On 9/17/2012 5:01 AM, firstname.lastname@example.org wrote: > On 2012-09-17, at 06:45, Jim Brain wrote: > >>> After thinking about it for a bit... You can also use a 74LS00 (or 74HCT00) which is fully used up in the process and has 2 pins less: >>> >> Looking at the original schematic, I sketched one up using a '148 and a '257. With the '257 as Gerrit suggested (the NAND idea would also work) and the 148 handling the 3 signals. The '148 takes care of OE/CE with it's GS pin (14), and the design requires just 2 ICs and 1 resistor (to hold the CHARROM switch position in high unless shorted through the switch/jumper. > The NAND version looks like this: > > https://dl.dropbox.com/u/58002657/rom_replacement_nand.png > > It has four pull-up resistors. One would of course be better. So you mean you used 148 in place of LS11 and 257 for the second one (interchangeable with 00), right? Yes, I hooked the BASIC/KERNAL/CHARROM to the 148, and the GS output feeds the CE/OE line of the EPROM. A12 is then handled as Gerrit and you have previously described. BUt, what does JP301 do in the schematic? I don't think I accounted for it, and I have just glanced at your schematic. I can draw it up again (the 257/148 option was easy to wire, nothing special to lay out), as I just drew enough of it in EAGLE last night to prove to myself it was doable. Jim > -- Jim Brain email@example.com www.jbrain.com Message was sent through the cbm-hackers mailing listReceived on 2012-09-17 18:00:36
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