On Thu, Jul 9, 2020 at 5:15 PM Gerrit Heitsch <gerrit_at_laosinh.s.bawue.de> wrote: > > On 7/9/20 3:44 PM, Jeffrey Birt wrote: > > Hi all, > > > > I pulled apart a C128 yesterday that someone sent in with a black screen > > complaint. Upon investigating I found a small PCB with a 74LS74 perched > > over top of U29 inside the VIC-2 RF shield box. I toned out how it was > > wired and discovered it was intercepting /CAS. I posted this on a FB > > group last night and someone said, ‘that looks like the VIC tower on > > schematic sheet 310378-4. Sure enough he was correct. > > > > http://www.zimmers.net/anonftp/pub/cbm/schematics/computers/c128/310378-4-left.gif > > According to the schematics it's intercepting /RAS though. > > It would be interesting to watch /RAS before and after the extra circuit > on the scope to see what kind of difference it makes. That flip flop is doing basically two things: 1) it lowers the Q output (/RAS to the RAMs) as soon (with a small delay) as the input /RAS goes low, the "original" /RAS is connected to the async reset of the FF 2) it raises the /RAS to the RAM at the next rising edge of the /DOT (I assume it's a dot clock negated?) input (this goes into the FF clock input). It was probably needed to re-align the rising edge of the /RAS signal or to lengthen/shorten its duration. Not having a C128, I can't take scope pictures, but the circuit is really straightforward. Might be needed to work around a timing issue on some IC revisions (CBM/MOS best way). Frank IZ8DWFReceived on 2020-07-09 18:02:03
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