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Altair 8800 front panel wiring

NeXT

NeXT

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Location
Kamloops, BC, Canada
An AltairClone has zero consideration for adding an S100 bus inside the extremely empty case if one so desired, but nothing says that with a LOT of diodes you could isolate the lamps and swiches and toggle between the onboard emulation and control of a real S100 machine. (or just disable the emulation completely) That being said, I'm not exactly sure how the front panel interfaces with the bus.

Referring to this photo I can see a number of lines interface with the bus directly and others feed into the CPU card.
Referring to this photo also indicates the front panel PCB isn't entirely passive.

Also, are there layouts of something like an 8-slot S100 bus somewhere I could use to send off to a board house for etching? I see photos of various people and their own designs but no actual layout files.
 
MITS packaged the cheapest white hookup wire that they could find with the kit. As I recall, it's pretty easy to break one of the wires with simple fatigue stress. Add to that the problem of nicking a conductor while stripping.

Then there's the issue of running live AC line traces on the front panel PCB. Let your hand probe around a bit in back of the PCB and you can get a nasty surprise. IMSAI had a much better design, to their credit.
 
Chuck(G) said:
MITS packaged the cheapest white hookup wire that they could find with the kit. As I recall, it's pretty easy to break one of the wires with simple fatigue stress. Add to that the problem of nicking a conductor while stripping.
Click to expand...

I concur... I've fixed, broke, and fixed again several wires on my Altair no more than an hour ago.

NeXT, you might want to look at the schematics and layout for the original front panel board that can be found here: http://www.classiccmp.org/dunfield/altair/altair6.htm

AFAIK the clone's front panel PCB lines up with the original panel, but the dress panel isn't a drop-in replacement. Still, I've thought about it myself and I'm sure it's do-able.
There's some S-100 backplanes for sale via the N8VEM project, I think the layouts are available too but I've not checked.
 
Those schematics were exactly what I was hunting for. Thanks TJ. Those will go a long way towards my plans. Don't worry, I'll be publishing the photographed project when it gets finished. ;)
 
Altair front pannel issue

Altair front pannel issue

I am working on what might be a bad logic chip on the front panel of an Altair 8800a.
A6-A11 go high when we first hit Examine.

Do you have any schematics for the front panel?

Please take a look at this video, and let me know if you have any ideas, of what we should be looking for.

https://drive.google.com/open?id=1Zeh6D76Aj0-NZFUOJlosx3T3kXKhML3T


Thank you
 
Take a minute and look at the schematic link I provided previously. In that file you’ll see a schematic for the front panel board. On that schematic you’ll see that IC-B drives switches A6-A11, which are the ones you’re having problems with.

Mike
 
Last edited:
deramp5113 said:
Take a minute and look at the schematic link I provided previously. In that file you’ll see a schematic for the front panel board. On that schematic you’ll see that IC-B drives switches A6-A11, which are the ones you’re having problems with.

Mike
Click to expand...

Thank you for clarifying, we found the IC-B chip you are referring to on the schematic. I will let you know, what we find out.

We will compare it to working machine.

Thank you again !!!
 
deramp5113 said:
Take a minute and look at the schematic link I provided previously. In that file you’ll see a schematic for the front panel board. On that schematic you’ll see that IC-B drives switches A6-A11, which are the ones you’re having problems with.

Mike
Click to expand...

Hey Mike,

Where are you located? I am planning a cross country trip in March, From Washington to Florida, I like to meet other collectors on my way home.
I met some great collectors when I made the same trip from CA back to FLA last year. Do you have a website?

www.theageofgaming.com

Jeff
 
Novell2NT said:
Hey Mike,

Where are you located? I am planning a cross country trip in March, From Washington to Florida, I like to meet other collectors on my way home.
I met some great collectors when I made the same trip from CA back to FLA last year. Do you have a website?

www.theageofgaming.com

Jeff
Click to expand...

Jeff,

I’m in the Dallas, TX area. My website for the vintage computing hobby is http://deramp.com

Come on by!

Mike
 
deramp5113 said:
Jeff,

I’m in the Dallas, TX area. My website for the vintage computing hobby is http://deramp.com

Come on by!

Mike
Click to expand...

Nice collection, love it... I would like to get a SOL-20 and a IMSAI, to round out my computer collection.

The trip from Seattle Washington back to Florida in March, unfortunately does not go through Texas this time.
I wanted to check out the NVM up there in Dallas, the last time we went through, but it was getting late, by the
time we got up that way from Austin.

