Re: Building a 6502 peripheral - timing

I had 7406 chip problems causing the black screens. One in a VIC20 and one 
in a Plus4.
I think those like to die also, so it always worth to check them.

Istvan

-----Original Message----- 
From: Francesco Messineo
Sent: Friday, March 16, 2018 7:53 AM
To: cbm-hackers@musoftware.de
Subject: Re: Building a 6502 peripheral - timing

On Fri, Mar 16, 2018 at 12:31 AM, Mia Magnusson <mia@plea.se> wrote:
> Den Thu, 15 Mar 2018 18:33:50 +0100 skrev Gerrit Heitsch
> <gerrit@laosinh.s.bawue.de>:
>> On 03/15/2018 05:52 PM, Mia Magnusson wrote:
>> > Den Thu, 15 Mar 2018 09:49:43 +0100 skrev Gerrit Heitsch
>> > <gerrit@laosinh.s.bawue.de>:
>> >> On 03/15/2018 09:21 AM, Baltissen, GJPAA (Ruud) wrote:
>> >>> Hallo Patryk,
>> >>>
>> >>>
>> >>>> No, it can't. PHI2 is nowhere to connect to there so it has to be
>> >>>> taken care of externally.
....

> Too bad that those chips are a bit rare/expensive nowdays. Back in the
> 90's when 8-bit stuff were almost for free it would had been
> interesting to leave a VIC-20 on running a short loop that just writes
> to two different ROM locations, with the values written and values in
> rom selected so all data bus lines clashes in both way for a cycle of
> two writes. Probably nothing would had happened but maybe something
> finally would give up?
>

I've repaired a few (tens or so) C64/VICs/PETs with bad roms recently
and one in the '80s too.
Data bus shorted by anything else than a ROM was and is common too.
Once the bus contention is eliminated, then almost everytime there was
nothing else having suffered damage, though
I could often know there was a bus contention by looking at the too
high current spikes in the bus lines with a HP current tra ofcer.

> What other reasons are there for ROMs to tend to go bad more often than
> for example CPUs?

good point, but also C64 PLAs go bad way more often than CPUs, and
there is no way to
make bus line contention on a C64 PLA that I know.
I think it's often poor manufacturing process.

> (I haven't replaced any bad ROMs myself, and I think I've never read of
> anyone replacing a bad ROM actually examining in what way the ROM is
> broken. Random bit rot or stuff related to the adress bus might just be
> that they are old. But if some bit is stuck at either 0 or 1, or maybe
> even works but with too low bus drive capability, it might stem from
> bus clashes. Although that doesn't explain bad ROMs in a C64 as the PLA
> makes sure there are no bus clashes when writing to ROM addresses as
> writes end up in RAM instead).

my replaced ROM samples, rather wide (Only 6526s samples are more than the 
ROMs)
include:

- one "non selectable" 6540, it doesn't drive the data bus with the
correct selects
- one "non selectable" 2332
- more than 5 "bus holders", they either have shorted data lines or
always think they're selected
- 3 bad contents, they have wrong memory bits, either on row/lines
(depends on how are internally organized I think) or have shorted data
lines to 0/1 but still obey to the select signal.

Other than ROMs, the second widest sample of "bus holders" are the
CIAs and I also have one SID that died holding the data bus on a C64
but I suspect that both die for some "external" connection to the C64.

Frank