S21 Hashboard – Tester detects 107/108 ASICs

[nighthawk91]

Hi everyone,

I’m working on an Antminer S21 hashboard and I’m running into an issue I can’t fully diagnose.

My tester detects 107 ASICs out of 108, but all the signals look good all the way down the chain:
CLK is present on every domain
RST is stable
BI / BO signals look clean
RO / RI chain looks continuous
All LDOs are outputting the correct voltages
No short circuits on any voltage domain

Because the tester stops at 107 chips, I know one ASIC is failing, but the problem is that everything looks correct up to the last chip.

❓ My question is:
If all the signals (CLK, RST, BO/BI, RI/RO), LDOs, and voltages look good until the end of the board, how can I identify which specific ASIC is causing the “107/108” detection issue?

🔧 Additional info:
I already checked for weak solder joints, missing resistors, and continuity issues around the chain.
I also verified that the last RO signal reaches the connector.
No overheating components detected.

👉 What’s the best diagnostic approach in this situation?

Should I:
Inject a low-voltage signal to trace the RO chain?
Replace the last detected ASIC + the next one as a pair?
Use thermal camera to identify the cold ASIC?
Check the small bypass resistors between chips more carefully?
Any advice or methods you use for S21 chain detection errors would be greatly appreciated.

Thanks in advance.

[philipma1957]

well first off lets get a know good board and use the tester on it.

If it shows 108 chips then you know the tester is not the issue.

Sometimes meters and testers are bad.

If the tester shows good I suggest buying a board like this one.

https://epicblockchain.io/wp-content/uploads/2024/02/ePIC-UMC-Data-2_29_2024.pdf
get it at

https://altairtech.io/product/epic-umc-universal-mining-control-board/

this board will show this image for chips

This is a 2 board t21 and it shows all the chips are working but that some are weak

in your case it should show a chip with a very low percent maybe 20% or less.
if you click on the chips it will tell you the chip number

good luck


note chip number 19

[BitMaxz]

It might be just a tester, just like the above said, but have you tried to run this on the unit if the hashboard is running normally?
If not, I suggest before touching any hardware you should try upgrading it first to the latest version and then test it again in low power and normal mode.

If it doesn't work on normal mode but works on low power, then it's possible that the last ASIC chip is overheating. It could be because of a loose heatsink since you said signals are perfectly fine.


If both modes are still the same, the next step is to dump the EEPROM from the working hashboard and flash it into that hashboard. Make sure you dump or back up the EEPROM of that hashboard just in case. Then test it again.

If none works, then check the diagram from this link below, and maybe the last ASIC chip has a loose heatsink or damaged chip. Check the image under that link where the 108 chip is located.

- https://www.zeusbtc.com/manuals/5042-antminer-s21-t21-hash-board-repair-guide