Firewire Port Dead, not recognized

Hallo,

sorry for my English.

Sad return from holidays: after 4 years of perfect work, now ONLY the firewire port do not work (not recognized).

The message is: no firewire port detected.

15 days ago, before holidays is all perfect, and now is gone...

I have tried:

- reset SMC

- reset PRAM

- repaired permission from OSX

- repaired permission from recovery partition (YOSEMITE 10.10.5)

- repaired permission from Web (CMD-R)

- try to reinstall another OSX, no way, port not recognized

- i have tried AHT short and long version: NO PROBLEM DETECTED.

I have three partition with three operating systems ( 2 Yosemite, 1 Snow Leopard) all perfect working. Now none of the OS recognize the port...

The iMac still perfect work now (4 hd attached, 2nd monitor in Thunderbolt, various usb devices attached, all work)

Have any suggestion to resolve my problem? I have tried all solution find in the net.

Thank you for any answer,

Mario, Italy

iMac, OS X Yosemite (10.10.5)