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Functional Description
SCSI Bus Interface
SYM53C810A Data Manual
2-9
SCSI Bus Interface
T he SYM53C810A supports single-ended opera-
tion only. All SCSI signals are active low. T he
SYM53C810A contains the single-ended output
drivers and can be connected directly to the SCSI
bus. Each output is isolated from the power supply
to ensure that a powered-down SYM53C810A has
no effect on an active SCSI bus (CMOS “voltage
feed-through” phenomena). TolerANT technol-
ogy provides signal filtering at the inputs of SREQ/
and SACK / to increase immunity to signal reflec-
tions.
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. Symbios recom-
mends active or regulated termination (also known
as Alt-2 or Alternative Two termination) to maxi-
mize the high performance of the SYM53C810A.
Figure 2-3 shows a Unitrode active terminator for
regulated termination. For additional information,
refer to the SCSI-2 Specification. TolerANT active
negation can be used with any ANSI-approved ter-
mination network.
(Re)Select During
(Re)Selection
In multi-threaded SCSI I/O environments, it is not
uncommon to be selected or reselected while try-
ing to perform selection/reselection. T his situation
may occur when a SCSI controller (operating in
initiator mode) tries to select a target and is rese-
lected by another. T he Select SCRIPT S instruc-
tion has an alternate address to which the
SCRIPT S will jump when this situation occurs.
T he analogous situation for target devices is being
selected while trying to perform a reselection.
Once a change in operating mode occurs, the initi-
ator SCRIPT S should start with a Set Initiator
instruction or the target SCRIPT S should start
with a Set Target instruction. T he Selection and
Reselection Enable bits (SCID bits 5 and 6,
respectively) should both be set so that the
SYM53C810A may respond as an initiator or as a
target. If only selection is enabled, the
SYM53C810A cannot be reselected as an initiator.
T here are also status and interrupt bits in the
SIST 0 and SIEN0 registers, respectively, indicat-
ing that the SYM53C810A has been selected (bit
5) or reselected (bit 4).