If it is the old PET with 2k ROM chips, thise 6550 (6540? Can't remember) had additional select lines. I'm sure the ROM has A11 connected and uses it internally as additional select, so it is only active in $e000-e7ff. André Am 16. April 2018 14:17:49 schrieb Francesco Messineo <francesco.messineo@gmail.com>: > Hi All, > looking at 320349 2001N schematics on zimmers.net I can't really > understand how the I/O space addressing don't make a data bus > conflict. > On schematic 4, the UD8 select is directly connected to /SELE, which > is low from $E000 to $EFFF, so UD8 must hold the databus even during > $E8xx accesses. > UD8 databus, like all other ROMs, appear to be directly connected to > 6502's databus, without buffers. > What am I missing? In the original 2001, there was no ROM mapped from > $E8000 to $EFFF, but in 2001N, the I/O chips are addressed with both > /SELE and X8XX signal that decodes (as the name suggests) > A11,A10,A9,A8 = $8. > I feel stupid, I know that 2001N works in real life, I have one > myself, but I can't think that schematic is correct. > > Frank IZ8DWF >Received on 2018-04-16 15:00:32
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