On 9/18/2012 10:53 AM, Gerrit Heitsch wrote: > On 09/18/2012 06:53 AM, Jim Brain wrote: >> On 9/17/2012 2:25 PM, Gerrit Heitsch wrote: >> >> Well, I sent BASIC, CHARROM into the 0,1 lines of the '148. !GS drives >> !CE and !OE on the EPROM, while the A and B lines go to the A13 and A14 >> lines of the EPROM. A12 is handled wither with the NAND gates or the >> '257, nothing new to report there. >> >> Setting up 2 KERNALS means bringing '148 lines 2 and 3 to the ends of a >> 3 pin header. Middle pin goes to !KERNAL shorting one half will shunt >> KERNAL to one of the two address lines. It does require 1 more resistor >> than the '11 solution, as it requires both the 2 and 3 lines of the '148 >> to be pulled weakly high, but the wiring is simpler. > > There is another problem with the solution with the '148 that is not > present when using the '11. In order to switch KERNALs, you have to > route _KERNAL to one or the other input. _KERNAL is the signal used to > select the ROM, meaning it changes state twice per every cycle the CPU > access the KERNAL ROM. That makes it high frequency signal, not > something you want to route through a mechanical switch in a > convinient position at the end of a long cable. As it was a late addition, I chose a quick and dirty solution. I defend it as: * I optimized for soldering time, not purity of circuit. * If this was the basis for a production-quality design, wire routing would not be an issue, so another option would be better. * Routing 1MHz signals through a switch is not the end of the world (people route IO1/IO2 through switches all the time :-) Not saying it won't cause issues, but if someone is doing this as a one-off, there are worse things. I was, though, assuming a short cable, etc. > I think a solution with all 2 input NAND gates is available as well, but that is getting overly academic. It basically boils down to how much the OP wants to solder. Jim > > Gerrit > > > > Message was sent through the cbm-hackers mailing list -- Jim Brain firstname.lastname@example.org www.jbrain.com Message was sent through the cbm-hackers mailing listReceived on 2012-09-18 19:00:52
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