Finally identified the B205 Half-Duplex problem I think. No packets involved here - everything is streaming. No Tags, no Messages.
My hypothesis is that when the B205 transmitter starts (the 2nd and later times) it tries to fill up something like a TX FIFO, sucking samples out of the flowgraph way faster than they can be provided. This causes the TX to underrun going back to receive. Then it catches back up and switches to transmit, bouncing back and forth. The first time the TX starts (on flowgraph startup) it seems to pace the TX correctly. After switching to RX then back to TX (by stopping TX samples the restarting) is when the problem occurs. A number of different approaches seem to yield the same behavior: 1) I used an Audio Source block (microphone) to feed a multiplier (frequency shifter) that feeds the TX. If I remove that and replace it with a signal source block sinewave (something that does not involve real time) then the TX can suck samples as fast as it wants, and everything T/R switches correctly. 2) The original flowgraph used a Signal Source Block as a L.O. feeding two multipliers: one in the RX chain, and one in the TX chain. The receiver is feeding samples to it's multiplier at a constant rate, which limits how fast the buffer on the L.O. has samples pulled. That same L.O. is also feeding a multiplier in the transmit chain, which drains all the buffers in the TX path up to that point. I created two gnuradio Signal Source blocks, with identical input parameters but instantiated as two separate blocks in the flowgraph. One feeds the receiver multiplier, one feeds the transmit multiplier - this avoids any common path that could slow down samples to the TX. As long as the TX side has no real-time sources then it T/R switches correctly. Using the Audio Source block in the TX chain causes the problem to resurface. This seems to confirm the hypothesis. My opinion is that this is probably a BUG in the B205 FPGA code. It should attempt to pull samples at a rate close to the sample rate of the transmitter when restarting. -- Tom, N5EG
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