Museum restoration of 8800

[Geoff Harrison]

Originally Posted by philrrtx

The 3P+S I/O board turns out to have been made by Processor Technology. So far I haven't found a schematic for it on line.

Dave Dunfield has a copy on line. Click on the S100 link.

Geoff.

[marty]

Grant, should be able to get the switches, either now or when He places his next order.. He should be able to add what ever you need on to a later order.
THANKS Marty

[shunt1]

Today, I stumbled upon this forum and never realized that other "old farts" like me were still around. I was simply trying to show people what my first computer looked like.

I purchased my MITS Altair 8800 in 1975 and still remember a whining college kid that wrote BASIC for it...

Ok, this machine is absolutly useless, but how can I get one?

When I ordered my first one, MITS allowed me to purchase modules for $200.

Since I was in the Army and stationed in Germany at the time, my total income was only $400 per month. However, I was still able to build my first Altair 8800.

Today, after all these years, I am still doing the exact same thing. This machine changed my life forever.

How do I purchase one of these reproductions?

[philrrtx]

Got my replacement ST1-1 switches in and replaced the three broken ones in the upper row. I still haven't found anyone with the ST1-3s in stock, and APEM, the current manufacturer, was less than helpful.

So, in a "what have I got to lose" effort, I removed the ST1-3 with the broken-off handle from the lower row, and carefully pried the case apart. The guts looked good, just a broken handle. I then took one of the old ST1-1s that I had replaced, which only had a bent handle, and pried it open. I straightened the spring-loaded actuating pin from the old ST1-1 and used that handle to reassemble the ST1-3. Amazingly, it works. The little U-shaped case, which holds the mounting stud, isn't as tightly crimped on to the insulating block as it originally was, but it is quite functional. If I find a source for them, I'll replace it with a new one, but I can at least continue troubleshooting with my cobbled-together replacement.

The power supply survived its smoke test, and seems to be working OK, although I still need to check the ripple levels. The filter capacitor for the –16V supply looks like it has blown its safety plug, but it wasn't shorted, so I proceeded to apply power. A replacement is on order. Now, can anyone tell me why the front-panel power switch was oriented with down for ON and up for OFF? Seems backward to me.

In other news, the 4K dynamic memory boards I have turned out to be Rev 2, and all I've found for documentation so far has been for Rev 0. Of most interest at this point is how the little slide switches are arranged for address block assignments. Has anyone seen Rev 2 documentation?

Phil

[marty]

Hi Phil;
I would NOT recemend the 4K Dynamic Memory Boards, Maybe by Rev. 2 , they fixed the Problems, I would use Static memory boards, I had a 16K Dynamic Memory Board, when I had my 8800b, and it never worked, right or well. It would pass Altair memory tests, but it would not hold any thing for very long.... This was back when I bought it new in 1977, I don't have it any more... nor the Altair 8800b, I have one of Grants kits with Static memory, they work much better. Thank You Marty

[philrrtx]

Originally Posted by marty

Hi Phil;
I would NOT recemend the 4K Dynamic Memory Boards

Actually, I misspoke when I said they were the 4K dynamic boards. They are actually 4K synchronous boards, which I believe use static RAM chips. In any event, I finally figured out how the address switches work and it appears that all four are functional.

I have a front panel LED that is dead, but with the CPU and memory boards working, I'm hoping to be able to wrap this project up without too much difficulty.

[philrrtx]

I reached the point where I wanted to run a memory test program and discovered two very annoying deficiencies (IMHO) with the 8080 instruction set.

The first annoyance is that the DCX (double register decrement) instruction doesn't set the condition flags based on the outcome of the subtract operation. That makes it less than convenient for use as a 16-bit loop counter. You have to explicitly check to see if both bytes are zero to find out whether you've reached the end of the loop. If the DCX automatically set the condition flags, you could just follow it with a JZ or JNZ to continue or exit the loop. Likewise the INX doesn't set condition flags, but the single register versions (INR/DCR) do. The 8080/8085 programming manual mentioned this as being a feature, but it's hard for me to figure out why they made it that way.

The other annoying thing I found was the behavior of the HLT instruction. I was basically transcoding a memory test I wrote for the PDP-8, where errors lead to halts, and a complete successful pass likewise leads to a halt. The idea is that when the program stops you can see what address it halted at to determine whether it detected an error or was successful. Unfortunately, when the 8080 enters a HLT on the Altair, the address bus gets tri-stated and you can't read the memory address you stopped at. You also can't restart the program without executing a Reset, because Intel made the HLT essentially a "wait for interrupt" instruction. The solution that I resorted to was to replace the HLTs with JMP $. When the program hangs in a tight (very tight) loop, you can at least press Stop and read what address it is looping on to see if you got an error or not.

In any event, my four 4KB synchronous memory cards all pass the test successfully, so it's on to trying to get the Processor Technology 3P+S I/O card working.

Has anyone heard from Grant lately? I sent him e-mail to the address on the web site before I joined the forums, then sent him a PM here, but haven't had a response to either. I'd like to find out what the prospects are of the museum buying a case and dress front panel from him for our box.