3.8
SCSI Interface
The SCSI interface on the VP7 consisting of the PCI Bus SCSI controller Symbios 53C875. This
controller chip drives the SCSI protocol in accordance to the ANSI X3.131-1986 (SCSI-1) and ANSI
X3.131-199x (SCSI-2) specification, with the following functions:
•
Arbitration
•
Selection
•
Message Handling
•
Disconnect/Reconnect
•
Data Bus Parity Checking
The SCSI controller can operate the SCSI bus at a maximum speed of 14 Mbytes/s asynchronously or 40
Mbytes/s synchronously and burst data 32bit wide to the host CPU at full PCI speeds.
•
Termination of the SCSI Signals
The SCSI interface is terminated on both sides of the connector cable. The terminator on the VP7 is
an active terminator. The other usually on the peripheral unit must be also an active type. If more
than one unit is connected to the host adapter, all terminators on the peripheral units must be
removed or disabled, except for the one at the far end of the SCSI cable chain.
•
Number of SCSI Devices
All units connected to the SCSI bus (also peripheral controller and the host adapter) must have an
address with which they may be accessed. Sixteen addresses (0..15), named ID numbers, can be
assigned on the SCSI bus. These ID numbers can be changed by jumpers or switches located on
the device. The SCSI controller on the VP7 usually reserves the ID number 7, meaning that ID
numbers 0 .. 6 and 8 .. 15 (Wide SCSI devices only) can be assigned to the peripheral devices.
The VP7 SCSI controller device ID can be altered from the SCSI BIOS configuration menu.
When using more than two devices on the SCSI bus make sure that they all have unique IDs. If
there are more SCSI hard disks in the system, the hard disk with the lowest ID number is the first
hard disk and is given the logical drive name "C:\". The VP7 tries to startup the operating system
from the first hard disk. SCSI scan order can be changed from the SCSI BIOS configuration.
EIDE versus SCSI scan order can be altered in the system BIOS Setup.
•
Cable characteristic
The characteristic impedance of the cable is shown below:
Characteristic impedance:
SCSI-2
90
Ω
to 132
Ω
Ultra Fast-20
90
Ω
+/-10
Ω
Signal attenuation:
0.095 dB maximum per meter at 5 MHz
Pair-to-pair propagation delay delta:
0.20 ns maximum per meter
DC resistance:
0.230
Ω
maximum per meter at 20
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