1394 debugging dead

I have never seen it fail before, in 20 years of kernel debugging, but I have a case now where the host cant talk to the target.

The host has windbg running with the amf 64 1394 debug driver installed correctly:

Opened \.\DBG1394_INSTANCE01
Timer Resolution set to 1000 usec.
Waiting to reconnect…
Wrote 0 of 1 bytes of the breakin packet.
Failed to write breakin packet to target. Win32 error 0n2
WARNING: The HOST cannot communicate with the TARGET!

The target is set up correctly and devmgmt shows the 1394 controller as “…been reserved for Windows Kernel debugger” so its set up OK.

Cables: tried three, two new, one know to work before.
1394 cards are PCI express, StarTech in each end, got 5 of them, brand new, tried them all. No joy.

Tried setting the bdf params on the target, no joy

Both OSs are windows 10

So what is it, windows glitch or hardware issue?

Does anyone know or has anyone seen this happen?