Pin 18 is /CS2 on the 2316 ROM and the 2516/2716 EPROM. On the Mainboard, Pin 18 is Connected to A11. It works unless you try to use a 2332 / 2532, which will give the named conflict. The reason for not using the x8xx signal to blank out only the 256 byte I/O window from the /CS for UD8 seems to be, that orignally it was planned to use another memory configuration, which eventually was abandoned to be compatibile with the orignal PET: On page 1 of the schematics (upper right, next to the memory expansion connector) you can find a jumper for a signal named I/O. It allows to move the I/O adress window from E8xx to 88xx. In this configuration, a 2332/2532 can be used for UD8. This memory configuration would have been much more flexible, it would give a continous ROM area from $9000 to $FFFF without the I/O window in between. Christian Am 16.04.2018 um 14:15 schrieb Francesco Messineo: > 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 > -- Christian Dirks Toast_r@Idealine.info Vorster Str. 66 47918 Tönsivorst 2. Stellv. Vorsitzender Verein zum Erhalt klassischer Computer e.V. http://www.classic-computing.de/
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