Tony,

Thanks for your interest, I was beginning to think this subject was a dead duck.

I think you must be correct with the communications issue. A few facts about the displays:

1. They are a commercially made item (not that this guarantees perfection, but it rules out obvious stuff-ups) from Futurlec.

2. Each display board has 8 digits and the MAX7219 driver.

3. The Iset resistors are 10k, not 'A maximum of 9.53k' as stated in the data sheet from MAXIM. Do you think the extra 470 Ohms matters?

4. There are decoupling capacitors on the boards: value 47mF, not '0.1mF' as stated in the MAXIM data sheet.

When the display fails it shows '88888888' without the decimal points.

I have just had a thought!

The supply voltages that I measured down the chain of three boards was OK - BUT this was whilst the displays were working OK.

Suppose that the three displays are commanded to display (worst case)
'88888888' '88888888' '88888888'
then the current drain will be much higher than (best case)
'-------1' '-------1' '-------1'
And the 5 Volt supply will be pulled down to a level where the MAX7219 chips won't work?

So it could be caused by an inability to suppy the current if all digits need to be on?

Any ideas/comments

Regards Bill Legge