RE: C128 'VIC tower' mod board

It seems this mod was not done on the original Rev 6 PCB but only on Rev7. The only difference I could see when comparing two Rev7 boards, is that the one with the mod board has slower, 200ns, DRAM.  

I put my cheap logic analyzer on it and captures what seemed like the relevant signals. Excuse the messy looking /DOT, it is a 24mhz capture so it can't quite capture /DOT properly.

The mod board pushes back the rising edge of /RAS (/RAS_FF is signal changed by mod PCB) to be closer to /CAS which makes me suspicious it was a workaround to be able to use 200ns DRAM instead of 150ns.  

Image: https://1drv.ms/u/s!AtH4vpaZnzX7kv19R15LKRhaqtRiSg?e=Uy3et2
Saleae Logic file: https://1drv.ms/u/s!AtH4vpaZnzX7kv1-yP-l4nFGsPauag?e=7TBRe1

I need to get the computer back to its owner but if you want any other signals captures, I have three channels left. Just let me know quickly, say by 19 July, 2020 _at_ 4:00PM Central Time (GMT-6).

Jeff Birt

-----Original Message-----
From: Francesco Messineo <francesco.messineo_at_gmail.com> 
Sent: Thursday, July 9, 2020 10:53 AM
To: cbm-hackers_at_musoftware.de
Subject: Re: C128 'VIC tower' mod board

> >>
> >> 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).
>

yes I stand corrected. The rising edge will happen at the next rising /DOT AFTER the "input" /RAS has risen too.

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.