Hey Marc, hope you are well and business is good.
I have tried exactly what you are trying. I was given documentation by Mazak to turn a USB-Serial device into a COM port that the NC side of the control recognises. This consisted of setting DPR16 to 1, and then registering the COM port in device manager to COM3 or COM4 and setting DPR14 to 2 or 3 accordingly.
I tried this on 2 machines with a Keyspan USA19HS adaptor.
I couldn't get it to work. I set DPR16=1 & DPR14=2 (COM3)
Mazak then provided me with a list of "tried and tested" adaptors. The Keyspan WAS NOT on that list - so I assumed the problem was the USB adaptor, not the Mazak procedure.
What you are saying goes against that in that you got a Keyspan to work (intermittently) but didn't use COM3 or COM4.
Anyway - this doesn't diagnose your problem specifically - and I will be surprised if you get much luck outside of Mazak but I am happy to exchange notes. The problem you have sounds very much like a handshaking or driver issue, though I wouldn't put it past being a flaky protocol. I don't suffer issues on Matrix controls but some Fusions need a longer wait time (DPR12), and 1 Fusion I have worked on ALWAYS produces an 887 eventually.
There is a new (cost) option from Mazak called "DPRNT Plus" (developed here in Europe) that uses virtual COM ports and bypasses the dreaded 887 alarm. I have done some tests at Mazak with "DPRNT Plus" and it is far more stable.
Let me know if you want to swap notes..