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2.3.2 LVD Link™ Technology
2.3.3 Programmed I/O Transfers
2-8
To support greater device connectivity and a longer SCSI cable, the
SYM53C040 includes 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 for 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 cost and
complexity compared to high-power differential designs. LVD Link lowers
the amplitude of noise reflections and allows higher transmission
frequencies.
The LSI Logic LVD Link transceivers operate in LVD and SE modes. The
SYM53C040 automatically detects which type of signal is connected,
based on voltage detected by the DIFFSENS pin.
The RBIAS+ and RBIAS − signals are the bias resistors for LVD
operation. A resistor value of 9.76 k Ω,=±= 1%, is recommended, as
illustrated in
Figure
Figure 2.4
LVD Resistor Value
3.3 V
9.76 k Ω
RBIAS +
RBIAS −
Programmed I/O is the most primitive form of data transfer. The REQ/
and ACK/ handshake signals are individually monitored and asserted by
reading and writing the appropriate register bits. This type of transfer is
normally used when transferring small blocks of data such as command
blocks or message and status bytes.
I/O transfer.
A Target Send operation begins when the ACD (Assert C_D/), AIO
(Assert I_O/), and AMSG (Assert MSG/) bits in the
Functional Description
2.4.
Figure 2.5
illustrates a programmed
Target Command
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