ACSI, SCSI and IDE

ASCI

ACSI is an acronym for Atari Computer Systems Interface. This peripheral bus is very similiar to the industry standard SCSI (Small Computer Systems Interface) bus. This peripheral bus allows more than one device to be connected to the computer by way of standard signal lines.

The ACSI is an 8 bit bidirectional data bus that allows data to pass from a peripheral, such as your hard disk, to the host Atari ST computer. The transfer of data may also flow over the bus from the Atari ST to the peripheral. Data may be transfered at a maximum rate of 1.25 million bytes per second and may be transfered to or from any of eight possible control devices attached to the bus. These control devices are addressed as devices 0 to 7.

ACSI makes use of the DMA.

Some important differences between ACSI and SCSI:

• Physical cable connection - ACSI uses a 19 pin shielded cable versus the 50 pin flat ribbon cable used by SCSI.
• The master/slave relationship - This defines who is in charge of bus operations between the computer and its peripherals. The ACSI bus specifies that the host computer is always in control of the bus as opposed to SCSI which allows both computers and peripherals to initiate control of the bus.
• Maximum cable length - ACSI allows a maximum of 2 feet between devices with a total length allowable of 88". However. total cable length should be minimized in the interests of signal reliability. SCSI has a total allowable length of approximately 18 feet.
• Maximum Number of Devices - System reliability requires that no more than four ACSI peripheral controllers be daisy-chained to an Atari ST.
• Utilization of the SCSI Message phase (extended status) - SCSI allows it and ACSI doesn't.
• Connect and Disconnect - the ability .for a device to disconnect after receipt of a command and connect only when data is to be transferred. ACSI does not utilize this. SCSI can.
• Bus Termination - SCSI uses open-collector bus drivers with 220/330 ohm termination resistors. ACSI uses CMOS or LSTTL level interfacing without terminating resistors.
• Request/Acknowledge - There is an important conceptual difference between ACSI and SCSI regarding the Request/ Acknowledge handshaking. SCSI uses a simple REQ/ACK procedure. whereas ACSI has CS/IRQ and DRQ/ACK procedures.
• Maximum Command Number - Because the ACSI controller device number is encoded in the most significant three bits of the command byte. the device can only use commands in the range $0Q-$1 F.
• Command/Status/Data Transfers - SCSI transfers all Commands, Status or Data information using its REQ/ACK (Request I Acknowledge) handshake. Whether this is done in a "DMA" mode or with byte by byte transfers through the controller processor, is a function of the SCSI controller used. ACSI provides for two methods of transferring information between the host processor and peripheral:
  1. Software Handshake Method where each byte is transferred under processor control. This method should nm be used for data transfer .
  2. DMA Method where the processor issues a command (using the Software Handshake Method above) specifying the peripheral device involved in the data transfer, the type of command, the maximum number of blocks (512 bytes) to be transferred.

(Source:ATARI ACSI/DMA. Integration Guide. June 28, 1991)

SCSI

SCSI makes use of the DMA.

DMA

DMA is an acronym for Direct Memory Access. When used in the context of a computer (such as the Atari ST) and its high performance peripherals (floppy and hard disks, laser printers, tape drives, CD ROMs, etc.). DMA refers to the direct transfer of data between the peripheral device and the computer's memory. This transfer is done without the direct intervention of the processor. The Atari ST provides an external DMA bus connection through the Atari Computer System Interface (ACSI) connector on the Atari ST computer.

Atari ST/TT/Falcon peripheral devices that use the DMA channel include

• the Atari floppy disk controller (which is connected directly to the DMA channel)
• internal & external SCSI of TT/MegaST/MegaSTE/Falcon
• external ACSI port of ST/STE/TT for the Laser printer through its APPC (Atari Page Printer Controller), the Hard Disk through its AHDI (Atari Hard Disk Interface), the CDAR Audio/ROM CD Unit, and the Removable Hard Disk.
• Sound system on STE, TT & Falcon
• other ?

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