> I have tried to read/write to a x24F128S EPROM, I think it is referred to
as
> I2C, but have never worked out from previous posts what the acronym is
for.

It stands for "Inter Integrated Circuit".  The people at Phillips that
invented the IIC acronym called it "I squared C" instead.  The "2" in I2C is
really supposed to be a superscript.  That's why I usually write IIC if I
don't have superscript capability (like this email message).

> The device X24F128S is a 3.5 volt device (I have easy access to these and
> not the 5 volt versions).
>
> I use a 2k resistor between the PIC port and the EPROM and have separate
> power supply for the PIC and EEPROM which is commoned to both.  Should
this
> be ok or does it sound like this is my problem.

Both IIC lines are driven with open collectors and passively pulled up.  The
threshold voltages are fixed and not dependent on the supply voltage.  5V
and 3V devices can therefore be mixed on the same bus.  You just want to tie
all SDA pins together, and all SCL pins together, then put a pullup resistor
on both lines.  Do no put series resistors on any IIC device.


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