
SYM53C895 Interfaces
2-17
In the differential wiring diagram example shown in
Figure 2.7, the
SYM53C895 is connected to the TI SN75976A2 differential transceiver
for Ultra SCSI operation. The recommended value of the pull-up resistor
on the REQ
,ACK, MSG, C/D, I/O,ATN, SD[7:0], and SDP0
lines is 680
when the Active Negation portion of Symbios TolerANT
technology is not enabled. When TolerANT is enabled, the recommended
resistor value on the REQ
,ACK, SD[7:0], and SDP0 signals is
1.5 K
. The electrical characteristics of these pins change when
TolerANT is enabled, permitting a higher resistor value.
To interface the SYM53C895 to the SN75976A2, connect the positive
pins in the SCSI LVD pair of the SYM53C895 directly to the transceiver
enables (nDE/RE/). These signals control the direction of the channels
on the SN75976A2.
The SCSI bidirectional control and data pins (SD[7:0]
SDP0, REQ,
ACK
, MSG, I_O, C_D, and ATN) of the SYM53C895 connect to the
bidirectional data pins (nA) of the SN75976A2 with a pull-up resistor. The
pull-up value should be no lower than the transceiver IOL can tolerate,
but not so high as to cause RC timing problems. The three remaining
pins, SEL
, BSY, and RST, are connected to the SN75976A2 with a
Table 2.4
High Voltage Differential Operation
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 direction of the
differential drivers for SCSI data and parity lines,
respectively
ACK+
Active high signal used to control direction of the
differential driver for initiator group signals ATN
and
ACK
REQ+
Active high signal used to control direction of the
differential drivers for target group signals MSG
,
C/D
, I/O and REQ/
DIFFSENS
Input to the
SYM53C895 used to detect the voltage
level of a SCSI signal to determine whether it is a SE,
LVD, or HVD signal. The result is displayed in STEST4
bits [7:6].