vladitx wrote:
On Mar 12, 12:17 am, "Michael J. Mahon" <mjma...@aol.com> wrote:The clocking of the Softcard is very clever, and a very unusual "syncopated" clock. It interacts with the Apple II by stealing DMA cycles, and therefore exercises areas of slot timing to their limits. The timing margin limits of some cards go beyond the timing margin limits of some Apple II's--particularly post-][+ Apple II's.My guess too, some timing violation must occur. Softcard must have been designed around 1980-1981, so is based around the venerable II+.
I'm not sure the issue is so subtle. What happens is that since the SoftCard and clones use Apple's main memory, they can only do so
during the 500ns when the 6502 normally does it but not the "video" 500ns. Applicard on the other hand has its own memory and only talks tothe 6502 via a pair of registers. The way the "busy" flags are implemented it doesn't matter if z80 runs at 20 MHz and 6502 at 1 MHz as is the case with my kind-of-replica card. By the same token the Apple can also run at any accelerated speed.
I have a ROM1 GS and a SoftCard clone. If you want I can test it. -Alex.