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TR909-V4-for-27C256-4000-to-5FFF.EPR
Here´s a technical description from Robin Whittle: (http://www.firstpr.com.au/rwi/tr-909/)
"I use a 27C256, with the firmware programmed into the 4000 to 5FFF locations, because 27C256s are more modern, faster and easier to obtain than 27C64s. This requires that pin 26 be grounded. That said, in 2018, no conventional supplier such as element14, RS Components, Mouser or Digikey supplies 27C256s, since they went out of production around the turn of century. Furthermore, the great majority of 27C256 EPROMs for sale on eBay are fakes, in that they are modern chips made by some unknown company and falsely marked as being ST or some other brand. I use EPROMs which I have had since the 1980s or which I have purchased more recently and whose the chip, package and markings exactly resemble the original devices I have had from those days"