Antminer S21 Firmware: Stock, Braiins OS+, VNish & LuxOS by Control Board

The Antminer S21 is the generation where the easy answer stopped working. The air-cooled units ship on Amlogic control boards with no SD-card slot, so the recovery routine every operator learned on the S9 and S19 simply does not exist here; the hydro units in the same family ship on Xilinx and still do. On top of that, the S21, S21 Pro, S21 XP, S21+ and T21 are not one chip — they are two. Almost every page ranking for “S21 firmware” today is either a vendor selling its own build or a distributor hosting downloads. This page is neither: it is the decision layer — every real firmware option including stock, matched to the board it supports, with the March 2024 install lock explained before you flash anything, and an honest account of which files we can and cannot hand you. We host no third-party firmware here and sell none; D-Central repairs this hardware in Montreal. For specs and buying context, see the Antminer S21 family owner’s hub and the S21 vs S21 Pro vs S21 XP vs S21 Hyd comparison.

Jump to: two chips, one family · identify your board · firmware options · the March 2024 lock · install reality by board · what we hold, and what we do not

Two chips, one family — and why that matters before you download anything

The S21 name spans two different ASICs, and getting this wrong is the fastest way to pick the wrong image. Our model corpus and D-Central’s own firmware source tree agree:

Variant	ASIC	Nameplate (corpus figure)	Control board per Luxor’s compatibility table
S21 (air)	BM1368	200 TH/s @ ~3,500 W	AMLogic, *CVITEK — hashboards BHB68601, BHB68603, BHB68606
T21	BM1368	190 TH/s @ ~3,610 W	AMLogic, *CVITEK — hashboards BHB68701, BHB68703
S21 Pro	BM1370	234 TH/s @ ~3,510 W	AMLogic — A3HB70601, A3HB70602, A3HB70603
S21 XP	BM1370	270 TH/s @ ~3,645 W	AMLogic — A3HB70501, A3HB70503
S21+	BM1370	216 TH/s @ ~3,564 W	AMLogic — A3HB70701
S21 Hydro	BM1368	335 TH/s @ ~5,360 W	Xilinx — HHB68501
S21 XP Hydro	BM1370	— (corpus figure not established)	Xilinx — H6HB70501
S21+ Hydro	BM1370	— (corpus figure not established)	Xilinx — H6HB70701, H6HB70702, H6HB70704

Two things in that table are load-bearing. The S21 and T21 are BM1368; the Pro, XP and + are BM1370. Sources that flatten the whole family to one chip are wrong, and we have corrected our own older material where it did. And every hydro variant is Xilinx while every air variant is Amlogic — so within one product family the install rail is completely different depending on how the machine is cooled.

We do not publish per-board chip counts for this family. Our own catalogue is internally inconsistent on whether the recorded figure is per hashboard or per unit, and an unresolved number on a page people flash from is worse than no number. Immersion variants exist (S21 Imm, S21 XP Imm) and Braiins lists images for them; the discriminator between air, hydro and immersion is the hashboard SKU prefix — BHB/A3HB air, HHB/H6HB/H1HB hydro, IHB immersion — not the control board, which is shared.

Step zero: identify your control board

On this generation there are three answers, not one. Amlogic on every air unit, CVitek on some S21 and T21 units (Luxor marks both with an asterisk in its table), and Xilinx on every hydro unit. Each has a different install rail and a different recovery story, and no amount of reading the model number tells you which you have.

So before reading the options matrix: run the control-board identifier — under a minute, works even with a dead board. Everything below is keyed to its answer. For the board internals, see the ASIC control board reference.

The firmware options

Dev-fee figures below are each vendor’s own published number, checked August 2026 — ranges are printed as ranges, not flattened, and a pool fee is never presented as a dev fee.

