I have some problems with my gigatron.

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

I have some problems with my gigatron.

Post by denjhang »

I need your help. Currently my gigatron is fully assembled, but when I connect the power supply, all four LED lights are on.
By the way, I currently cannot buy 6.25mhz crystals (I use 8mhz instead) and 4606X resistors (I use discrete resistors to build).
And all the diodes were soldered in-line single-pin female sockets by me (actually I want to try to replace these diodes with light-emitting ones).
The following picture is the result of repeatedly turning on and turning off the power. Each time the power is turned on, the four LED lights will have different lights on. And keep the power on, the four LED lights will not change, and I tried to connect the gigatron to the vga monitor, and found that there was no signal.
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denjhang
Posts: 23
Joined: 02 May 2021, 01:25

Re: I have some problems with my gigatron.

Post by denjhang »

I use a multimeter to test and refer to https://gigatron.io/wp-content/uploads/ ... oting.xlsx
In diode mode, I found that all ICs are normal.
But in the voltage detection mode, there are many IC pin voltages that do not match the data in the table.
For example, in my gigatron U3, the voltage of pins 11-13 is 5V, and the voltage of pin 14 is 0.
The 7 pin of U14 is 0, and the 13 pin is 5V.
at67
Posts: 329
Joined: 14 May 2018, 08:29

Re: I have some problems with my gigatron.

Post by at67 »

Looking at your photos it looks to me like you have used turned pin headers for the diodes, are you sure that the diodes are making full contact with both sides of the PCB? If even one of the diodes is not making 100% connection to both sides, then the instruction decoding will fail and the boot up process will hang.

Also I would be very surprised if you could just replace fast switching signal diodes with LED's, there are 3 main issues I can think of, (but there are probably more):
  • Voltage drop, signal diodes have voltage drops ranging from 0.2v to 0.7v, (depending on the type). LED's have voltage drops ranging from 1.4v to well over 2v.
  • Current limiting resistors, even though the maximum sink and source currents of TTL devices is limited, you really would need to use series resistors to both protect the LED's and the TTL outputs, as LED's represent a non linear resistive load.
  • Switching speed, I would imagine the switching speed of LED's would be far slower that a signal diode, that with the required current limiting resistor mentioned above, would probably play havoc with the timing, (setup, hold, propagation delays, etc), of the signals.
If you really wanted to add LED's to the diode farm, you would probably have to tap the diodes with buffers driving the LED's, (with current limiting resistors), so an extra switching transistor, resistor and LED per diode, (which may not be practical given the board space limitations).
denjhang wrote: 14 May 2021, 14:54 I use a multimeter to test and refer to https://gigatron.io/wp-content/uploads/ ... oting.xlsx
But in the voltage detection mode, there are many IC pin voltages that do not match the data in the table.
For example, in my gigatron U3, the voltage of pins 11-13 is 5V, and the voltage of pin 14 is 0.
The 7 pin of U14 is 0, and the 13 pin is 5V.
Your Gigatron is most likely halted/crashed on an incorrectly decoded instruction, hence U3, (part of the program counter), will most likely have halted. There isn't much point looking at voltages of data/address pins if your Gigatron is in this state. I would look at all the GND/VCC connections of every IC and verify that every chip is powered up correctly. After that I would then probably check every diode's connections and also checking for PCB shorts/open circuits, (with the power off), on every PCB track/IC/diode pin.
Hans61
Posts: 27
Joined: 29 Dec 2020, 16:15
Location: Saxonia
Contact:

Re: I have some problems with my gigatron.

Post by Hans61 »

If the crystal is not 6.25mhz there will probably be no picture. The function can then only check with the flashing Led's.
I have already built a few Gigatrons without problems. Also one with a socketed crystal to play a bit. My Gigatron does not run with 8mhz anymore.
I would try a crystal smaller than 6.25mhz if available. Your EPROM is a M27C1024-25F1 if I see it right in the picture.
That should mean 250ns access time. The Gigatron has a clock of 160ns (at 6.25mhz). So this should not work.
I've also noticed a general problem with various EPROM's on my end. I wanted to investigate it again, but had no time until now.
All EPROM with square chip in 4 segments do not work for me. The ones with rectangular chip do work. And it is not a problem with the access times.
not work (for me)
20210217-211132-029-gigatron-geht-nicht - Kopie.jpg
20210217-211132-029-gigatron-geht-nicht - Kopie.jpg (266.22 KiB) Viewed 430 times
Works (and similar)
20210217-210658-027-gigaton-geht - Kopie.jpg
20210217-210658-027-gigaton-geht - Kopie.jpg (46.69 KiB) Viewed 430 times
at67
Posts: 329
Joined: 14 May 2018, 08:29

Re: I have some problems with my gigatron.

