Silicon Graphics 1450 User Manual page 41

SCSI Controller
The Adaptec AIC-7899 SCSI controller contains two independent SCSI channels that
share a single PCI bus master interface as a multifunction device, packaged in a 352-pin
ball grid array (BGA). Internally, each channel is identical, capable of operations using
either 16-bit Single-Ended (SE) or Low Voltage Differential (LVD) SCSI providing 40
MBps (Ultra-wide SE), 80 MBps (Ultra 2), or 160 MBps (Ultra 160).
Both channels attach to 68-pin 16-bit differential SCSI connector LVD interfaces. Each
channel 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 Adaptec AIC-7880 controller contains a single SCSI channel 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 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 a 16-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 attached to 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 attached to only one connector (either wide or narrow), all on-board termination
is on.
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