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Functional Description
SCSI Bus Interface
SYM53C825A/825AE Data Manual
2-13
resistor. T he pull-down resistors are required when
the pins (nA) of the 75LBC976 are configured as
inputs. When the data pins are inputs, the resistors
provide a bias voltage to both the SYM53C825A
pins (SEL/, BSY/, and RST /) and the 75LBC976
data pins. Because the SEL/, BSY/, and RST / pins
on the SYM53C825A are inputs only, this config-
uration allows for the SEL/, BSY/, and RST / SCSI
signals to be asserted on the SCSI bus. T he differ-
ential pairs on the SCSI bus are reversed when
connected to the 75LBC976, due to the active low
nature of the SCSI bus.
8-bit/16-bit SCSI and the Differential
Interface
In an 8-bit SCSI bus, the SD15-8 pins on the
SYM53C825A should be pulled up with a 1.5 K
resistor or terminated like the rest of the SCSI bus
lines. T his is very important, as errors may occur
during reselection if these lines are left floating. In
the SYM53C825AJ, the SDIRP1 pin is replaced
by the T CK JTAG signal. If the device is used in a
wide differential system, use the SDIRP0 pin to
control the direction of the differential transceiver
for both the SP0 and SP1 signals. T he SDIRP0
signal is capable of driving both direction inputs
from a transceiver.
Terminator
Networks
T he 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 ter-
minators installed and active. SCSI host adapters
should provide a means of accommodating termi-
nators. T he terminators should be socketed, so
that if not needed they may be removed, or there
should be a means of disabling them with software.
Single-ended cables can use a 220
pull-up to the
terminator power supply (Term-Power) line and a
330
pull-down to Ground. Because of the high-
performance nature of the SYM53C825A, Regu-
lated (or Active) termination is recommended.
Figure 2-4 shows a Unitrode active terminator. For
additional information, refer to the SCSI-2 Speci-
fication. TolerANT active negation can be used
with either termination network.
Note: If the SYM53C825A is to be used in a
design with only an 8-bit SCSI bus, all 16
data lines still must be terminated or pulled
high.