Embarking on an Altair 8800C build

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

Re: Embarking on an Altair 8800C build

Postby TheoAU » September 17th, 2022, 4:38 am

AltairClone wrote:Yes - good looking scope shots and it sounds like you have your head wrapped around the issue.

The minimum phi2 high time is 220ns, so I don't like to take all the time we need from phi2 - that leaves us too close to the 220ns parameter.

The minimum phi1 high time 70ns, so we can take some time from there as well, which adds to the time between phi2 fall and phi1 rise that we need.

Assuming R42 for phi2 is presently 6.2K, then 5.6K will take you from 280ns on phi2 down to around 250ns. That gets us about 30ns

Assuming R41 for phi1 is presently 13K, then 10K will take you from 125ns on phi1 down to around 95ns. This gets us another 30ns. This give us 60ns total additional time between phi2 fall and phi1 rise.

Mike


Thanks Mike, this should be a pretty straight forward mod. I'll head to the store tomorrow and get the required resistors.
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Re: Embarking on an Altair 8800C build

Postby BillO » September 17th, 2022, 12:20 pm

TheoAU wrote:Hi BillO,

Here's a capture of IC-N, pins 2 and 8. CH1 is connected to pin 2 and CH2 to 8. Voltages seem good, 13v on both.

Excellent!
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Re: Embarking on an Altair 8800C build

Postby TheoAU » September 18th, 2022, 7:20 am

I checked the existing resistors and they were 6.2k and 13k at R42 and R41. So, today I replaced those with the recommended resistors 5.6k and 10k and I think we have success!

In testing, I kept the order of the boards as before: CPU and front panel in slots 4 and 5; FDC+ in 9. The very first thing I tested was running kill the bit from the front panel injection. That worked. Then I plugged in the 882SIOJP in slot 1 as before, and loaded CP/M using AMON and the Serial disk drive. That worked. Then I set up the boards in the configuration that didn't work initially. CPU and Front Panel in slots 1 and 2; 882SIOJP in 7; FDC+ in 9. I'm happy to report that the slot sensitivity issue seems to be gone!

Just to recall what the issue was. Whenever I tried typing in AMON, I would get inconsistent characters appearing on the terminal. This is now gone, I get a good solid echo back of what is typed.

Here's an updated scope capture of IC-Q. Things look much better. Width of phi2 is 259ns and gap between phi2 falling edge and phi1 rising edge is around 84ns.

Big thanks to both Mike and BillO for their help on this.
Attachments
1.png
IC-Q, pins 13 and 5
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Re: Embarking on an Altair 8800C build

Postby AltairClone » September 18th, 2022, 7:56 am

That’s great!
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Re: Embarking on an Altair 8800C build

Postby Wayne Parham » September 18th, 2022, 9:23 am

Good show, Theo! Congrats!
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Re: Embarking on an Altair 8800C build

Postby BillO » September 18th, 2022, 9:43 am

Perfect!

How are you liking that Fnirsi tablet scope?
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Re: Embarking on an Altair 8800C build

Postby TheoAU » September 19th, 2022, 2:11 am

BillO wrote:Perfect!

How are you liking that Fnirsi tablet scope?


It seems good. It was a good choice for me as it's simple to use and portable. The price was good from Aliexpress, with free delivery to Australia. I've never used them before. It took a little while to arrive, but I was happy with their purchase process.

I looked at lots of Youtube reviews for both that scope and the Hantek range. Both get very good reviews. A friend of mine also has a Hantek model and he recommends it. This guy on Youtube does a really good review of the Fnirsi scope:

https://www.youtube.com/watch?v=kBH5WO9IOMU&list=FL1_m7Mx4aTwFqO1fOHA6kvg&index=1
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Re: Embarking on an Altair 8800C build

Postby BillO » September 19th, 2022, 7:56 am

Yes, I've seen that review. I had thought of getting one as a portable, but I already had a Hantek 4ch USB scope which I moved over to that role. Not quite as convenient as the tablet, but gets the job done, plus I do like the 4 channels.

There sure is a lot of neat new technology being put into play with lower cost diagnostic equipment.
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