@equinox1 - everything was working with no problem a few days ago. Nothing has really changed other than whatever is happening inside the DX softwarewise. I've verified that the driver is present and my comm programs are seeing and connecting to it as a serial port. One program, coolterm, goes as far as to have led-like status indicators for tx, rx, cts, dtr, etc., and I can see activity on the TX line when I type on the keyboard, but no activity on RX, i.e. no character echo back from the DX.
@geekmaster - aware of the battery problem, and have been careful to keep the DX connected to a charger. I have a yellow power led, for what that's worth. I did plug the DX usb into the mac to improve the ground path, but not a fix for the communication problem.
I should note that previous to this (just a few days ago), this was working without a problem (as far as serial comms) and I cannot fathom what might have changed hardware wise.
@knc1 - I've been using a spare ipad charger, which has a higher output current spec to allow the ipad to fast charge. I believe this is equivalent to the high current usb ports on the macbook pro I have. Now have the DX connected to the mac in order to have a reinforced ground for the TTL converter, as geekmaster suggested.
Nice "solution" to the charger problem from Amazon...