Big Book of Amiga Hardware

The DMAC and SDMAC are the C= custom chips which front the WD33C93A SCSI controller IC, and perform DMA transfer functions for that SCSI interface chip.  It actually presents an 8-bit PC-XT-like interface to a common PC chip like the WD33C93A, and provides custom glue-logic and busmaster functions to communicate over the Amiga expansion buses.

On the A2091/A590 SCSI controlers, that bus is the 24-bit Zorro I/II expansion bus protocol of the original 68000-based Amiga machines.  DMAC-01 and DMAC-02 are applicable parts to these products.  DMAC-01 has been seen mostly on early A590 products, and may exhbit problems with heat.  The DMAC-02 may be required for some 3rd party enhancements which implement the faster Synchronous SCSI mode of the 33C93A SCSI chip. 

In the A3000, the SDMAC can access the larger address space of the Zorro III expansion space.  

SDMAC-02 is intended for use with Ramsey-04 in the A3000D/T.  DMAC-04 is intended for use with Ramsey-07, which came out to address issues found with the longer signal runs on the A3000T motherboard. The RAMSEY/SDMAC pairings address issues which require each other.  As per Dave Haynie (designer), "You can't mix them up".

The A4000T uses the NCR 53C710 3rd generation SCSI and DMA Controller, and is not a related chip to the DMAC/SDMAC.