Re: Ribbon Cable

On 2013-12-17, at 22:02, Gerrit Heitsch <gerrit@laosinh.s.bawue.de> wrote:

>>> I sometimes wonder if one could improve stability if one removed the pullups for A12 to A15 and replaced them with a 74LS125. The inputs connected to +5V, the enable signals to AEC. That way A12 to A15 would no longer slowly float up to +5V when VIC takes over the bus but get pulled HIGH quickly. Maybe the same for R/_W.
>> 
>> BTW - I wondered why is it done like that in the first place? I mean the address lines, not R/_W.
> 
> Well, once the CPU is taken off the bus via AEC going LOW, there is no one left to drive the upper address lines. VIC and the 74LS373 only supply up to A11. Everyting else stays in the multiplexed area between the 74LS257, VIC and the DRAMs.
> 
> So you have A12 to A15 floating. This should not confuse the PLA when AEC is LOW since _CS for the Char-ROM does not depend on A12 - A15 with AEC LOW, but you don't want open signals, just in case. Simplest way to fix it are pullups. Cheaper than a 74LS125 for sure.

That’s what I was wondering - like which part actually cares about it being pulled up or not. As you mentioned to supply CHAROM data to the VIC even the PLA doesn’t need them. So - if “nobody cares” about those lines, why making them behave more “digital” when the CPU let’s them off the hook could improve stability.. Although - on the second thought - maybe something on the EXPANSION port could benefit from them being cleaner..

> Another detail I'd take a closer look at would be the 7406/pullup combo that tristates the output the 74LS257 multiplexers when it's VIC's turn. Replace it with a real driver (74LS04).
>> 
>> That “nice” 180 on the OC output. Right - another interesting idea. It was probably a good (read: cheaper) solution and a good use for the otherwise unused 7406 gate.
> 
> Yes, and on the older boards R31 was 1KOhm and it still worked.

Maybe less “usually” then ;-)

> Oh, BTW, it seems 180 Ohm is a bit low, not too long ago I had 2 C64 boards with the same fault, the OC driver in the 7406 that handles the 180 Ohm pullup was dead. Both 7406 were made by NatSemi, maybe they at a dodgy run at one time.
> 
> I agree that it was probably using that leftover driver and a low value pullup or use another IC, so they used the leftover driver. With 180 Ohm that alone eats more than 20mA when pulled low. Thats more than the power draw on a CMOS EPROM.

That’s what I mean by “nice”. Whenever I see (a need for) pull-ups below k ranges, it triggers a little warning lamp that this should have rather been done differently.

-- 
SD!
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