Building a Clone MITS 8800

[wperko]

Hi, While building the computer, you might want to add a SciTronics RTC board ... "Real Time Clock" ... A small team of us have created reproductions and the Gerber files are FREE for downloading ... Falls under the "Creative Commons Non-Commercial" ... That is you can get a license from us to manufacture and sell the boards, but you can have them fabricated for personal use for FREE.

http://www.brainless.org/Altair/Repository/ReproductionSciTronicsRTCGerbers+Manual.zip

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[troj]

Hi, While building the computer, you might want to add a SciTronics RTC board ... "Real Time Clock" ... A small team of us have created reproductions and the Gerber files are FREE for downloading ... Falls under the "Creative Commons Non-Commercial" ... That is you can get a license from us to manufacture and sell the boards, but you can have them fabricated for personal use for FREE.

http://www.brainless.org/Altair/Repository/ReproductionSciTronicsRTCGerbers+Manual.zip

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Thanks - I'll add that to the list of things to consider, once I have the base system up and running!

-Kevin

 

[troj]

Fuses (PTC type) for the backplane arrived today, so continuing to assemble that. This lead to a lesson - there are two types of S100 connectors. Since I don't have 9 matching ones, I was going to use a mix of my blue and green to make an alternating pattern....only to discover that the rows of pins on the ones that fit my 4 slot backplane are closer together than the ones that fit the 9 slot.

-Kevin

 

[wperko]

Fuses (PTC type) for the backplane arrived today, so continuing to assemble that. This lead to a lesson - there are two types of S100 connectors. Since I don't have 9 matching ones, I was going to use a mix of my blue and green to make an alternating pattern....only to discover that the rows of pins on the ones that fit my 4 slot backplane are closer together than the ones that fit the 9 slot.

-Kevin

Hi,

IF you use modern switching power supplies like the Mean-Well power supplies you don't need fuses and it's better to just short out those fuse points. The Mean-Well power supplies will save you better than the fuses.


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[troj]

IF you use modern switching power supplies like the Mean-Well power supplies you don't need fuses and it's better to just short out those fuse points. The Mean-Well power supplies will save you better than the fuses.

That's what I have for power supplies, but I was planning on going ahead and installing fuses as well.

-Kevin

 

[wperko]

That's what I have for power supplies, but I was planning on going ahead and installing fuses as well.

-Kevin

Hi,

IF you blow a fuse, you have to remove the whole 9-slot back plane to repair it, with jumper wires, the Mean-Wells take care of everything and it's about impossible to blow them out, they just shut down until the short is fixed.


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[RetroAND]

Hello

I am also interested in building a clone of the original Altair computers (or their clones, the Imsai). Seeing the prices, I had two roads before me: one - take a Z80; two - use a soviet microprocessor. As I already had dozens of computers with Z80 I decided this time to switch on to the soviet ones. I already have the basic three-ic set of CPU, clock generator and bus controller. Is there a simple design I could check in order to build a CPU card around it?

Thank you very much

 

[pbirkel@gmail.com]

For reference: http://www.s100computers.com/My System Pages/8080 CPU Board/8080 CPU Board.htm

Not simple, but a proven design that is complete and which can operate as a plug-in replacement for the original ALTAIR & IMSAI systems.

 

[Hugo Holden]

Hi,

IF you blow a fuse, you have to remove the whole 9-slot back plane to repair it, with jumper wires, the Mean-Wells take care of everything and it's about impossible to blow them out, they just shut down until the short is fixed.


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Remember though, while the Meanwell supplies do have overload protection they are not as good as the original analog supplies in other aspects, that people often don't think about.

SMPS supplies specifically designed for computers, if you look at their designs, not only have over-current (overload ) protections, which help to deal with the scenario of shorts, they also have over-voltage protection systems, many are quite elaborate, involving voltage windows and SCR crowbars.

The reason is, in a computer, with a myriad of IC's on a board, an over voltage event is a catastrophic disaster.

For example you might want to look at the design of the IBM-5155 switch mode power supply and the voltage window system they created for the supply outputs. Consider why they did this, it is quite a lot of extra parts.

Or the SOL-20 with an SCR crowbar system on the +5V supply that powered the Mobo.

