SCSI ControllerThe baseboard includes two SCSI controllers. A dual function SCSI controller
(Adaptec AIC-7899) is on the PCI-B bus, and a PCI wide SCSI controller (Adaptec AIC-7880) is on the PCI-C bus.
The Adaptec AIC-7899 SCSI controller contains two independent SCSI controllers that share a single PCI bus master interface as a multifunction device, packaged in a 352-pin ball grid array (BGA). Internally, each controller is identical, capable of operations using either 16-bit SE or Low Voltage Differential (LVD) SCSI providing 40 MBps (Ultra-wide SE), 80 MBps (Ultra 2), or
160 MBps (Ultra 160/m).
In the SKA4 implementation, both controller A and controller B attach to a 68-pin 16-bit differential SCSI connector LVD interface. Each controller has its own set of PCI configuration registers and SCSI I/O registers. As a PCI bus master, the AIC-7899 controller supports burst data transfers on PCI up to the maximum rate of 266 MBps using on-chip buffers.
The AIC-7880 controller contains a single SCSI controller with full-featured PCI bus master interface in a 160-pin Plastic Quad Flat Pack (PQFP). The controller supports either 8- or 16-bit Fast SCSI providing 10 MBps or 20 MBps (Fast-10) throughput, or Fast-20 SCSI that can burst data at 20 MBps or 40 MBps. As a PCI 2.1 bus master, the AIC-7880 controller supports burst data transfers on PCI up to the maximum rate of 133 MBps using the on-chip 256-byte FIFO.
The SKA4 AIC-7880 implementation offers 8-bit or 16-bit SCSI connectors and operation at data transfer rates of 10, 20, or 40 MBps. The AIC-7880 controller also offers active negation outputs, controls for external differential transceivers, a disk activity output, and a SCSI terminator power- down control. Active negation outputs reduce the chance of data errors by actively driving both polarities of the SCSI bus, avoiding indeterminate voltage levels and common-mode noise on long cable runs. The SCSI output drivers can directly drive a 48-mA single-ended SCSI bus with no additional drivers. The SCSI segment can support up to 15 devices.
The AIC-7880 controller can be used as an 8-bit controller via the narrow, 50-pin connector and as
a16-bit controller via the wide, 68-pin connector. As a result, the AIC-7880 controller is not always at one end of the SCSI bus, and termination is controlled through some simple circuitry. The circuitry senses whether there is a device attached through the narrow 50-pin connector or the wide 68-pin connector. When there are devices off both connectors, the termination is on for the upper 8 bits of data and the parity bit associated with these data lines. All other signals are not terminated on board and are terminated by the devices attached through the connector. When there is a device on only one connector (either wide or narrow), all on-board termination is on.