On 09/15/2012 12:59 AM, silverdr@wfmh.org.pl wrote: > > On 2012-05-14, at 12:56, Gerrit Heitsch wrote: > >> - Build an adapter to use a 27xxx-EPROM in the socket of a 24pin 2364 ROM. Most of it is 1:1, just a few lines need to be rerouted. Do not connect the _CS line directly. Place it either into the KERNAL or BASIC socket. >> >> - Connect all 3 _CS signals for the ROMs to the 3 inputs of one of the gates of the 74LS11. The output goes to _CS and _OE of the EPROM. >> >> - Also connect two of the _CS-Signals to A13 and A14 of the EPROM. >> >> The 74LS11 is a 3-input AND gate. So when the CPU wants to talk to one of the 3 ROMs, one of the inputs of the AND goes LOW, resulting in the output going LOW and activating the EPROM. Which part of the EPROM gets selected is also done by 2 of the _CS lines. You just need to put the 8 K BASIC and KERNAL and the 4 K char into the right parts of the EPROM. > > Could you have a look at: > > https://dl.dropbox.com/u/58002657/rom_replacement.png This might work since JP2 set means A13 will go low if either _KERNAL or _BASIC go low. That might make it possible to have 2 KERNALs in the EPROM. Looks like a good use for one of the other 2 AND gates. Oh... Depending on EPROM speed it might be necessary to use the old trick of tying _CE to GND and only use _OE to control the EPROM if the system doesn't run, is unstable or the charset looks corrupted. 200ns should be fast enough though. >> The 8 K block where A13 and A14 are both low stays empty. > > I didn't like this sentence ;-) Especially that second KERNAL (DolphinDOS, Jiffy, ..) is really good thing to have. That's why I put the jumper and used the second gate. I am somewhat unsure about the pullup R302 but generally the idea should do the trick, right? Any comments? TNX in advance. The pullups can be about any size as long as they make sure the input circuits of the 74LS11 sees them as HIGH. I'd go for 10KOhm. Both signals are static and since you usually switch KERNALs only when the system is turned off or in RESET, the longer time it takes for the LOW -> HIGH transition with a larger resistor is irrelevant. > P. S. I am still thinking of dropping in a second charset too. Like the one from C128 and have ASCII/DIN switch. A second charset means a 27C512 and a more complicated circuit since I assume you want to be able to switch charset and KERNAL independant from each other. Gerrit Message was sent through the cbm-hackers mailing listReceived on 2012-09-15 17:00:05
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