Hi,

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README Files for MPLAB IDE v7.31
http://ww1.microchip.com/downloads/e...eadmeMP731.zip

The above link is from this page:
http://www.microchip.com/stellent/id...cName=en019469

UNZIP the file and open the file "Readme for PICSTART Plus.htm"

Release Notes for PICSTART Plus® Device Programmer

MPLAB® IDE v7.31
Software DLL Version v4.30.02
Operating System Versions v4.40.01 (pspls44001.hex)
February 8, 2006

In the device support List you will see:

PIC18F1320#

Where the # = Some revisions of these parts fail to program.

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See also this thread:

http://forum.microchip.com/tm.aspx?m=151295

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Best regards,

Luciano