kyuusaku wrote:
I think there was a thread about this already. Kevtris mentioned the PGM pin and how different EPROMs require different voltage on it to be stable, try reversing your level.
Yes, PGM *must* be pulled high when it is not being used. Also, VPP must be pulled high. Never "float" (leave disconnected) either or ground either. The only exception is when VPP is shared with a control signal such as /CE or similar. In that case, the chip has circuitry to handle it.
I have seen some chips do really odd things when VPP was just floated. Stuff like the ROM would work but if you moved your hand near it, the code running would crash (due to noise pickup from your body, etc).