I'm sorry I've been away for a few days at the cottage.
I was waiting to get my Altair before continuing with this project.
It came 7 weeks ago, but before it did, I got swamped with a bunch of projects.
I'm still working on 2 projects at the moment, so this project has been sitting on the side lines.
Readers digest version:
I built a prototype to get an IMSAI at my local museum up and running CP/M.
We discussed it here and came up with a list changes to make it more versatile.
I was going to continue the project based on those changes when I got the Altair, but life happens.
If I recall correctly, the changes were:
-Allow the board to be Phantom-able.
-Deselect blocks of on board RAM to allow for external RAM access on the bus.
-Narrow I/O addressing to allow bus I/O
-Add a real UART
Other changes I had in mind to improve transparency of the PIC was to add a buffer chip and extra SPI SRAM for Flash ROM buffering.
This board's biggest feature is in how the PIC is doing 3 tasks (Boot ROM, UART and Floppy Emulation).
John, to answer your question. The local bus wasn't taken out to the S-100 bus for two reasons, 1. It was extra work to hard wire. 2. This board already had 64K RAM, which was all the ram any classic 8080 system would ever need (my guess). (Sounds like that famous saying by Mr Gates, "640K is all that anyone would need")
DO5 was the only one taken out because the IMSAI front panel needs it to create an early M1 signal. The M1 signal that is on the S-100 bus is too late to put the 8080 in to a wait state cycle. The front panel uses this early M1 on it's RUN/STOP flip flop so when you stop, you're on an M1 cycle. For Z80 cpu's, the STOP button will stop the CPU on any cycle, you then must single step to the next M1 cycle before using "Examine" or "Examine/Deposit Next" buttons.
Hopefully, within the next week, I should be able to bring 1 of my current 2 projects to a milestone and then I can start on building another prototype with all those extra features so I can get my Alair running.
I haven't even plugged my Altair in yet... guess I should do that soon to start reforming those caps?
*For anyone not familiar with old electronics, it's best to reform electrolytic capacitors. On linear power supplies, I think I'm safe to power the unit through a variac. 2% Voltage for the first day, 5% next day, 10% on the 3rd day, 25%, 50% then 100%.
Subject: RE: [N8VEM-S100:1772] IMSAI 8080 CPU card replacement
Date: Thu, 8 Aug 2013 11:28:27 -0700
Andrew here is my first pass at trying to hammer the 8080 “Front panel” board into IEEE-696 shape.
I’m not entirely comfortable with it as I don’t understand some aspects.
Josh why for example is the Data out line (S100 pin 39) the only one coming off the board to the S100 bus.
As the board looks it will never access any other off board RAM/ROM.
It’s unclear to me also how I/O ports are addressed. Looks like ALL ports go into PIC. Is there a range we could select for so that are outside that range and go to the S100 bus/other cards.
I think the only way we will get INT’s to work would be to play with a prototype.
Attached are my initial “suggestions”
Hi John! Thanks! Here are the most latest S-100 8080 CPU board schematic and PCB layout. Please review and send me any changes and/or corrections.
Ignore the J1 (DB25M) since I don’t know where to put it and it doesn’t make a lot of sense where it is. Maybe this should be a 13x2 dual row header? DB25M has nowhere to go with the present layout and wouldn’t most builders just use a ribbon cable? Also ignore the extra filter capacitors since those are just spares for any additional chips that will probably get added later.
I have not started trace routing since it may be a couple iterations yet before we’re ready to set an initial PCB layout.
Any input from anyone is greatly appreciated. Also what is the Altair FP connector look like? How is it connected?
Guys, I’m wondering if it would make sense to modify that board more. I see there is a decent demand for it. If people are not in a hurry it might make sense to modify it could act as a bus master like the old style IMSAI/Altair accepting DMA etc. This would require adding a true data bus in and out lines, 2MH clock etc. so it could actually be used with other S100 cards in the bus.
I (or others), could take a stab at this, but it probably would be best to do a prototype board first.
We need to have both Altair and Imsai front panel connector.... Same signals, different connector
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