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SCSI Functional Description
2-33
To support LVD SCSI, all SCSI data and control signals have both
negative and positive signal lines. The negative signals perform the SCSI
data and control function. In the SE mode they become virtual ground
drivers. In the HVD mode, the positive signals provide directional control
to the external transceivers. TolerANT technology provides signal filtering
at the inputs of SREQ/ and SACK/ to increase immunity to signal
reflections.
2.2.13.1 LVD Link Technology
To support greater device connectivity and a longer SCSI cable, the
SYM53C896 features LVD Link technology, the LSI Logic implementation
of LVD SCSI. LVD Link transceivers provide the inherent reliability of
differential SCSI, and a long-term migration path of faster SCSI transfer
rates.
LVD Link technology is based on current drive. Its low output current
reduces the power needed to drive the SCSI bus, so that the I/O drivers
can be integrated directly onto the chip. This reduces the cost and
complexity compared to traditional HVD designs. LVD Link lowers the
amplitude of noise reflections and allows higher transmission
frequencies.
The LSI Logic LVD Link transceivers operate in LVD or SE modes. They
allow the chip to detect a HVD signal when the chip is connected to
external HVD transceivers. The SYM53C896 automatically detects which
type of signal is connected, based on the voltage detected by the
DIFFSENS pin. Bits 7 and 6 of the
SCSI Test Four (STEST4)
register
contain the encoded value for the type of signal that is detected (LVD,
SE, or HVD). Please see the
SCSI Test Four (STEST4)
register
description for encoding and other bit information.
2.2.13.2 HVD Mode
To maintain backward compatibility with legacy systems, the
SYM53C896 can operate in the HVD mode (when the chip is connected
to external differential transceivers). In the HVD mode, the SD[15:0]+,
SDP[1:0]+, SREQ+, SACK+, SRST+, SBSY+, and SSEL+ signals control
the direction of external differential pair transceivers. The SYM53C896 is
placed in the HVD mode by setting the DIF bit, bit 5, of the
SCSI Test
Two (STEST2)
register (0x4E). Setting this bit 3-states the SBSY
,
SSEL
, and SRST
pads so they can be used as pure input pins. In