This is shared Antminer S21 Series reference material covering 10 models. Components, supply voltage/phase, PSU, cooling and service procedures vary by model and revision. Use the exact model nameplate and OEM manual; shared examples do not establish compatibility.
Every unit is built from the same handful of parts. Learn what each does, how to identify it, and - most importantly for diagnosis - what its failure looks like.
The S21 generation moved from the S19's Xilinx Zynq to an AMLOGIC application-processor control board. The hardware-UART/FPGA approach was dropped for SOFTWARE UART to each hashboard, and per-domain voltage is now set over I²C via a DAC from the control board.
Same brain role as S19's CB, but a different architecture that changes how faults present.
Amlogic-based board with RJ45, SD slot, reset + IP-report buttons, and three ribbon sockets. No FPGA hardware-UART.
Unlike S19, the S21 hashboards have NO PIC microcontroller. Voltage enable/control moved to the control board over I²C plus a DAC. This is a key diagnostic difference.
Voltage/enable is now centralized on the CB instead of per-board.
You won't find a PIC16F-class chip on an S21 hashboard.
Hashboards contain the mining ASICs. Count, silicon, domain layout and connectors vary by exact model/revision. The local BM1370 process-node claims conflict and remain unverified; do not infer them from the family name.
The hashing muscle - much better efficiency than S19 (17.5 → 13.5 J/TH).
Silkscreen PCB number (e.g. BHB68603) identifies the exact revision. Chips hidden under heatsinks.
Power supplies differ by model and revision. The cited air S21 and S21 Pro specifications list 220-277 V AC; S21 Hydro lists 380-415 V three-phase. The Bitmain PSU table lists APW111721c for S21 Hydro/XP Hydro and APW11I1216-1a for S21 Immersion. Verify the exact PSU/nameplate before installation or replacement.
Feeds the hashboards and control board; output and connector requirements are model-specific.
Read the exact miner and PSU labels; connector fit alone does not establish compatibility.
Air-model example: fan cooling and tach feedback support safe operation. Hydro uses a liquid loop; immersion uses approved dielectric fluid. Fan count, connector and protection settings require exact-model documentation.
Removes heat through the cooling system fitted to the exact model.
Identify the actual fan assembly or liquid cooling system from the exact model manual.