The Meanwell supplies, which are generic universal SMPS supplies, suited to power a range of appliances, are not specifically intended for computer applications and do not have all these features and failure modes exist in the voltage regulation system where they could potentially over-voltage a computer board, even though they might have good overload/current limiting functions.

Also the old Analog regulators had over-current and over-voltage and thermal protective systems and very rarely, if ever, damage a computer PCB.

So, I'm not saying there is anything wrong with Meanwell supplies, I like them and I use them a lot for various applications, but when it comes to supplying power to vintage computer boards, I either use the original line transformers with Analog regulators or an SMPS supply specifically intended to power a computer.

But of course, without thinking this through, its easier to grab the quick option. And , like a lot of these things, "most of the time" you will probably get away with it.

If I was building a replica vintage computer, the above would give me two reasons to want to replicate the original power supply system design and keep the analog regulators on the S-100 boards as they were when new.

 

[maxtherabbit]

Hi,

IF you blow a fuse, you have to remove the whole 9-slot back plane to repair it, with jumper wires, the Mean-Wells take care of everything and it's about impossible to blow them out, they just shut down until the short is fixed.


.

He's using PTC fuses, they reset themselves

 

[maxtherabbit]

If I was building a replica vintage computer, the above would give me two reasons to want to replicate the original power supply system design and keep the analog regulators on the S-100 boards as they were when new.

Each S-100 card having its own linear regulator is a feature not a bug. I can't imagine wanting to do away with that

 

[hmb]

Hello

I am also interested in building a clone of the original Altair computers (or their clones, the Imsai). Seeing the prices, I had two roads before me: one - take a Z80; two - use a soviet microprocessor. As I already had dozens of computers with Z80 I decided this time to switch on to the soviet ones. I already have the basic three-ic set of CPU, clock generator and bus controller. Is there a simple design I could check in order to build a CPU card around it?

To answer your question as to whether there a simple S100 design that uses the 3 ICs you have...
8080-based S100 CPU board designs do not use a dedicated "bus controller" chip since they do not meet the requirements for interfacing the 8080 to the S100 bus. If by "clock generator" chip you are referring to the 8224 - with the exception of the original Altair CPU board most other boards (like the JAIR) do use it.

I too strongly recommend starting with the "JAIR" 8080-based S100 CPU board that PBirkel provided a link to above. The 8080 chip it uses is not expensive if you avoid the people trying to sell them on eBay to "collectors" at ridiculous prices, whereas it costs under $10 from suppliers of new old stock parts like Unicorn Electronics. That "JAIR" board is available on eBay direct from the person who designed it. Since it is a standalone computer board it is the least expensive way to get an Altair/IMSAI clone up and running. You can begin with nothing more than this board and a power supply, later adding an S100 backplane, front panel, and enclosure as finances allow.

 

[RetroAND]

To answer your question as to whether there a simple S100 design that uses the 3 ICs you have...
8080-based S100 CPU board designs do not use a dedicated "bus controller" chip since they do not meet the requirements for interfacing the 8080 to the S100 bus. If by "clock generator" chip you are referring to the 8224 - with the exception of the original Altair CPU board most other boards (like the JAIR) do use it.

I too strongly recommend starting with the "JAIR" 8080-based S100 CPU board that PBirkel provided a link to above. The 8080 chip it uses is not expensive if you avoid the people trying to sell them on eBay to "collectors" at ridiculous prices, whereas it costs under $10 from suppliers of new old stock parts like Unicorn Electronics. That "JAIR" board is available on eBay direct from the person who designed it. Since it is a standalone computer board it is the least expensive way to get an Altair/IMSAI clone up and running. You can begin with nothing more than this board and a power supply, later adding an S100 backplane, front panel, and enclosure as finances allow.

It's something I have been preparing for some time, therefore I already have the soviet equivalents of the 8080 and 8224. Why have a 8080 for 10$ when I can have two for the same price?
The Jair board seems a good place to start. My only concern at this point is compatibility, as this processor is known to have a bugged interrupt system (they copied it wrong).

Thank you very much

 

[Hugo Holden]

Each S-100 card having its own linear regulator is a feature not a bug. I can't imagine wanting to do away with that

Yes, I agree, it was a good idea because it also distributes the overall heat dissipation to a number of boards & locations.

