On 2012-09-15, at 18:12, Gerrit Heitsch wrote: >> 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. Yeah.. Still one to go ;-) > 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. I shall try with 200 then. I prefer lower power consumption if possible. Not that the difference would be really worth it but I happen to have 200s at hand too. So the choice is obvious. >>> 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. That's what I was wondering about. Whether to put a higher value. 3.3k looks like a good compromise (and is used around the PCB in many places) but if not necessary then higher is better. >> 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 Why? CHAROM is 4KiB, isn't it? I'd put it next to the original one in the same 8KiB segment. Would need to select the halves only whenever _CHAROM is LO /me thinks. > and a more complicated circuit since I assume you want to be able to switch charset and KERNAL independant from each other. Right, and I have still one gate to use, which is a good start ;-) -- SD! Message was sent through the cbm-hackers mailing listReceived on 2012-09-15 20:00:27
Archive generated by hypermail 2.2.0.
