run basic 802.15.4, ZNet, DigiMesh or ZigBee. I have been using ZigBee since my application requires many sensor locations. There are many ways these modules/protcols can be set-up and used used (too many to go into here and now).
Download the documents on the different XBee modules/firmware and read carefully.
I may be wrong, but I thought that series 2 is ZigBee only, for legal reasons with Ember.
By waltr
#81039
I never heard that.

Check and read the threads on Digi's Forum and Digi's Knowledge Base. There are instructions on how to change the firmware from one type of network to another. Also its is suggested to a poster to use a different firmware (network type) due to one type being better at what the poster wishes to do.

http://www.digi.com/support/forum/categ ... egoryID=59

Digi's series 1 modules are based on the FreeScale chip set whereas the series 2 modules are the Ember chip set. That's the only difference between series 1 and 2 modules. The firmware makes the hardware run a specific network protocol. Digi no longer sells series 1 modules, all are series 2 and are available with different network firmware.
http://www.digi.com/technology/rf-tips/2007/05.jsp

http://www.digi.com/products/wireless/zigbee-mesh/

http://www.digi.com/products/wireless/z ... e.jsp#docs

http://www.digi.com/technology/wireless/products.jsp

All of the modules with a part number starting with XB24- or XBP24- are series 2 that can be used to run 802.15.4, ZigBee, ZNet2.5 or DigiMesh firmware. Check the Digi docs and the firmware update options in X-CTU.
http://www.digi.com/support/supporttype.jsp?tp=2
http://www.digi.com/technology/rf-tips/2005/09.jsp

I know there is much confusion as to which module does what and the interchangeability of the firmware. Digi International is the source of most of this confusion since nothing is clear from their web site. Its only after digging deep into all of the documents and reading the questions and answers on their forum does a clearer picture of their product line emerge.
By ttabbal
#81042
Integrate wrote: ttabbal - I was reading the section on UART in the manual, and it seems to be able to do what I want. However, I have a few questions:

I plan to get this xbee, but it is a series 2. How would the mesh network design interfere with my application?

I'm also still wondering if multiple Xbees with different ATIDs would interfere and slow the baud rate or not. The tutorial says that the Xbees do not "see" each other, but are they just filtering out collisions or do they actually operate at different frequencies? I probably wont be able to get more Xbees anytime soon, but it might be something I want to pursue if it would work.
That unit has the transparent serial support. I have two of them here that talk to each other just fine that way. I was thinking of the units marked "2.5" on Sparkfun. I'm not sure if they can do that or not, from reading on this forum, it sounds like not.

As for running more than one. You can do that, with restrictions. There is a command to set the channel, use that to get them on different frequencies. They also need to be spaced away from each other to prevent interference. Even then, being close to each other might overload the RF front end if one is trying to receive while the other is transmitting. If you really want to try it, I suggest going with a 2.4Ghz and a 900Mhz module. That should prevent most of those types of issues.
By stevech
#81060
Perhaps I have it backwards - series 1 cannot by legal issues, run ZigBee. Series 1 I believe changed from Freescale to something else due to the politics of T.I. acquiring the ZigBee stack vendor that Freescale was using.

I for one am glad that Digi has a lot of sales of series 1 and didn't drop it when adding the series 2 Ember ZigBee-centric product.

As ever, the root of all this confusion is a buch of greedy lawyers/clients fighting over the intellectual property rights between TI and Freescale.
By waltr
#81084
stevech wrote: As ever, the root of all this confusion is a buch of greedy lawyers/clients fighting over the intellectual property rights between TI and Freescale.
That would explain a lot.
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