RE: C128 'VIC tower' mod board

Good tip about checking the exact datasheet Gerrit. I did find the datasheet for the exact DRAM used and the -15, -20 did refer to the access time. After some more time spent looking at the data from the LA it does seem the issue was with the /CAS low to /RAS high timing (tRSH). It was a bit too short for the 200ns DRAM. C128 with mod board had Rev 4 VIC and 200ns DRAM.

I checked my C128 without the mod board, which has 150ns DRAM and Rev5 VIC and I did not see a difference in /RAS or /CAS from the newer VIC version.

I did a quick video if anyone is interested

https://youtu.be/SvCESp13ba4

Jeff Birt

-----Original Message-----
From: Gerrit Heitsch <gerrit_at_laosinh.s.bawue.de> 
Sent: Monday, July 20, 2020 11:30 AM
To: cbm-hackers_at_musoftware.de
Subject: Re: C128 'VIC tower' mod board

On 7/20/20 5:58 PM, marko.makela_at_iki.fi wrote:
> On Sun, Jul 19, 2020 at 08:58:15PM +0200, Gerrit Heitsch wrote:
>> I have never seen any 64Kx1 DRAM slower than 200ns though.
> 
> My oldest PAL C64 is equipped with 350ns DRAM chips and a VIC-II in 
> ceramic case. So, they do exist.

Please check tha datasheet for that RAM. It's likely that they printed the RAS cycle time on the RAM instead of the RAS access time. The former is the whole cycle including RAS precharge time. Hitachi and NEC did that for a while, the chips had a '-3' printed on the case. They still had a 200ns RAS access time though.


DRAM with 350 ns RAS access time would be too slow, together with the RAS precharge time it would be more than 500ns.

I do have a few C64 with VIC in ceramic, all 6569R1 come this way, they ran too hot for plastic. You can also find the 6569R3 in ceramic.




>> Maybe they thought they need it but then found out they don't and 
>> omitted the circuit on the newer revision? I mean the 250466 board for 
>> the C64 has an unused place U11 for a 74LS139 next to the RAMs. It 
>> will work fine with U11 populated and J3 cut or U11 empty and J3 
>> closed. If I remember right, the purpose of U11 is to make R/W go HIGH 
>> as soon as PHI0 goes down.
> 
> Should I check it on that board?

On the 250466? I did that, it makes no difference. But if you have the 
newer C128, you could check if the '74 is really gone or if they 
integrated it on the board.

  Gerrit