Thanks for replying.
Yes that's actually the conclusion I came to, but I wondered if anyone else had any similar problems with these devices, as I have not had
that much experience with using them.
The control pins are held high at power up via some programmable logic. This provide address decoding / mapping.
Originally the 5v rail was poor, and used to rise very slowly (30ms) and there was a slight delay around 3v (because of a loading effect in
another part of the circuit and a switch mode PSU coming up none of which I can change).
I altered the supply circuit, using a 5v regulator with an enable pin, so it only came on when the incoming supply was above the correct
level. That got the supply rise time to a much cleaner 1-2ms, and I hope has fixed it.
The device datasheets says "powerup supply to read operation" < 10ms but I found it had to be much less than that to work.
So I am still worried that the devices are still capable of getting into a latched state which I can not find a way to free. I can see that if there is a PSU glitch this still maybe a problem.
It seems like the address is being latched internally, but I do not believe that the during the read operation, the devices use an internal address
latch. However, I might be wrong.
The devices does have 'command mode' whereby it is possible to interrogate the device etc. There is a reset command which I tried to no effect.
Thanks