On Mon, 29 Dec 1997 20:51:28 +0000 ken <ken@WELWYN.DEMON.CO.UK> writes:
>I have recently purchased two JW parts of the 12C508,

> So I read each
>device to make a record of this value for each chip for future
>reference, but each one only has a NOP (000) at that location. Anyone
>have any idea's why this should be.

MPLAB version 3.22 only shows valid values up to 1FE in the program
memory window.  The value at 1FF is always shown as 000.  You need to
open the Calibration Data window to see the calibration value.  It is
shown there only as a hex number, hopefully MPLAB converts it to a MOVLW
instruction and places it at the otherwise invisible 1FF location.

MPLAB version 3.30 does show the calibration data correctly as a MOVLW
instruction in the Program Memory window.

I think in either case, in order to load the correct value back into the
chip after erasing it, use the Calibration Data window.