Re: C128 'VIC tower' mod board

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.


> 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.

  Gerrit