Tony,

Not much luck with my idea that the supply was being pulled down by the displays attempting to show '88888888' '88888888' '88888888'

I did three tests:

1. Bypassed the MCU/PIC 5Volt supply and provided 5V from my bench-top PSU. No luck - problem remains.

2. Set the intensity register low to minimise current drawn by the displays - No luck.

3. Measured the Vcc pins on the displays whilst showing '88888888' '88888888' '88888888' - and the voltages are 'healthy' i.e. about 4.8 Volts

I think I'll just have to revert to one port per MAX7219?

I also made an error in understanding the Iset resistance, it's a MINIMUM of 9.5k so 10k is OK. The circuit I am using is identical to the one in the MAX spec sheet for cascading devices but with the decoupling capacitor 47mF not 0.1mF

Regards Bill Legge