> -----Original Message-----
> From: wrcooke(a)wrcooke.net <wrcooke(a)wrcooke.net>
> Sent: 08 October 2023 04:15
> To: rob(a)jarratt.me.uk; Rob Jarratt via cctalk <cctalk(a)classiccmp.org>
> Subject: Re: [cctalk] VT100: Failing 2114 Chip Replaced With One With The
> Same Fault
>
>
>
> > On 10/07/2023 5:35 PM CDT Rob Jarratt via cctalk <cctalk(a)classiccmp.org>
> wrote:
> >
> >
> > I find this really hard to explain. It can't be the chip selection
> > logic because then the addresses 0x2400-0x2407 would also fail and I
> > checked the CS signal with the logic analyser just to be sure. I also
> > checked the address lines directly on the RAM chip for any stuck bits
> > and they seemed fine too.
> >
> >
> >
> > What are the chances of two 2114 chips failing at exactly the same address?
> > Is there some failure mode I might not be considering?
> >
> > Rob
>
> Perhaps it isn't the 2114 or its associated circuit at all. Maybe some other
> device is being incorrectly selected by that address and driving (half) the bus
> low? Just a thought.
Many thanks for the suggestion. This hadn't crossed my mind, so I checked. All the things that I could identify on the schematic that connect to the bus (UART, interrupt vector, flag buffer and modem signals) seem not to be enabled. I have looked at what is sinking the data bus, there is a buffer which seems to be OK and the 8251 PIC. The PIC is harder to check but I can see it is not selected and the input pins don’t appear to be shorted.
Not really sure what else to consider.
>
> Will
>
> If you want to build a ship, don't drum up people to collect wood and don't
> assign them tasks and work, but rather teach them to long for the endless
> immensity of the sea.
>
> Antoine de Saint-Exupery
Hi
I have had so many inquiries for the boards on my list its
difficult to respond to them all.
Please make an offer for what you want.
Allow an additional minimum of $50 for UK to US shipping
I'll keep the offer list open for a few days.
Rod Smallwood
I posted recently that I had identified a faulty RAM chip in my VT100 and
having replaced it the terminal seemed to get further into the self test.
After further analysis with the logic analyser I have realised that it is
still failing the RAM test, despite an apparent change in behaviour. I think
the change in behaviour could be simply due to the slightly suspect keyboard
cable.
But here is the puzzle. When I first identified the faulty RAM chip if found
that address 0x2408 would read back as 0x0A instead of 0xAA. I reckon this
equates to E50 in the schematic, as it is the upper nibble of the second
bank of RAM. I replaced the chip with one I bought recently. It turns out
the self-test is still failing at the SAME address.
I find this really hard to explain. It can't be the chip selection logic
because then the addresses 0x2400-0x2407 would also fail and I checked the
CS signal with the logic analyser just to be sure. I also checked the
address lines directly on the RAM chip for any stuck bits and they seemed
fine too.
What are the chances of two 2114 chips failing at exactly the same address?
Is there some failure mode I might not be considering?
Thanks
Rob
Quote:
> I could be remembering incorrectly but I think the Gould PN6080 mini we had exclusively for third year
> comp sci at Macquarie Uni in the mid/late 80s was 32-bit made up of AMD2900 family logic (2901 ALU's).
Find attached two pages of the CPU drawings of a Concept 32/67 and
PowerNode 6000. Here the AMD 2901s show up. You remembered correctly!
Geert Rolf
owner of a PowerNode 6040 -- see
https://geerol.home.xs4all.nl/DownLoad/UTX-paper.pdf
There was a PDP-8 (rack straight 8 with asr33) that was on ebay that
disappeared..anyone know if it was sold? I can't find it, maybe the seller
pulled the auction to sell privately.
Bill
Here's a question for all our Computer Museum curators.
Have any of the old GEAC Library systems ever been salvaged and put into
a museum? They were curious boxes and I think would make a nice addition
to a collection.
bill
Hi all
I re-discovered some eurocards I found in a box at a swapmeet long ago.
It looks like a complete 8085 system. Lots of RAM, 2K EPROM, I/O, FDC.
The strange thing is that the EPROM is mapped at F800, the code in there
looks like 8085 code, and looks like it wants to live at F800. And there's
RAM at 0000.
The DIN41612 A/C 64 pin bus has provision for 16 bits data.
Pics here http://retro.co.za/8085/Microcom/
Anyone maybe have more information on this? I'm curious.
W
I have my faulty VT100 now passing the RAM test, but it is still not
producing any output to the screen. Looking at the DC012 chip it seems to be
permanently asserting the interrupt line and not generating any DMA HOLD
REQUEST signals. It seems that all the clock inputs are running (DOT CLK,
CHAR CLK, VERT RESET). Unless there are other clock inputs not shown on the
schematic, I think this suggests that the DC012 is faulty.
Does anyone know of any other reason why the DC012 might behave this way?
Anyone have a spare DC012 chip?
I have posted a bit more detail here:
https://robs-old-computers.com/2023/10/01/vt100-ram-fault/
Thanks
Rob