One of these days I will get back up that way, so look for a knock on your door. As I would love to see the vintage
computers that you have restored back to life.

Do you have any suggestions, on testing the floppy drive unit that came with my Altair. I am worried about trying to apply
power to it.

https://drive.google.com/open?id=1197OVi0OuEAg0k7n2Arh3aOdeOli8QS8
 
Based on the pictures, your Altair is configured closer to a Sol-20 than to an Altair. Your Altair has the ”Subsystem B” board set from Processor Technology that makes any S100 computer pretty much the same as a Sol-20. Looks like you’ve got the North Star single density floppy controller as well. My guess is your disks are North Star DOS and/or CP/M 1.4?

The disk images and transfer utilities at the link below can be used to archive the disks you have and to create new disks. I’d like to know what your disk labels say is on your disks and possibly archive them with you before putting them at risk in your drives.

http://deramp.com/downloads/processor_technology/sol-20/software/northstar_sd_controller/

Mike
 
deramp5113 said:
Based on the pictures, your Altair is configured closer to a Sol-20 than to an Altair. Your Altair has the ”Subsystem B” board set from Processor Technology that makes any S100 computer pretty much the same as a Sol-20. Looks like you’ve got the North Star single density floppy controller as well. My guess is your disks are North Star DOS and/or CP/M 1.4?

The disk images and transfer utilities at the link below can be used to archive the disks you have and to create new disks. I’d like to know what your disk labels say is on your disks and possibly archive them with you before putting them at risk in your drives.

http://deramp.com/downloads/processor_technology/sol-20/software/northstar_sd_controller/

Mike
Click to expand...


This is getting funnier by the minute. The guy who I have working on my Altair lives in Austin. He has actually purchased a couple of your drop in floppy controllers, and that is what he is using to
work with on my Altair. I told him, if he gets up that way, that he should stop by and meet you in person.

I have not received the Altair, so I do not know, what disks are included.

I did notice on your website that you do have a Northstar system. I have two NorthStar Advantage 8/16 systems, one that boots up and one that I use for parts. Could not get the floppy drives to work properly.
and of course the keyboard pads were dissolved. So, I created new ones, probably not has good as the originals.
I also have a huge collection of unopened software packages for NorthStar and a bunch of opened binders as well. If you are interested in any of these items, just let me know, and I will work on
mailing them out to you.

Ricky is doing a good job with my Altair, but I do not think he will be able to work with the floppy drive or the old memory boards. So, when he is done, I might ask him to mail it up to you, if you have
the time and are willing to take a look at it.


If you send me your email address, I will send you some pics of the NorthStar systems and software packages that I have here. They are just taking up room in my garage right now. It would be nice
to give them a good home.
 
I was looking at your posted problem. It isn't clear that you understand fully how the front panel works. The address displayed is from the CPU, not the front panel. The way it works is that when you hit Examine, it sequences the instruction C3, Low, High onto the data port of the CPU as a fetched value, by the processor. This is done by pulsing the 7405s onto to the CPU at the right time. The CPU attempts to read this location but put into WAIT by the front panel. C3 is the 8080 jump instruction.
Examine Next put a NOP on the CPU's data ports as the instruction.
The address displayed should be the address lines on the S100 addresses, by the CPU, not the address switches, when something doesn't work right.
It helps to have a digital scope or logic analyzer to see the enabling pulse sequences to the 7405s when doing the Examine. A logic probe with a pulse detect can be used but may not always give you the timing information needed in some cases.
On the 10th sheet you can see the 7405s that are used to force the CPU data port.
I hope this information help you on your debugging.
Dwight
 
Dwight Elvey said:
I was looking at your posted problem. It isn't clear that you understand fully how the front panel works. The address displayed is from the CPU, not the front panel. The way it works is that when you hit Examine, it sequences the instruction C3, Low, High onto the data port of the CPU as a fetched value, by the processor. This is done by pulsing the 7405s onto to the CPU at the right time. The CPU attempts to read this location but put into WAIT by the front panel. C3 is the 8080 jump instruction.
Examine Next put a NOP on the CPU's data ports as the instruction.
The address displayed should be the address lines on the S100 addresses, by the CPU, not the address switches, when something doesn't work right.
It helps to have a digital scope or logic analyzer to see the enabling pulse sequences to the 7405s when doing the Examine. A logic probe with a pulse detect can be used but may not always give you the timing information needed in some cases.
On the 10th sheet you can see the 7405s that are used to force the CPU data port.
I hope this information help you on your debugging.
Dwight
Click to expand...

