16 channel relay

How to connect 4 pcs of 16 channel relays
on an arduino and how to control them, I must have 64 outputs

Your best bet would be to daisy-chain four 595s. You need three pins.

Make that eight 74HC595 chips. Two per 16-channel relay module.

Did you buy the boards with or without the buck converter parts fitted.
Don’t know what will happen if you put four buck converter outputs in parallel.

16-channel boards do have optos, but no opto isolation (design fault).
Connecting them to a single Arduino will put the supplies in parallel.
Good luck.
Leo…

Wawa:
Make that eight 74HC595 chips. Two per 16-channel relay module.

Adieu!

how to connect 16 pcs of 16 channel relay boards to
arduino mega or raspbery pi - please sketch

256 relays ?

The way you ask your question and the fact that you already received a good answer on the same question two weeks ago gives the impression that you’re just lazy and want other people to do everything for you.

Pieter