Firmware	S21-family coverage	Dev fee (vendor-published)	Status
Bitmain stock	Every unit ships with it. Signed images, warranty intact, no dev fee. On this generation stock is a stronger default than it used to be — the factory tuning is closer to the practical ceiling than it was on the S19 line, and the install locks make a reversal harder.	None	Verified (it is the factory state)
Braiins OS+	The broadest coverage of the four: S21, S21 Pro, S21 XP, S21+, S21++, T21, S21 Hyd., S21+ Hydro, S21 XP Hyd., S21e Hyd., S21E XP Hyd. U3, S21 Imm and S21 XP Imm — each a distinct image on the official download page	“2-2.5% dev fee (depending on the hardware model)”, collected by directing that share of hashrate to Braiins’ pool. Mining on Braiins Pool earns a 0% effective pool fee and a 100% pool-fee refund — that is a pool-fee rebate, and it is a separate line from the dev fee	Verified (vendor download page + vendor fee page, Aug 2026)
VNish	VNish’s own model menu names “ANTMINER S21, T21”. Coverage of the Pro, XP, + and hydro variants is not called out on the page we read — published as an observed absence, not as a verdict on support	“This is 2.8% MAXIMUM of the total hashrate or lower!” — VNish’s own wording	Supported (vendor site, Aug 2026); per-variant coverage Unknown
LuxOS	The most precisely documented of the four on this family: S21, S21 Pro, S21 XP, S21+, T21, S21 Hydro, S21 XP Hydro, S21+ Hydro and U3 S21e XP Hydro, each with its control board and hashboard SKUs listed — per Luxor’s own compatibility table	2.8%, always collected. Mining on Luxor’s own pool is free pool usage; it is not a dev-fee waiver	Verified (vendor docs, re-checked Aug 2026)
DCENT_OS (ours)	Registered as real work targets, not offered as a public install. See the row below this table.	No mandatory dev fee. What ships is an optional donation: on by default at 2%, set in the first-boot wizard, adjustable 0–5% with 0% as a one-click preset, and switchable off entirely. Voluntary, transparent, disableable — and the only number in this column you can read in the source	Verified (our own public GPL-3.0 tree)

DCENT_OS on the S21 family, stated exactly. D-Central’s own firmware is the only fully open-source option on this table — GPL-3.0 end to end, where Braiins OS+’s mining binary is closed and VNish and LuxOS are closed — and it is in public beta. Its public tree registers am3-s21, am3-s21pro, am3-s21xp and am3-t21 as real serial work engines, and the source comment on the S21 target reads “mining evidence exists; public install lab-gated”. That second half is the part that matters to you: exactly one board target in the entire tree is open for a public-beta first install, and it is the Antminer S9. Nothing in the S21 family is. A flashable image also reuses vendor boot components we cannot redistribute, so what exists to build today is the daemon, not an image. If a page anywhere tells you to flash DCENT_OS onto a producing S21, it is wrong and we would rather say so here. Current state: DCENT_OS for Antminer S21.

One consumer-protection note that belongs on every firmware page: VNish’s own FAQ warns that “free / no-devfee” builds of its firmware circulating on lookalike domains are fake and may carry hidden fees — that is the vendor’s warning, and it generalises. Download only from a vendor’s own listed domains, and verify what you downloaded: how to verify mining firmware authenticity and the mining firmware security guide.

The March 2024 lock: this generation is the one it was built for

The S21 shipped straight into the lock era, which is why so many owners find the install advice written for older Antminers simply does not apply:

Verified, from Luxor’s compatibility docs (re-checked August 2026): “Remote Install is required for: Machines with CVITEK control boards” and for any machine running “Bitmain Firmware dated March 2024 or newer.” Both conditions are common on this family — a CVitek-boarded S21 or T21 is remote-install regardless of date, and a machine bought new since spring 2024 almost certainly ships with firmware past the cutoff.
Supported, from third-party-vendor installation notes: from around March 2024, Bitmain shipped Amlogic control boards with firmware locks that block third-party installation outright. Braiins maintains an Amlogic remote-installation guide for exactly this case, and aftermarket unlocking kits exist. Reportedly, stock firmware released after September 2025 additionally locks the SD-card downgrade path and disables the micro-USB port on affected boards — on a board with no SD slot, that second half removes the last local rail. Bitmain does not publish these lock dates; the record is assembled from the vendors who work around them, which is why this bullet is labelled Supported, not Verified.

The practical rule, in one line: read the stock firmware date off the dashboard before you plan anything, and budget for a remote install rather than an afternoon. We track lock events as they land on the ASIC firmware lock & security event tracker.

Install reality, board by board

Your board	What installation actually looks like
Amlogic (every air unit)	No SD-card slot exists. The local recovery rail is a USB flash drive through the micro-USB port with an OTG adapter, and third-party installs go through the vendor’s management tool — BOS Toolbox, LuxOS Commander, or the vendor’s remote-install service where the board is locked. If your unit’s stock firmware is March 2024 or newer, assume remote install.
CVitek (some S21 and T21)	No SD boot rail either — the CVitek BootROM is eMMC-first, so an SD install is structurally impossible rather than merely unsupported. Luxor routes every CVITEK unit through remote installation regardless of firmware date, and that policy is stated on Luxor’s own page.
Xilinx (every hydro unit)	The one self-serve rail left in this family: an SD-card slot on the board, and the classic SD-image install path. If you own a hydro S21 you have more local options than an air owner does — which is the opposite of what most people assume.