One thing I am against, is to replace the original linear regulators with switching modules. They do not have the same protections that are built into the analog regulators and I think they pose a risk to the vintage IC's. It is better to put up with the heat dissipation, and to mitigate that, run the regulator inputs at around 7.5 to 8.5V to allow for some line fluctuations and add additional ventilation if required. This is what I do for my SOL-20.

 

[troj]

Interface.....interface? board is done!

I realized later than I would've liked that I didn't have all the sockets I needed, so had to wait for those to arrive.

Next up, the CPU board. I actually have parts to build two of them.

Also waiting to heard back from Martin Eberhard about an 88-2SIOJP

-Kevin

 

[Hugo Holden]

One thing to think about with any replica boards or new boards that plug into edge connectors:

Most of the original S-100 boards, and even smaller boards (like the personality module in the SOL-20) had a taper machined on the leading edge of the pcb where it enters the connector. If there isn't much of one, or none, the forces from all the socket claws add up, and it takes a lot of initial force to get the board into the edge connector. Then, once it enters, it abruptly slams home into the connector shell which is not good for it, and over time that can encourage the shell to crack.

If there isn't much of a taper on the leading edge of any s-100 board, you can easily add one, carefully, by hand with a file. I cannot tell from the photo if there is one there on this board, or not, it may have one.

 

[maxtherabbit]

One thing to think about with any replica boards or new boards that plug into edge connectors:

Most of the original S-100 boards, and even smaller boards (like the personality module in the SOL-20) had a taper machined on the leading edge of the pcb where it enters the connector. If there isn't much of one, or none, the forces from all the socket claws add up, and it takes a lot of initial force to get the board into the edge connector. Then, once it enters, it abruptly slams home into the connector shell which is not good for it, and over time that can encourage the shell to crack.

If there isn't much of a taper on the leading edge of any s-100 board, you can easily add one, carefully, by hand with a file. I cannot tell from the photo if there is one there on this board, or not, it may have one.

any of the major PCB houses can provide a card edge bevel in manufacturing, you just have to ask for it

 

[ef1j95]

Been following along, enjoying the discussion. I'm in the final assembly stages of an Altair 8800c, too. I'm waiting on Q1 (2N4410), which I unfortunately forgot to order.

I powered up the unpopulated CPU board (minus Q1) and one thing I'm seeing is that the unloaded 12V on the CPU board is 13V (measured at R29). I'm a little worried about plugging in the 8080, since this is at the max specification. Perhaps it drops a little under load, though?

Second, the POC* circuit that 2N4410 is part of suggests that other transistors could be used as a substitute. I have a few MPS3904 NPNs with specs that aren't too different in terms of dissipation and current. Max voltages are about half, but I don't see why that would matter? Given the circuit is a simple RC timer with a transistor to switch the 8T97 once C6 charges, I'm curious about this.

Interesting to compare the POC* circuit used in the Altair to the IMSAI, which uses the 8224 to control that line.

 

[hmb]

Been following along, enjoying the discussion. I'm in the final assembly stages of an Altair 8800c, too. I'm waiting on Q1 (2N4410), which I unfortunately forgot to order.

I powered up the unpopulated CPU board (minus Q1) and one thing I'm seeing is that the unloaded 12V on the CPU board is 13V (measured at R29). I'm a little worried about plugging in the 8080, since this is at the max specification. Perhaps it drops a little under load, though?

Second, the POC* circuit that 2N4410 is part of suggests that other transistors could be used as a substitute. I have a few MPS3904 NPNs with specs that aren't too different in terms of dissipation and current. Max voltages are about half, but I don't see why that would matter? Given the circuit is a simple RC timer with a transistor to switch the 8T97 once C6 charges, I'm curious about this.

Interesting to compare the POC* circuit used in the Altair to the IMSAI, which uses the 8224 to control that line.

Under the typical 40mA load the 8080 draws on its Vdd 12V supply, the unloaded output of the resistor/Zener diode "regulator" on the Altair CPU board will drop a bit more - so you will be fine. Further, the absolute maximum 8080A spec for Vdd with respect to Vbb (the -5V supply) is 20V, so worst case scenario you will also be fine.

For the POC circuit an MPS3904 would be an acceptable substitute for a 2N4410.

 

[troj]

Reading the S100 email list/Google group, sounds like I timed it well on acquiring my Altair case from Mike Douglas. Sounds like he no longer has any inventory. Makes me wonder a bit if I didn't luck into the last one he had.

-Kevin