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SCSI Bus Interface 2-11
2.6 SCSI Bus Interface
The LSI53C810A supports SE operation only. All SCSI signals are active
LOW. The LSI53C810A contains the SE 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 LSI53C810A has no effect
on an active SCSI bus (CMOS “voltage feed-through” phenomena).
TolerANT technology provides signal filtering at the inputs of SREQ/ and
SACK/ to increase immunity to signal reflections.
2.6.1 Terminator Networks
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 to ground. Because of
the high-performance nature of the LSI53C810A, regulated or active
termination is recommended. Figure 2.3 shows a Unitrode active
terminator. TolerANT active negation can be used with any ANSI
approved termination network. For additional information, refer to the
SCSI-2 specification.
2.6.2 Select/Reselect During Selection/Reselection
In multithreaded SCSI I/O environments, it is not uncommon to be
selected or reselected while trying to perform selection/reselection. This
situation may occur when a SCSI controller (operating in the initiator
mode) tries to select a target and is reselected by another. The Select
SCRIPTS instruction has an alternate address to which the SCRIPTS will
jump when this situation occurs. The analogous situation for target
devices is being selected while trying to perform a reselection.