exidysorcerer.net

I found the cause of the tearing / jitter during screen access. I looked at the HSYNC circuit on an oscilloscope and saw that, during screen RAM accesses (Monitor command TE FE00 FF00), there is a one cycle jitter in timing between HSYNC pulses. That's one cycle of the 6 MHz clock, which is the time it takes for two pixels to clock out.

It appears to be a bug in Exidy's original circuit. The low bit of the horizontal counter gets loaded from the output of a flipflop which clocks the E256 signal. The problem is that, during screen access, that flipflop gets set in order to briefly blank the pixels. That's by design, and it's why we see the brief, short, black lines. But a side effect is that, if the access happens when the counter reloads, the wrong value gets loaded into the counter, causing a short (by 2 pixels) scan line, and horizontal jitter on a CRT monitor. When I lifted IC3A pin 3, the jitter on the scope went away.

Presently, IC3A (IC22A on the II) pin 3 connects to IC7B (IC17A on the II) pin 9. The fix is to connect it to IC7B (IC17A on the II) pin 12 instead. This has been tested with CRT displays on a Sorcerer I and a II.