Re: C128 'VIC tower' mod board

From: Gerrit Heitsch <*gerrit_at_laosinh.s.bawue.de*> Date: Sun, 19 Jul 2020 20:58:15 +0200 Message-ID: <a91eb81a-e27d-5dd9-e511-0b18d4638478_at_laosinh.s.bawue.de> · gerrit_at_laosinh.s.bawue.de

On 7/19/20 8:43 PM, Jeffrey Birt wrote:
> I have looked at th(RHrd), tsu(WRH) and tCLRH and they all seem fine with either /RAS for read or writes. It is rather curious. Maybe there is some odd edge case where you need to know exactly where and when to look to see the problem.
> 
> I also swapped in the R4 VIC2 into my C128 which does not have the mod board and it worked fine. So, it is back to possibly being about the slower RAM?

I have never seen any 64Kx1 DRAM slower than 200ns though.

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.

  Gerrit