Steve,

I have tried the TI chips in many other circuits where the Phillips ones work (PCF8574).

We manufacture several different products using the Phillips PCF8574 with the PIC16F876 using I2C and Picbasic and thought that a viable substitution would be the TI part.

It has to be something obvious however, trying different pull-up resistors on the I2C bus has had no effect. I still can't write to the TI chip. I can read it fine.

I was wondering if, for some reason, the TI chip was waiting for an acknowledge from the PIC chip or something - something that the Phillips chip didn't require before writing to the output of the chip.

Larryt