Overhead Panel, Arduino and SPI

[heywoodp]

I'm building an overhead panel using an arduino as a controller. Because of the number of inputs, I've built a shift register based on 74hc165's. My first card runs fine with 6 165's hence 48 inputs. I've currently run out of inputs so have built another card but this time using 9 165's to give 71 inputs. I use a gate signal from the arduino to select whichever card I want. Tested out on the bench and all works ok as expected. I've encountered a switch that doesn't now work. By elimination, this appears to be due to the second card. Removed half the 165's from the second card and all now works. I'm using SPI library to read in the data from the cards. I'm suspecting the fault is due to loading on the SPI SCK signal. Has anybody encountered similar ? How many 74165's could the SPI SCK feed ?

[heywoodp]

Just in case anybody is reading this thread, I managed to get it working. It must have been loading of either the SPI SCK or the latch. I fed these to a couple of hex inverters then onto the secondary boards and its cured the problem