Post by at67 »

Hans61 wrote: 14 May 2021, 21:03 I would try a crystal smaller than 6.25mhz if available. Your EPROM is a M27C1024-25F1 if I see it right in the picture.
That should mean 250ns access time. The Gigatron has a clock of 160ns (at 6.25mhz). So this should not work.
Well spotted, also it looks like his SRAM is 120ns, you need 70ns, (or lower), SRAM for the Gigatron to run reliably at 6.25MHz.
denjhang
Posts: 23
Joined: 02 May 2021, 01:25

Re: I have some problems with my gigatron.

Post by denjhang »

Thank you for your help. I have placed an order to buy a lower frequency crystal and a faster ROM.
For RAM, I cannot buy 62256 within 70ns (currently I can only find HM62256BLP-12, UT62256CPC-70LL, these may not be useful.), is there any alternative? I currently plan to use HM628128DLP5 and follow the ram doubling method: viewtopic.php?f=4&t=9.
denjhang
Posts: 23
Joined: 02 May 2021, 01:25

Re: I have some problems with my gigatron.

Post by denjhang »

:roll: Maybe I can also buy this AS6C62256A-70PCN with -70 or -55 chips, is that okay?And I also want to try AS7C256-12JC.
The chips I currently own are HM62256BLP-8, HM62256BLP-10, HM62256BLP-12, HM628128DLP-5. Will these work?
at67
Posts: 329
Joined: 14 May 2018, 08:29

Re: I have some problems with my gigatron.

Post by at67 »

The AS6C62256A at 70ns or 55ns should be fine.

The HM62256BLP-8 would be borderline at 6.25MHz, it may work, but it would probably be temperamental if it did.

The HM628128DLP-5 will work at 6.25Mhz and 8Mhz, but you would need to wire it up according to this thread, (use the 128Kx8 diagram as a reference):
viewtopic.php?f=4&t=9

Anything over 80ns, i.e. -8 or bigger will not work.

Always check the pinouts and voltage specifications of any devices you choose, (use the thread I posted above as a reference).
denjhang
Posts: 23
Joined: 02 May 2021, 01:25

Re: I have some problems with my gigatron.

Post by denjhang »

Hans61 wrote: 14 May 2021, 21:03 If the crystal is not 6.25mhz there will probably be no picture. The function can then only check with the flashing Led's.
I have already built a few Gigatrons without problems. Also one with a socketed crystal to play a bit. My Gigatron does not run with 8mhz anymore.
I would try a crystal smaller than 6.25mhz if available. Your EPROM is a M27C1024-25F1 if I see it right in the picture.
That should mean 250ns access time. The Gigatron has a clock of 160ns (at 6.25mhz). So this should not work.
I've also noticed a general problem with various EPROM's on my end. I wanted to investigate it again, but had no time until now.
All EPROM with square chip in 4 segments do not work for me. The ones with rectangular chip do work. And it is not a problem with the access times.
not work (for me)
20210217-211132-029-gigatron-geht-nicht - Kopie.jpg
Works (and similar)
20210217-210658-027-gigaton-geht - Kopie.jpg
I asked the dealer. They said that the M27C1024-10FI chips that are not working are refurbished chips. The words on the chips have been tampered with, and they are actually counterfeit with other speed grades.
walter
Site Admin
Posts: 116
Joined: 13 May 2018, 08:00

Re: I have some problems with my gigatron.

Post by walter »

You can check by rubbing some acetone on it. If the text disappears, they are rebranded. Buying several and them all having exactly the same numbers is also a giveaway. Plenty of video's on the subject on YouTube.

E.g. https://www.youtube.com/watch?v=12u_hBkHB88

https://www.youtube.com/watch?v=j9nasajve1o
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