9XTend RF Modem

Jay Kickliter

Does anyone have any experience with Digi's 9XTend RF modems? Is the latency too long to control an RC Plane? They quote latency numbers in the data sheet, but they also mention something about the buffer having to be full before it transmits data.

I'm this setup: [noparse][[/noparse]joystick, nunchuc, or rc trainer] -> Propeller -> RF Modem -> RF Modem -> Propeller -> Servos

and reversed for some telemetry.

We still have to launch our balloon project first (in about 2 months), but I'm just doing a little thinking ahead right now.

Post Edited (Jay Kickliter) : 12/3/2008 1:34:19 AM GMT

Mike Green

I believe these are functionally similar to the xBee devices in that they're packet modems. They assemble a packet of information which is transmitted either when full or when a user configurable timer expires. If you make the packet size too small or the expiration time too short, the packet overhead, both space and time, becomes prohibitive. What you get for the overhead and latency is error detection and correction. Note that, if there's an error in the received packet, there's additional latency due to the packet being retransmitted automatically.

stamptrol

The Maxstream/Digi radios should work fine in your application. As Mike pointed out, the latency is somewhat adjustable. Considering the minimum amount of data you'll be sending, I think you'll be OK.
I've done a few applications which required a lot of send/receive alternations (modbus) and the radios performed flawlessly at 2400 and 9600 baud.

Cheers,

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Tom Sisk

http://www.siskconsult.com
·

T Chap

I tested an XbeePro once to move an object up and down from two buttons on a handheld remote. The two buttons were connected to an SX, if one button was pressed, the SX sent a loop of the same serial command to the receiver, in other words, repeat while button1 = 1 ser.tx($01). The receiver read it and sent commands to a mosfet which allowed power to flow through a relay in it's N.C. mode, which turned on the motor in one direction, or if the other button was turned on another mosfet turned on the relay and the motor ran in reverse. I found it to be quite responsive up to about 150 feet outdoors with nothing but onboard chip antennas. The Xbee acted just like straight wire, whatever you send it, it transmits. I suspect the Xtend versions would be similar. In the past the company was very good about answering questions over the phone or email, you should call them and explain what you are wanting to accomplish to get their opinion.

stamptrol

For those interested, Digi is having an XBee webinar on Dec 10.

https://www1.gotomeeting.com/register/725291498

The link came to me via email, hopefully it will work for anyone interested.

Cheers,

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Tom Sisk

http://www.siskconsult.com
·

Jay Kickliter

Thanks for the link. I was able to sign up.

The guy I'm working on the project with and I can't agree on· platform, so I think we may turn my 8' PortaBote with a 4 hourse outboard· into a ROD (remotely operated dinghy). We do go to a maritme college after all, so I guess it makes sense. But that's a few months off.

steve_b

Just got a set of these modems for work.
They look pretty good and are quite a bit less $$ than many others out there.

To note: I did try an "extended range" blue tooth modem, but found the drivers to have problems in XP and it wouldn't work at ALL in linux (their software has to create the infamous "virtual com ports"...ack!)

Haven't hooked up the 9XTends yet!

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<FONT>Steve



What's the best thing to do in a lightning storm? "take a one iron out the bag and hold it straight up above your head, even God cant hit a one iron!"
Lee Travino after the second time being hit by lightning!

Jay Kickliter

Steve, please let us know how they work for you.· I'm sure I'm not the only one who caould find these modems usefull. ALso, elaborate on any of these points if you can:

Range
Latency
Reliablity at 115 kbps vs 9600 bps
Antenna choice

steve_b

I've played with these a little bit.
I DEFINATELY haven't gone through all the settings.
But I'm hooking up to a remote datalogger, so am really only interested in receiving minutely data.

Anyhow, with minimal setup, it worked. I set the serial ports to 9600 (cuz I just don't need to worry about noise nor do I need the faster speed); the RF link was at 116k....thinking I'll try to get them all to match though.
So, using the defaults, what I end up getting is a minutely message that does NOT fill the buffer, therefore it does NOT force a transmission. Instead, I end up with a buffer timeout, which then forces a packet transmission.
(I don't have the radio's with me at the moment, so can't load up what my settings are)
Anyhow, the bonus is that I am able to program the loggers remotely. Again, if I don't fill the packet buffer, it will eventually timeout and force the transmission....so remote loading is a bit slower.

Now, something I ran in to is partly a logger issue....I think! I get garbage on the transmission....the logger has an openport/closeport parameter; I opening and closing is causing me some noise....so I may try leaving it open! Not sure!

Anyhow, I did try the sleep on rs232 traffic....not sure if it was my noise issue, but I seemed to get some lost packets (not necessarily always the beginning of a message though.

Overall, I'm impressed by the number of options and the price. I think I could decrease the timeout and get it to send faster.
I believe the manual described the timeout as....if the RS232 buffer still has entries when the timeout occurs, then it'll go in to streaming mode (not sure what that does for full duplex though).

I just use the stock antennas....but haven't "gone for a walk" with max power (although the manual describes 14km or so, with stock antennas).

I'll let ya know how I make out when I'm done!

Cheers

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<FONT>Steve



What's the best thing to do in a lightning storm? "take a one iron out the bag and hold it straight up above your head, even God cant hit a one iron!"
Lee Travino after the second time being hit by lightning!