Thanks for the information Dwight, we are looking at the IC-B (7405) on the front panel on page 10, because we know the CPU card and memory card are good.
The IC-B 6 inverters (7405) just happen to be connected to SA6-SA11. of the front panel.
If the switches were locked into the up position, and you hit examine, and you do not have the correct signal coming out of all 6 inverters of the IC-B chip, you would get all highs out.

If we were not sure that the CPU card was good or not, I would check the buffer chips. on the CPU board 8T28s The only problem with that, is that A6-A9 are on Buffer driver C and A10-A11 are on buffer drive D. There are two other address lines on buffer C (A12, A15) and buffer driver D has four other lines it handles.

My first thought was to check the address lines as well, but there is not a single chip that handles A6-A11 specifically. The only one that does, is the IC-B (7405) on the front panel.

I believe that Page 10, that you are referring to, is the Front Panel Logic board. and the 7405 that you are referring to, is the IC-B that we are looking at.

I do appreciate how you were able to explain, the steps involved in doing a fetch, to get the address information as well as the data information onto the buss and then get the LEDs to light up.

Thank you
 
Novell2NT said:
Thanks for the information Dwight, we are looking at the IC-B (7405) on the front panel on page 10, because we know the CPU card and memory card are good.
The IC-B 6 inverters (7405) just happen to be connected to SA6-SA11. of the front panel.
If the switches were locked into the up position, and you hit examine, and you do not have the correct signal coming out of all 6 inverters of the IC-B chip, you would get all highs out.

If we were not sure that the CPU card was good or not, I would check the buffer chips. on the CPU board 8T28s The only problem with that, is that A6-A9 are on Buffer driver C and A10-A11 are on buffer drive D. There are two other address lines on buffer C (A12, A15) and buffer driver D has four other lines it handles.

My first thought was to check the address lines as well, but there is not a single chip that handles A6-A11 specifically. The only one that does, is the IC-B (7405) on the front panel.

I believe that Page 10, that you are referring to, is the Front Panel Logic board. and the 7405 that you are referring to, is the IC-B that we are looking at.

I do appreciate how you were able to explain, the steps involved in doing a fetch, to get the address information as well as the data information onto the buss and then get the LEDs to light up.

Thank you
Click to expand...

You are most likely right about which chip to replace. It is clearly the only one that can be an issue.
There is another thing that always catches people and that is the DO5 signal coming in( on the previous page ). I had to think about this one for some time. What it is is that the CPU sends out status information. The front panel need to know the difference between an instruction fetch and an address fetch. DO5 bit tells the front panel that.
The reason I go through how it works is because just looking at the schematic, one often doesn't understand the function of the processor. When the front panel seems to fail, it is good to verify that the processor is healthy. If you have another board CPU board it is good to try swapping it to see if the problem goes away.
Another thing that causes problem is that the Data connector cable can be connected backwards. The C3 works OK because even if the bits are swapped end to end it is still C3. The addresses will be mixed up.
Anyway, replace the 7405 and it sounds like you are on your way.
 
Dwight Elvey said:
You are most likely right about which chip to replace. It is clearly the only one that can be an issue.
There is another thing that always catches people and that is the DO5 signal coming in( on the previous page ). I had to think about this one for some time. What it is is that the CPU sends out status information. The front panel need to know the difference between an instruction fetch and an address fetch. DO5 bit tells the front panel that.
The reason I go through how it works is because just looking at the schematic, one often doesn't understand the function of the processor. When the front panel seems to fail, it is good to verify that the processor is healthy. If you have another board CPU board it is good to try swapping it to see if the problem goes away.
Another thing that causes problem is that the Data connector cable can be connected backwards. The C3 works OK because even if the bits are swapped end to end it is still C3. The addresses will be mixed up.
Anyway, replace the 7405 and it sounds like you are on your way.
Click to expand...

Sorry, I did not receive this message.

I did have a crazy idea of something to check, maybe you can tell me, what would happen, if instead of toggling Examine, which causes A6-A11 to go high,
we toggle "Examine Next" instead of Examine, while all the address lines are at Zero?
In theory, this could show a problem with the Examine switch process.
 
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