In modicon quantum consists of main rack in which the cpu 140cpu43412A is located

Main rack contained power supply 140cps52400 and 140cpu43412A and RIO HEAD 140CRP93100 in addition to another I/o cards . Another remote rack containe power supply 140 cps51100 and RIO DROP(140CRA93100) in addition to another i/o cards digital and analog.
As we know RIO HEAD and RIO DROP connected to each other using coaxial table.
The problem occurs suddenly no power on the i/o cards of remote rack and error ERROR A appeared on the RIO HEAD OF THE MAIN RACK.
For the first time i think the power supply in remote rack damage and shut off all related cards in that rack but the power supply tell me pwr ok as on it's display. Then i tightin the coaxial cable on the RIO DROP and the power back to the i/o cards, i can't understand why when tighten the coaxial cabe the power pack to i/o cards.

What is the type of signal transmitted between RIO H. and RIO D i dont think such signal employed for powering remote I/O cards, but why when retightin the coaxial cable the remote i/o cards turnning on.

It could very well be that the connector has come loose from the cable or was improperly crimped to begin with. I suggest verifying your network components and other practices against the manual (https://www.mroelectric.com/static/application/product/pdfs/MA-0185-100.pdf).

People take shortcuts with these networks by treating them as one would a cable TV network. If you've any doubt, get new connectors, the proper crimping tool, and redo the connections/network per the manual.
Good luck

Thank you very much

I've had an issue before with a quantum PSU failing but still showing power ok light. I would change the PSU before messing around with anything else.

Did you mean pwr ok appeared on PSU of modicon but the cards beside it in off state, i did not understand.

Yes, Power ok still showing on psu but nothing on the io cards

Similar thing occurs with me but when tightin the drop cable to RIO drop all i/o cards turn on again and that thing which i can't understand.
Yes PSU is the responsible for suppling all cards beside it, but why this happen for me.

But also (ERROR A) was latched on RIO head processor.