Thank you Amberwolf, you're a lighthouse in a stormy sea of BS non-compatibility "looseness". I'm super grateful for all your work, I've been "sponging" your knowledge for a while. I sent you some $, rock on please.
You're welcome, and thank you, too.
I initially ordered just the display (EN06) thinking it'd communicate with the existing controller, but I got the E10 error code ("communications receiving failure" because of wrong COM). I emailed vendor and then purchased a recommended controller from them. I assume they are compatible...
Now with the (presumed) correct "set" I get the Error code 07 (motor fault) as described above.
Given those, it's probable that they are at least communications compatible, and likely that they are matched.
It is still possible that they have different sets of menus and values that are within those settings, or other differences that could prevent stuff from working the way you expect it to based on the settings you see in the menus, but it probably wouldn't cause a fault / error code, just unexpected results while operating.
Such as....say it has four presets for wheel size, 20, 24, 26, 29, but the controller's five presets are 16, 20, 24, 26, and it just uses a code 0, 1, 2, 3 for the dipslay to tell the controller which one to use for speed/distance measuring--none of the display's choices would match with any of the controller's actual numbers...but you have no way of knowing that, until you see a wrong speed in the display while riding, or a wrong distance measurement. AFAICT with most or all of these systems, the display does not read what actual choices the controller has, they just share a I don't think this specific thing is actually done that way, I think the display does all this internally, but it's just an example.
Another example would be that the controller supports user choice of current limits, and defaults to say, half of it's full capability. But the display version firmware doesnt' happen to have a menu for choosing current limit, so you can't change the option in the controller (you don't even know it is there!), and thus never get full power.
Or the display menu settings, when sent to the controller, use "address 0123" for "speed limit" but the controller firmware uses "address 0123" for "throttle disable", so some value of speed limit ends up enabling throttle, and other values disable throttle, and it doesnt' make any sense to the user why this happens.
While I don't know how any of the firmwares in any of these are actually written, these are the kinds of behavior I have seen with the Lishui controllers I have from different systems, vs the Kingmeter displays from each of the systems--the hardware looks the same on all of them, but the firmware is almost certainly different, as they don't behave the same when mixed as they do when matched. I expect this is similar for other systems given the results posted by various members over the years when troubleshooting mismatched stuff.
I was thinking that somehow the ebrake was preventing throttle so I connected the brakes but still no motor run. On the display I can see the ebrake symbol come on when I squeeze the lever.
Given these other test results, it pretty much eliminates all the other common things I could think of.
To be certain, does the motor still work normally using the original controller? If it doesn't operate because the display is not working, you can bypass the display to let the controller work in default mode (whatever that happens to be) by connecting the pins on the controller side of the controller/display connector for Battery + and KSI (also called lock, doorlock, ignition, keyswitch, etc). There's usually five wires, B+, KSI, data TX & RX, ground, but there's more than one possible wire order in a connector (and wire colors don't necessarily express anything other than their color).
You can also do this test with the new controller. If it works without the display, it means the display is telling the controller something that is interfering with normal operation.