Scope note: each vendor documents its own procedure, and the vendor’s document outranks this page for the vendor’s image. This page tells you which doors exist for your board. If you are going the other way, back to factory, the procedure is on restore stock firmware on an ASIC miner. When the machine is throwing codes rather than refusing a flash, start at Antminer S21 error codes.

What we hold, and what we do not

This is the part most download pages will not tell you, so we will: D-Central mirrors no S21-family firmware at all. Zero files. Our archive holds 112 files with a published SHA-256 for every one of them, and it is deep on the S9, S17, T17 and S19 generations precisely because those machines outlived their vendor download pages. The S21 has not. Every current image for this family is on a live vendor channel, linked in the options table above, and that is where yours should come from.

We say this plainly because the alternative is what the rest of this corner of the internet does: mirror a binary with no hash and no stated origin, or list a model with a download button that goes nowhere. Our archive is a mirror, not an official channel, it is excluded from search indexes, and Source: Unknown appears on 107 of its 112 records because that is the true answer. You can read the whole record, including every hash, on firmware mirror provenance. For an S21, use it to learn what a verifiable download looks like — then go to the vendor and check what you get against their published hash. The method is on how to verify mining firmware authenticity.

When it stops being a firmware question

A chain that died mid-flash, a board that boots nothing, an S21 that hashes on two chains no matter what image it runs — that is not a firmware decision anymore, it is a bench job. D-Central repairs this family at component level in Montreal: S21, S21 Pro, S21 XP and T21 repair, plus S21-family hashboard repair for chip-level work. If the control board itself is the casualty, identify the board first, then match it against the control board parts category.

Record last verified: 2026-08-19.

Antminer S21 firmware questions

Which firmware can I run on an Antminer S21?

Four real options: Bitmain stock (the legitimate default, and a stronger one on this generation than on older Antminers), Braiins OS+ (which publishes distinct images for thirteen S21-family variants including the hydro and immersion lines), LuxOS (which documents every S21 variant with its control board and hashboard SKUs), and VNish (whose model menu names the S21 and T21). D-Central’s own DCENT_OS registers the S21, S21 Pro, S21 XP and T21 as real work targets but does not offer a public-beta install for any of them. Which of these you can actually install depends on your control board and your stock firmware date — identify the board first.

What ASIC chip does the Antminer S21 use?

The S21 and the T21 use the BM1368. The S21 Pro, S21 XP and S21+ use the BM1370. Both are recorded that way in our model corpus and in D-Central’s own open-source firmware source tree, where the board targets carry the chip identity explicitly. Any source that gives the whole S21 family a single chip is flattening two different ASICs into one row.

Does the Antminer S21 have an SD card slot?

The air-cooled units do not. They ship on Amlogic control boards, and the local recovery rail is a USB flash drive through the micro-USB port with an OTG adapter instead. The hydro variants are a different story: Luxor’s compatibility table lists S21 Hydro, S21 XP Hydro and S21+ Hydro on Xilinx boards, which do have an SD-card slot and the classic SD-image install path. So within one product family, whether you have a local install rail depends on how the machine is cooled.

Why won’t my S21 accept third-party firmware?

Almost always the install lock. Luxor’s own docs require remote installation for any machine on Bitmain firmware dated March 2024 or newer, and for every CVITEK-boarded unit regardless of date. Amlogic boards shipped from around March 2024 carry locks that block third-party installation outright, and this generation shipped straight into that era — a machine bought new since spring 2024 is very likely past the cutoff. Check the stock firmware date on your dashboard; if it is March 2024 or newer, the route is the firmware vendor’s remote-install or unlock service, not a file you upload.

Does D-Central host S21 firmware downloads?

No — we mirror zero S21-family files, and we would rather say that than list a model with a dead button. Our archive is deep on the S9, S17, T17 and S19 generations because those machines outlived their vendor download pages; the S21’s vendor channels are all live, and that is where your image should come from. Every file we do mirror carries a published SHA-256 and an honest provenance record, including Source: Unknown on 107 of 112 files. It is a mirror, not an official channel, and it is excluded from search indexes.

Related products, repair, and setup paths

Last reviewed August 20, 2026.