Thanks, John. A matrix layout is great: 3 shift registers to control the columns and 2 shift registers to control the rows. Does that mean I need an OR gate at the row-column junction, i.e. relay input trigger? Or can I simply connect the output from the row/column shift registers to the relay input trigger?
*** Correction. I will need an AND gate, e.g. 7408, at each row column junction. With 360 outputs, I will need 90 7408 AND gates. I am wondering if I am better off with 45 shift registers instead.