7i78 power

I have a system 7i92+ 7i76 (on P2 connector of 7i92) +7i84 (connected to RS422 of 7i76).

I am using separate (not FPGA) 5V power for 7i76 TB3 pin 21,22 (W2 in right position).

Now, I want to add one 7i78 (on P1 connector of 7i92).

If I do not use FPGA power, how to power the 7i78?

You can power the 7I78 via one of the step/dir connections,
but typically the 7I78 draws such little current (<100 ma) that
it makes  mores sense to power it via the FPGA cable.

That mean 5v and ground pin of any step/dir to be connected?

Yes, that will work (but it's easier to just power it from the FPGA card)