On 2012-09-16, at 12:57, Gerrit Heitsch wrote: >> https://dl.dropbox.com/u/58002657/rom_replacement.png > > Doesn't look quite right to me... > > Lets recap what you plan on doing: > > If _CharROM is HIGH, A12 must be passed to the EPROM unchanged, otherwise BASIC and KERNAL run into problems. Exactly. > if _CharROM is LOW, you want to be able to control its level by a jumper or switch to allow 2 charsets. Exactly. > The simplest way to achieve that is a 74LS257. Yes, or 157.. as we don't need the cutoff of the output anyway. > Tie _OE to GND, connect _SELA to _CharROM. A12 goes into I0b, your switch goes into I0a and A12 of the ROM is connected to output Za. Tie all unused inputs to GND. Don't forget the pullup on I0a. This also makes sure that the one pullup you need is again on a static signal. > > The LS257 is a bit overkill, but it does the job with a single IC and one pullup and you can be sure that all logic levels are correct. Yup, levels are very much my worry here but I wanted to avoid adding another IC, especially one with most pins unused. However if I start adding more transistors and resistors to the circuit, I may quickly exceed both power and footprint requirements of a 257.. -- SD! Message was sent through the cbm-hackers mailing listReceived on 2012-09-16 12:00:24
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