Ubuntu 16.04 FireWire suddenly disappeared

Question

I have been controlling a cable set-top-box with a FireWire channel changer script running on Ubuntu 16.04 for years and it has been working fine.  Yesterday it stopped working for no apparent reason.
lspci lists the controller
06:07.0 FireWire (IEEE 1394): Texas Instruments TSB43AB23 IEEE-1394a-2000 Controller (PHY/Link) but plugreport  returns nothing. The changer script likewise reports that it can detect no ports or nodes.  I have checked the cable and rebooted both the linux box and set-top-box box.  No help.  Can anyone suggest how to troubleshoot this?
UPDATE
I was getting nothing from plugreport because I wasn't running it as root. When I run it properly I get:
sudo plugreport
Host Adapter 0
==============

Node 0 GUID 0x00e6bc100000241d
------------------------------
libiec61883 error: error reading oMPR
libiec61883 error: error reading iMPR

When I searched that error I found this, which talks about a permission problem with the firewire device.  Checking the permissions on /dev/fw0 I find the same permissions as in the link
crw------- 1 root root 241, 0 Jul 27 18:31 fw0

I'm not sure what to do about this, and that bug is reported in a Fedora forum so I'm reluctant to try the fixes they propose.

Steve · Answer

I'm fortunate enough to have another Ubuntu box, so I ordered an inexpensive PCI-E firewire card for it, which allowed me to test.  On the test box plugreport showed the set-top-box.  The card came with a full-size to mini-size firewire cable, which I connected to the other 1394 socket on my original machine, and voila, plugreport on that machine showed the set-top-box.  So it seems that either the mini-size socket on my original machine went bad, or the mini-to-mini cable went bad.  I have no idea how that can happen to things sitting there undisturbed, but there you have it.  So this is SOLVED.
UPDATE:  No it's not!  :-( the day after the above post, it revered to the original situation even using the new socket.  Once again plugreport shows no node 1.  I have no idea what is going on.

Ubuntu 16.04 FireWire suddenly disappeared

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