The 27SF512 is a replacement for the 29C256.  There are some differences when programming the 27SF512 using the pocket programmer:

1. Ensure that you have the latest software version from http://xtronics.com/memory/updates.html
2. Check the chip type is set to 27SF512
3. Parallel port (non-USB) Pocket Programmer only:
   1. Click on ‘Vpp 12V’
   2. Press ‘T’
   3. Click on ‘Port Delay’ and enter 120
   4. Click on ‘Quit Test’
4. Use ‘Erase device’. This must be done every time the 27SF512 is programmed.
5. Click on ‘Buffer Start’ and enter 8000
6. Use ‘Load Buffer’ to load the ROM image.
7. Click on ‘Buffer Start’ and enter 0
8. Use ‘Program Device’ to program the EEPROM.
9. Use ‘Verify Device’ to verify the EEPROM.