On Thu, Jul 9, 2020 at 5:48 PM Gerrit Heitsch <gerrit_at_laosinh.s.bawue.de> wrote: > > On 7/9/20 5:26 PM, Francesco Messineo wrote: > > 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 cannot raise the output until /RAS on the input is HIGH again. Only > then will the rising edge of /DOT do anything. yes I stand corrected. The rising edge will happen at the next rising /DOT AFTER the "input" /RAS has risen too. > > > > It was probably needed to re-align the rising edge of the /RAS signal > > or to lengthen/shorten its duration. > > This should lengthen /RAS being low. yes, it lengthens and re-align the rising edge of /RAS to a rising edge of /DOT, so probably the edge align was the real necessity. FrankReceived on 2020-07-09 18:02:38
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