
2-16
Functional Description
function. In HVD mode, the positive signals provide directional control
and in SE mode they are virtual ground drivers. TolerANT technology
provides signal ltering at the inputs of SREQ/ and SACK/ to increase
immunity to signal reections.
2.6.3.1 LVD Link Technology
To support greater device connectivity and a longer SCSI cable, the
SYM53C895 features LVD Link technology, which is 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 (high power) differential designs. LVD
Link lowers the amplitude of noise reections and allows higher
transmission frequencies.
The Symbios LVD Link transceivers operate in LVD and SE modes. They
allow the chip to detect a high voltage differential signal when the chip is
connected to external high voltage differential transceivers. The
SYM53C895 automatically detects which type of signal is connected,
based on voltage detected by the DIFFSENS pin. Bits 7 and 6 of the
STEST4 register contain the encoded value for the type of signal that is
detected (LVD, SE, or high voltage differential). Refer to the STEST4
register description for encoding and other bit information.
2.6.3.2 High Voltage Differential Mode (HVD)
To maintain backward compatibility with legacy systems, the
SYM53C895 can operate in HVD Mode (when the chip is connected to
external differential transceivers). In high voltage differential mode, the
SD+ [15:0], SDP+ [1:0], REQ+, ACK+, RST+, BSY+, and SEL+ signals
control the direction of external differential-pair transceivers. The
SYM53C895 is placed in differential mode by setting the DIF bit, bit 5 of
the STEST2 register (0x4E). Setting this bit 3-states the BSY
, SEL,
and RST
pads so they can be used as pure input pins. In addition to
the standard SCSI lines, the SYM53C895 uses these signals during high
voltage differential operation as shown in
Table 2.4: