A silly question perhaps.
But, if you connect an allstar node configured as half duplex to a repeater (capable of full duplex), it seems that node is quite capable of running in full duplex mode. In fact, if you link it to another allstar node that is configured as full duplex, then the node essentially runs full duplex. Is there a way to force a node to run half-duplex no matter what it is connected to? Is the only way to guarantee half-duplex operation to replace the repeater with a simplex radio? (that way the receiver will be dead while transmitting).
My point is that you have configured the node as half-duplex but that does not mean it is going to run as half duplex.
OK - answering my own question (as I often do for some reason) it seems that you need to set the duplex= parameter in the URI config to half-duplex also. Then you get a “real” half-duplex functionality. The use of this parameter has never been properly documented AFAIK.
I would note that if you set up the node for full duplex (the default) and the URI is set for duplex=0 it will not repeat audio.
I just installed a new RPI image from the ASL download site and the default configurations for usbradio.conf and usbsimple.conf were both duplex=0
So - accepting the defaults - ASL is non-functional. When a new distro is rolled out, I hope this is fixed.