Avago Technologies LSI53C895A - Manual (Page 61)

SCSI Functional Description 2-35
The LSI Logic LVDlink 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 LSI53C895A 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
LSI53C895A 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]+, REQ+, ACK+, SRST+, SBSY+, and SSEL+ signals control
the direction of external differential pair transceivers. The LSI53C895A 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 BSY
−
,
SEL
−
, and RST
−
pads so they can be used as pure input pins. In
addition to the standard SCSI lines, the signals shown in Table 2.6 are
used by the LSI53C895A during HVD operation.
In the example differential wiring diagram in Figure 2.5 , the LSI53C895A
is connected to TI SN75976 differential transceivers for Ultra SCSI
operation. The recommended value of the pull-up resistor on the REQ
−
,
Table 2.6 HVD Signals
Signal Function
BSY+, SEL+,
RST+ Active HIGH signals used to enable the differential drivers as outputs for SCSI
signals BSY
−
, SEL
−
, and RST
−
, respectively.
SD[15:0]+,
SDP[1:0]+ Active HIGH signals used to control the direction of the differential drivers for
SCSI data and parity lines, respectively.
ACK+ Active HIGH signal used to control the direction of the differential drivers for the
initiator group signals ATN
−
and ACK
−
.
REQ+ Active HIGH signal used to control the direction of the differential drivers for target
group signals MSG
−
, C_D
−
, I/O
−
and REQ
−
.
DIFFSENS Input to the LSI53C895A used to detect the voltage level of a SCSI signal to
determine whether it is a SE, LVD, or high-power differential signal. The encoded
result is displayed in
SCSI Test Four (STEST4) bits 7 and 6.