rudolpe@upas.CS.ORST.EDU (RUDOLPh ERIC) (10/17/90)
Here goes again: I installed a 16mhz 68000p12F (F!) in the rev 5 amiga 500. For the 14mhz signal, I tried CDAC* XOR 7mhz and (NOT) CDAC* XOR 7mhz with a 74Fast86. The leads fron agnus to the 86 are about two inches long, solid wire. The leads from the 86 to the 68k are about 2 inches long also, with ferrite bead. Now when I turn on the computer, it ALWAYS comes up dark grey first, then bright green replaces it. I get 10 short flashes, and one long flash, and it tries to restart. I am under the impression that DTACK will hold off weird bus cycles. What is the problem here, will somebody explain why this is going bad and how I might get it to work with some extra logic? In my opinion, this is a very very poor Hack. Also, please include the number of your server. My server has a HARD time identifying names. I.E. umbra.cs.orst.edu=128.???.???.??. Thanks.
daveh@cbmvax.commodore.com (Dave Haynie) (10/17/90)
In article <21053@orstcs.CS.ORST.EDU> rudolpe@upas.CS.ORST.EDU (RUDOLPh ERIC) writes: >Here goes again: >I installed a 16mhz 68000p12F (F!) in the rev 5 amiga 500. For the 14mhz signal, >I tried CDAC* XOR 7mhz and (NOT) CDAC* XOR 7mhz with a 74Fast86. If that's the only logic, you're doomed to failure. Even ignoring the problems with the E/VPA*/VMA* logic for now, any accelerated 680x0 must still respond properly to the standard 68000 bus signals at 7MHz. So, for example, that means that you don't drive AS* on the bus until S2, in 7MHz time. Same for the DS* lines on read, and on write you wait until S4, again in 7MHz timeframe. You don't look at DTACK* until the S4->S5 transisiton, 7MHz timeframe, and you don't count on any valid data until the S6->S7 transition. That's the simple basics. From there, you have to worry about the E/VPA*/VMA* logic; you can't feed an 8520 the E from a 68000 clocked at 14MHz and expect it to work right, if at all. If you're trying this with other hardware, you really need BR* and BG* to be clocked in and out of the 68000 on the right clocks, or DMA devices may lock up. In short, there's a reason accelerator boards cost more than 74F86s and 74F74s, and there's also a reason even some commercial accelerator boards have trouble with expansion hardware. >Now when I turn on the computer, it ALWAYS comes up dark grey first, then >bright green replaces it. I get 10 short flashes, and one long flash, and it >tries to restart. Your 68000 is having trouble communicating either with the 8520, Chip RAM, or very likely both. >I am under the impression that DTACK will hold off weird bus cycles. DTACK* gates 68000 access to Chip RAM, true. But you have to play by its rules, as described above. >In my opinion, this is a very very poor Hack. In that, we are in complete agreement. -- Dave Haynie Commodore-Amiga (Amiga 3000) "The Crew That Never Rests" {uunet|pyramid|rutgers}!cbmvax!daveh PLINK: hazy BIX: hazy Standing on the shoulders of giants leaves me cold -REM