I have some problems with my gigatron.

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denjhang
Posts: 23
Joined: 02 May 2021, 01:25

Re: I have some problems with my gigatron.

Post by denjhang »

Now I have replaced the RAM and ROM, using HM62256BLP-8 and M27C1024-15F1, and the crystal oscillator uses 6MHZ.
Unfortunately, my gigatron still does not work properly. After turning on the power, Blinkenlights quickly lights up the first led, and then quickly turns all four led lights on.
I changed the crystals of different values and found that when using a low frequency (such as 2MHZ) crystal, the above process becomes slower, and using a high frequency is faster. The result is the same every time.
denjhang
Posts: 23
Joined: 02 May 2021, 01:25

Re: I have some problems with my gigatron.

Post by denjhang »

The new ROM I bought looks correct. The chip inside the round glass is a flat rectangle instead of the previous square.
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bmwtcu
Posts: 54
Joined: 01 Nov 2018, 12:02

Re: I have some problems with my gigatron.

Post by bmwtcu »

Do you have an EPROM and SRAM with faster access times that you can try? The EPROM part in your picture has 150ns access time. At 6MHz, your clock period is ~167ns. That means the PC needs to put out an address, wait 150ns for the EPROM to put out valid data, and read the EPROM output into IR and D in less than that 167ns window and there isn't much margin for error. Similarly, from his notes on page 5 of the schematic, Marcel had intended for 1 SRAM access and 1 ALU operation in that same window, and at 80ns, you're probably riding the hairy edge. If you have a slower crystal, you could try increasing your clock period to see if you at least get correct Blinkenlight behavior. Note that TTL logic parts cannot drive HC parts reliably because HC CMOS and TTL logic levels usually aren't compatible. Other than the 595 shift register which is only available as a HC part, you should confirm that all the other parts have TTL compatible logic levels. From the screenshot above, it looks like your Bus Access Decoder is using a 74HC139. You should replace it with a 74HCT139 part. This is likely part of the problem.
Hans61
Posts: 27
Joined: 29 Dec 2020, 16:15
Location: Saxonia
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Re: I have some problems with my gigatron.

Post by Hans61 »

It's not so bad with the M27C1024s that don't work. I have enough that work. My negative experiences with Chinese goods are within limits.
The M27C1024 can be written, read and compared with the TL866II without any problems. My first thought was also the access time. My slowest crystal I had there was 2 Mhz, that's 500ns.
With that the LED'S flash with the working ROM but not with the other one. But probably I have some kind of thinking error there. I just wanted to understand it, but it's not important.
denjhang
Posts: 23
Joined: 02 May 2021, 01:25

Re: I have some problems with my gigatron.

Post by denjhang »

I changed 74HC139 to CD74HCT139E, and tried 2M/4M/6M crystals, and the ROM was changed to AT27C1024-45PI. However, the result was disappointing, and there was no change from the last time.
The four LED lights of Blinkenlights are still on after the power is turned on, and the first light is turned on for a short time, and then quickly turned to full on. :roll:
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bmwtcu
Posts: 54
Joined: 01 Nov 2018, 12:02

Re: I have some problems with my gigatron.

Post by bmwtcu »

Are you able to rule out defective/fake ICs? Some PROM programmers have built-in 74xx test capability. I don't think you had said so, but have you already definitively ruled out connectivity issues at each of your diodes like at67 suggested? I'd be extra careful of that since AFAIK you're the first person to try that there. Use a DMM in continuity check mode and make sure you have connectivity across the diode when testing at your headers. Make sure you have your red lead on the anode side header and black lead on the cathode side header. Don't touch the diode itself in case the weight of your test leads makes a difference with contact. Usually when you build from a purchased kit you're really only dealing with bad solder joints, but you definitely have more things to check when you DIY or deviate from the build instructions. Also, just thinking outside the box, the EPROM is a 16-bit part, so it's possible to mess up the byte order by clicking byte swap when programming. It sounds like you're at least past a few instructions if the LED state changes, so that might not be a concern.
at67
Posts: 329
Joined: 14 May 2018, 08:29

Re: I have some problems with my gigatron.

Post by at67 »

Can you post a video of your Gigatron as it boots, (specifically on the Blinkenlights), that should tell us pretty quickly if your Gigatron is actually running any native code instructions.

The LED's change state pretty slowly relative to how many thousands of native code instructions are run per 60Hz frame, so if your board is running the first few steps of the LED sequence then your issue will be something trivial most likely. I'd be highly surprised if your board was indeed doing this, but it's worth a shot.
denjhang
Posts: 23
Joined: 02 May 2021, 01:25

Re: I have some problems with my gigatron.

Post by denjhang »

I found that TL866II-Plus has the function of testing 7400 series chips, so I will soon test all the 7400 series ICs I have.
I must find out the correctness of these ICs as soon as possible, because in addition to Gigatron, I also produced Omega MSX2, and this project also uses a large number of 7400ICs.
I very much suspect that it is the problem with these 7400ICs. As for the diodes, I tested them one by one with a multimeter.
denjhang
Posts: 23
Joined: 02 May 2021, 01:25

Re: I have some problems with my gigatron.

Post by denjhang »

I used the TL866II-Plus programmer to test all the 7400 series ICs I own and found that they are all normal. Those discrete diodes were also tested with a multimeter, and there was no problem. I have recorded a few videos so far.
https://www.youtube.com/watch?v=SpnsXJP4F38

Then I removed the crystal and something interesting happened.
https://youtu.be/aagaJgz1yuA
at67
Posts: 329
Joined: 14 May 2018, 08:29

Re: I have some problems with my gigatron.

Post by at67 »

The second video without the crystal looks like the MCP100 is continually causing your Gigatron to reset 3 times a second in a loop, (entering a random Blinkenled state). I'm not sure why it would do that without a crystal, probing crucial signals with an Oscilloscope would probably find an explanation.

It's starting to look more and more like a ROM issue, as @bmwtcu hinted at; see this thread, (if you haven't already), for a comprehensive debugging session with Marcel that turned out to be a dud ROM: viewtopic.php?f=4&t=137

P.S. If you private message me your address I can send you a spare working ROM that I have, I have no idea how long you would take to receive it, as I am located in Australia.
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