There was a very comprehensive thread here perhaps four or
five years back on this topic.  I think (?) the general 
conclusion was that the 74xx595 lacks a gate internally
which would be needed to prevent a race condition when
the chips are daisy-chained as you are doing.  At the
time, it was pointed out that more modern alternatives 
such as the TPIC6595 family do have the proper internal
architecture to allow trouble-free series operation.

My brief search for this thread has come up empty :-(
Maybe someone else remembers more about it and can post
the link.

Brian Aase

> Hello,
> 
>      I'm glad to report that I'm using 6 cascaded 74HC595 to light 48 
> LEDs, with 8 bit software PWM individual brightness control. I'm 
> connecting the LEDs on the '595 outputs with 100R resistor and I'm 
> getting, aprox, 9.5mA on the Green and Blue LEDs, 20mA on the Red LEDs. 
> No '595 blown untill now. :)
> 
>      Now, other problem <grin>. Let's say I turn on LEDs 1, 4, 7 and so 
> on. They turn on correctly, but I got ocasional blinks on the other LEDs 
> (2, 3, 5, 6 and so on).
> 
>      It's very weird. I'm scratching my head, looking at my scope, 
> modifying my source code... no conclusions. The LEDs on the first and 
> second '595 are ok, on the third it's happening, on the fourth it's 
> worse, on the fifth it's even worse, on the sixt it's like sh*t.
> 
>      Someone can give any clues? Please, if you need more informations 
> just ask.
> 
>      Thanks,
> 
>      Brusque

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