2 Xbee S1 to 1 coordinator communication speed/rate

Hi there,

I have two Minibee's with Xbee S1's (not pro). Minibee's are arduino variants: http://www.sensestage.eu/?page_id=48 They both send accelerometer and 2 analogin sensor data to a Xbee coordinator USB board connected to the computer. It runs a python app which routes the data to MaxMSP via OSC (on the same computer). Xbee and Minibee baud rate is 57600.

Right now I'm getting lots of dropouts (every couple of seconds) when sending sensor data at rates higher than say 10Hz. This is with the nodes and coordinator pretty much next to each other. Dropouts get worse with distance. Dropouts are much less frequent when only 1 node is sending. Ideally I'd like to get 60Hz or more with 2 nodes at up to 5 meters but I don't know if it's realistic.

Does anyone have empirical data on stable transmission speeds? Would Xbee pro's solve my issue or is it something else?

Best and tanx, Dieter

Those are awfully fast data rates. The trouble has less to do with the local baud rate of the XBee's UART (although faster is usually better) and more about the communications between the XBees themselves. With one pair of S2 modules in API mode, it can take 20-50ms to send a data packet (smaller is better) and get the acknowledgement back. I'm assuming S1 modules wouldn't be significantly different in that regard.

So 1/50ms gives us a maximum transmission rate of 20Hz, but of course that would be unrealistic as there's no slack time for retries or other network overhead. I'd start by cutting that maximum at least in half, and then divide by the number of modules sending data.

That gets us down to 5Hz for your setup. Not having tried such rapid data rates, I wouldn't be too surprised if it had to be slowed down more still. Would be interested to hear more on your results!

Edit: Using XBee Pros wouldn't change any of this. Maybe someone else can suggest another technology; XBees aren't real high-bandwidth devices.