LSI LSI53C896, Technical Manual

Page: 64 / 370

2-38 Functional Description
To interface the LSI53C896 to the SN75976A, connect the positive pins
in the SCSI LVD pair of the LSI53C896 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 LSI53C896 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
when the pins (nA) of the SN75976A are configured as inputs. When the
data pins are inputs, the resistors provide a bias voltage to both the
LSI53C896 pins (SSEL
−
, SBSY
−
, and SRST
−
) and the SN75976A data
pins. Because the SSEL
−
, SBSY
−
, and SRST
−
pins on the LSI53C896
are inputs only, this configuration allows for the SSEL
−
, SBSY
−
, and
SRST
−
SCSI signals to be asserted on the SCSI bus.
The differential pairs on the SCSI bus are reversed when connected to
the SN75976A due to the active low nature of the SCSI bus.
8-Bit/16-Bit SCSI and the HVD Interface – In an 8-bit SCSI bus, the
SD[15:8] pins on the LSI53C896 should be pulled up with a 1.5 k
Ω
resistor or terminated like the rest of the SCSI bus lines. This is very
important, as errors may occur during reselection if these lines are left
floating.
2.2.13.3 SCSI Termination
The terminator networks provide the biasing needed to pull signals to an
inactive voltage level, and to match the impedance seen at the end of
the cable with the characteristic impedance of the cable. Terminators
must be installed at the extreme ends of the SCSI chain, and only at the
ends. No system should ever have more or less than two terminators
installed and active. SCSI host adapters should provide a means of
accommodating terminators. There should be a means of disabling the
termination.
SE cables can use a 220
Ω
pull-up resistor to the terminator power
supply (Term Power) line and a 330
Ω
pull-down resistor to ground.
Because of the high-performance nature of the LSI53C896, regulated (or
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