Hi Andre, On Mon, Apr 16, 2018 at 2:36 PM, And Fachat <afachat@gmx.de> wrote: > 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. No, as I said, the old (first) 2001 design with static RAMs and 2K ROMs (either 6540 or 2316) has a big I/O space from $E800 to $EFFF with shadows of the same I/O ports. I'm talking about the 2001N schematic, the one with dynamic RAMs and 4K ROMs, they added the X8XX signal to enable I/O chips only in the $E8xx addresses, but I can't see from the schematic how they prevented the UD8 ROM to be "quiet" when the I/O is addressed since it's enabled only by /SELE signal. I think the schematic is wrong, and one of these days I'll check in the actual PCB to find differences. I thought I'd ask here if anyone already noticed the "error" in the schematic (or in my reasoning). > > 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 16:00:02
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