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Aug 24, 2008, 06:13 AM
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Death of an XBee


No-one read the last blog post. How about this. It's "cloud" autopilot.

Remarkably, Google's entire XBee search database is advertisements or people trying to sell XBee's but no tests of its performance. Now for the first time, we have discovered the XBee sux at full duplex.

With the link saturated & no flow control, U get 88000 bits/sec in 1 direction. This seems to be the limit of the PIC.

Saturate in both directions & U get nothing even with the flow control.

The only way to get full duplex is to have 1 be the master & the other wait for a master packet, then send as much as possible & back off just before the next master packet is due. In this arrangement, the master packets need to be 20ms apart & U get 32000 bits/sec each way.

This isn't the 115200 bits/sec low latency we wanted & no-one wants to spend $70 on double XBees for every aircraft.

Well the XBee's got less & less reliable over time. The ground XBee occasionally stopped receiving while still transmitting successfully. Finally 1 or the other died.

Contrary to many social network hits & according to 1 of the many datasheets floating around, the $32 ones were not 5V tolerant. Only the new $36 ones are. Fortunately, a web page for a future $39 900Mhz XBee has appeared. This one has been upgraded to 115200 & the range downgraded to 1 mile. Probably not available until 2009.
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