
2-34
Functional Description
addition to the standard SCSI lines, the signals shown in
Table 2.6
are
used by the SYM53C896 during HVD operation.
In the example differential wiring diagram in
Figure 2.5
, the SYM53C896 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
when the Active Negation portion of LSI Logic 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
. The electrical characteristics of these pins change when TolerANT
technology is enabled, permitting a higher resistor value.
To interface the SYM53C896 to the SN75976A, connect the positive pins
in the SCSI LVD pair of the SYM53C896 directly to the transceiver
enables (DE/RE/). These signals control the direction of the channels on
the SN75976A.
The SCSI bidirectional control and data pins (SD[7:0]
, SDP0
, SREQ
,
SACK
, SMSG
, SI_O
, SC_D
, and SATN
) of the SYM53C896
connect to the bidirectional data pins (nA) of the SN75976A with a
pull-up resistor. The pull-up value should be no lower than the transceiver
I
OL
can tolerate, but not so high as to cause RC timing problems. The
three remaining pins, SSEL
, SBSY
and SRST
, are connected to the
SN75976A with a pull-down resistor. The pull-down resistors are required
Table 2.6
HVD Signals
Signal
Function
SBSY+, SSEL+,
SRST+
Active high signals used to enable the differential drivers as outputs for SCSI
signals SBSY
, SSEL
, and SRST
, 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.
SACK+
Active high signal used to control the direction of the differential drivers for
the initiator group signals SATN
and SACK
.
SREQ+
Active high signal used to control the direction of the differential drivers for
target group signals SMSG
, SC_D
, SI_O
and SREQ
.
DIFFSENS
Input to the SYM53C896 used to detect 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.