majenko:
How long a piece of ribbon cable are you using?
In general, ribbon cable used for digital communication should be alternately signal and ground (i.e., something like SCK,GND,MISO,GND,MOSI,GND,SS) so that the ground lines reduce crosstalk between the wires.
I think the problem is probably that it's ribbon cable, not that it's stranded wire.
It's about 4 inches long. I have two cables, set up as (GND,+5V,AnalogRead,GND,SDA,SCL) and (SCK,MISO,MOSI,DRDY,RESET,CS). So it's probably the alternating ground absence that's leading to problems.
Waruma:
I am in no way an expert with these two modes of communication, but from what I do understand about them I'm going to blame crosstalk as the reason the ribbon cable isn't working. What are the data rates you're using? I know that ribbon cable has minimal insulation and minimal distance between wires - which would only make crosstalk more of an issue for you. Crosstalk issues would get much worse with higher data rates.
It's probably crosstalk. I'm using standard SPI and I2C data rates, not scaling anything.
Graynomad:
Easy to test, split some wires from the ribbon and see if the problem persists.
Rob
The crazy thing is after I did that, the problem persisted. Only when I switched to short jumper wire did the problem vanish.
floresta:
Maybe he should just go out and buy some of that $5 per foot Monster cable.
Don
What about this? http://www.amazon.com/Diamond-Digital-Audio-Ethernet-Connection/dp/B003CT2A6I