Embarking on an Altair 8800C build

Discuss construction, troubleshooting, and operation of the Altair 8800c computer

Re: Embarking on an Altair 8800C build

Postby Wayne Parham » August 18th, 2022, 9:01 am

Very good. Onward! Now you're cookin' with gas!
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Re: Embarking on an Altair 8800C build

Postby TheoAU » August 20th, 2022, 7:35 am

I made quite a bit of progress today. The front panel Interface board is complete. The Front Panel board is ready to have the switches soldered on. I've got it set up in the jig ready to solder tomorrow. So, just switches. LED's and connectors to go. I found using spray can lids works well for keeping the jig off the desk. I also came up with a novel way of holding the front panel PCB onto the switches. A rubber band and a spray can lid to hold it down.
Attachments
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Re: Embarking on an Altair 8800C build

Postby TheoAU » August 20th, 2022, 7:38 am

Front Panel Interface board.
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Re: Embarking on an Altair 8800C build

Postby AltairClone » August 20th, 2022, 12:57 pm

Looks good! Getting close :)
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Re: Embarking on an Altair 8800C build

Postby Wayne Parham » August 20th, 2022, 4:28 pm

Good stuff! The coffee's starting to brew!
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Re: Embarking on an Altair 8800C build

Postby TheoAU » August 20th, 2022, 9:06 pm

Switches and LED's installed. They all look nice and straight. I was bit nervous about these steps but they turned out well.
Attachments
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Re: Embarking on an Altair 8800C build

Postby BillO » August 21st, 2022, 2:01 pm

Nice going! Can't wait for the first boot.
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Re: Embarking on an Altair 8800C build

Postby TheoAU » August 21st, 2022, 10:17 pm

The news is generally good. I have a working system. Although there are some issues. After my last post, I installed the connectors on the front panel. I got impatient and tested the front panel with a 50 pin cable that I already had. However, I didn't realize that this cable connected the pins in reverse on the front panel side. The LED's did come on but nothing else worked (obviously). Hopefully connecting the cable that way didn't cause any problems?

I then made the correct 50 pin cable and the 8 pin cable to connect the interface board with the CPU. I did an initial power-on test to check voltages and everything seemed ok. Then I tested the system with the following boards installed:

- Replica CPU
- Front panel interface
- FDC+ with memory enabled and PROM disabled
- 882SIOJP with PROM enabled and serial port 1 connected to Teraterm

I tested this configuration in my original machine and it jumped to AMON on power up. So I was hoping to get the same result with the 8800C. But unfortunately it didn't work. The good news is that the following things worked:
- Stop/reset
- Memory examine and deposit, including examine next and deposit next.
- Run/stop

That's where I left things yesterday, some good signs of life but not perfect.

Today, I had a little more success. I changed the configuration to:
- Replica CPU
- Front panel interface board
- California Computer Systems 64k Dynamic ram
- MITS 882SIO

With this configuration I was able to successfully run the serial echo program on port 1. I was also able to load 4k Basic v4 via tape file from Teraterm. I was really happy with the outcome as it shows the machine is generally working!
I have discovered a couple of issues though. I'll post those up shortly. See pics below of bench test setup.
Attachments
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Bench test setup with CPU, front panel interface, CCS64k and MITS 882SIO
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Loading 4k Basic from tape file through Teraterm
Last edited by TheoAU on August 22nd, 2022, 2:42 am, edited 2 times in total.
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Re: Embarking on an Altair 8800C build

Postby TheoAU » August 21st, 2022, 10:35 pm

Now that I've experimented with the machine, these are the things I've observed:

Issues:
1) I can't access the PROM on the FDC+. When I access the PROM region all data lights are on.
2) Communication through the 882SIOJP serial port doesn't work properly. Data sent out to the terminal by the computer seems ok. But data typed at the keyboard doesn't always work. I've found switching to 7 data bits helps a little bit but still not perfect.

Both of these boards work fine in my original Altair. So the issue must be related to the new front panel and/or motherboard.

Things that work:
- The MITS 882SIO board works fine as a terminal. Serial echo is perfect. Works fine in Basic.
- I enabled the code snippet injection feature and that works great. Very nice feature Mike!
- FDC+ disk loading works fine. I can load CP/M using the disk boot loader in the 882SIOJP.
- I can access the PROM on the 882SIOJP. AMON runs when I power on and reset the machine. But I have the same issue with terminal quality.
Attachments
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Loading CP/M with disk boot loader in the 882SIOJP and FDC+
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Re: Embarking on an Altair 8800C build

Postby Wayne Parham » August 22nd, 2022, 11:40 am

I had similar troubles with one of my front-panel interface boards. Mine ended up being caused by one of the NOR gates in the 74LS02 chip in location "L."

If you socketed your chips and have spares, you might swap 'em one at a time and retest. You can always break out the 'scope too, but you may find it just by swapping parts.
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