Promise PDC20621

• Type: PCI ATA/RAID accelerator ASIC (388-pin BGA)
• Introduced: ~2003
• Used on: FastTrak S150 SX4 / SX4-M (4-port SATA RAID)

Overview

PDC20621 (aka SATA150 SX4) is Promise’s host-assisted RAID engine. It integrates four ATA channels, a DIMM memory controller, a host<->board DMA (“HDMA”) engine, and a hardware XOR unit for RAID-5 parity. Cards built around it expose 4× SATA-150 ports (typically via on-board PATA↔SATA bridges), plus a 168-pin SDRAM DIMM slot used as controller cache.

Unlike “IOP” RAID chips, the 20621 does not embed a general-purpose CPU. High-level RAID logic lives in the PC BIOS/OS driver, while the ASIC executes packetized DMA/XOR/ATA sequences. This makes the SX4 a hybrid: hardware-assisted (XOR + cache) but host-orchestrated.

Interfaces & Clocks

• Host: PCI 2.2, 32-bit 33/66 MHz bus mastering.
• Drive side: 4 independent ATA engines (PIO/MWDMA/UDMA up to Ultra100); boards present SATA-150 (1.5 Gb/s) per port.
• Memory: SDRAM DIMM, ECC optional (SX4-M boards commonly support up to 256 MB).
• Clocking: Internal controller clock derived by PLL and DIMM SPD: typically 66/100/120 MHz depending on module; PC100+ DIMMs required for full speed.

RAID & Features

• Hardware building blocks for RAID 0/1/10/5 (and JBOD).
• XOR offload for RAID-5 parity/gen/check; mirrors can be written from DIMM without extra PCI copies.
• Packetized/linked commands allow batching: HDMA → XOR → multi-disk ATA with a single completion interrupt.
• Option ROM (“FastBuild”) provides boot-time array setup; OS driver handles advanced operations (expansion/migration, monitoring).

Identification

• PCI VID:PID: 105A:6622 (common for SX4).
• Often labeled “SATA150 SX4” on the package/heatsink.

Notes

• Performance is bounded by PCI bandwidth and driver design; optimal use chains HDMA/XOR/ATA to minimize interrupts and PCI traffic.
• BIOS validates DIMM type/speed (unbuffered; PC100/133 recommended) before enabling the controller.
• Contemporary Linux support: sata_sx4 driver; Windows via Promise FastTrak drivers.