Avago Technologies LSI53C896 - Manual (Page 64)

2-36 Functional Description
Version 3.3 Copyright © 1998–2003 by LSI Logic Corporation. All rights reserved.
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).
Refer to the
SCSI Test Four (STEST4) register description for encoding
and other bit information.
2.2.14.2 HVD Mode
To maintain backward compatibility with legacy systems, the LSI53C896
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 LSI53C896 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 addition to the
standard SCSI lines, the signals shown in Table 2.6 are used by the
LSI53C896 during HVD operation.
In the example differential wiring diagram in Figure 2.5 , the LSI53C896
is connected to TI SN75976 differential transceivers for Ultra SCSI
operation. The recommended value of the pull-up resistor on the SREQ
−
,
SACK − , SMSG − , SC_D − , SI_O − , SATN − , SD[7:0] − , and SDP0 − lines is
680
Ω
when the Active Negation portion of TolerANT technology is not
enabled. When TolerANT technology is enabled, the recommended
resistor value on the SREQ
− , SACK − , SD[7:0] − , and SDP0 − signals is
1.5 k
Ω
. The electrical characteristics of these pins change when
TolerANT technology is enabled, permitting a higher resistor value.
Table 2.6 HVD Signals
Signal Function
SBSY+, SSEL+,
SRST+ Active HIGH signals that enable the differential drivers as outputs for SCSI
signals SBSY
−
, SSEL
−
, and SRST
−
, respectively.
SD[15:0]+,
SDP[1:0]+ Active HIGH signals that control the direction of the differential drivers for
SCSI data and parity lines, respectively.
SACK+ Active HIGH signal that controls the direction of the differential drivers for the
initiator group signals SATN
−
and SACK
−
.
SREQ+ Active HIGH signal that controls the direction of the differential drivers for
target group signals SMSG
−
, SC_D
−
, SI_O
−
and SREQ
−
.
DIFFSENS Input to the LSI53C896 that detects the voltage level of a SCSI signal to
determine whether it is a SE, LVD, or HVD signal. The encoded result is
displayed in
SCSI Test Four (STEST4) bits 7 and 6.