Went to plug my Roland CAMM-3 mill in and it's meandering all over the place. Tried my Warco and it has the same problem.
I think I have it, apparently modern Chinese USB to serial adapters work at 0-6V, despite RS232C serial being defined as somewhere between +-3V and +-18V
Probably explains why all my USB to serial cables mysteriously disappeared and had to be replaced.
Anyway, I just ordered 2 from Farnell spec'd as +-5V minimum. Cost me 23 each plus tax but I guess you get what you pay for.
Oscilloscope suggests the Chinese are also now diode clamping any negative input to GND. Luckily my poor milling machines did not succumb to this cruel inhuman treatment but hey, let's be careful out there