RE: Ribbon Cable

From: Bil Herd <>
Date: Tue, 17 Dec 2013 16:59:15 -0500
Message-ID: <>
Several of the times when  I tried to "clean up" this kind of thing for
the C128 I would come across a compatibility issue; we found that third
parties had used the glitches on IO lines to latch R/W and that some carts
like the Magic Voice cart would drive the bus at unexpected times
including the high address lines (They would toggle /GAME and /EXROM
dynamically when the system had been designed for them to be solder
jumpers).  Eventually I stopped trying to "improve" it and try to learn to
"live with" it.


-----Original Message-----
[] On Behalf Of
Sent: Tuesday, December 17, 2013 4:27 PM
Subject: Re: Ribbon Cable

On 2013-12-17, at 22:02, Gerrit Heitsch <> 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.

       Message was sent through the cbm-hackers mailing list

       Message was sent through the cbm-hackers mailing list
Received on 2013-12-17 22:04:38

Archive generated by hypermail 2.2.0.