Drive Select; Signal Values; Or-Tied Signals; Signal Sources - Seagate SCSI Interface Product Manual

SCSI Interface Product Manual, Rev. J
2.1.1

Drive select

For SCSI ID selection, install drive select jumpers as shown in configuration selection figure in the individual
drive's Product Manual, Volume 1. Refer to the "Physical Interface" section of the individual drive's Product
Manual for the location of the drive select header. The drive using the eight bit data interface can have one of
eight ID bits selected by installing 0 to 3 jumpers in a binary coded configuration on the drive select header.
Drives using the 16 bit data interface can have one of sixteen ID bits selected by installing 0 to 4 jumpers in a
binary coded configuration on the drive select header. Drives that support SCAM protocol (SCSI Configured
Automatically) can have their drive ID assigned via the SCSI interface (see Appendix A).
2.1.2

Signal values

Signals may assume true or false values. There are two methods of driving these signals. In both cases, the
signal shall be actively driven true, or asserted. In the case of OR-tied drivers, the driver does not drive the sig-
nal to the false state, rather the bias circuitry of the bus terminators pulls the signal false whenever it is
released by the drivers at every SCSI device. If any driver is asserted, then the signal is true. In the case of
non-OR-tied drivers, the signal may be negated. Negated means that the signal may be actively driven false, or
may be simply released (in which case the bias circuitry pulls it false), at the option of the implementor.
2.1.3

OR-tied signals

The BSY and RST signals shall be OR-tied only. In the ordinary operation of the bus, these signals are simulta-
neously driven true by several drivers. No signals other than BSY, RST, and DB(P) are simultaneously driven
by two or more drivers, and any signal other than BSY and RST may employ OR-tied or non-OR-tied drivers.
DB(P) shall not be driven false during the Arbitration phase. There is no operational problem in mixing OR-tied
and non-OR-tied drivers on signals other than BSY and RST.
2.1.4

Signal sources

Table 1 indicates which type of SCSI device is allowed to source each signal. All SCSI device drivers that are
not active sources shall be in the passive state. Note that the RST signal may be sourced by any SCSI device
at any time. The drive functions as a target.
2.2

SCSI bus timing

Unless otherwise indicated, the delay time measurements for each SCSI device, defined in Paragraphs 2.2.1
through 2.2.16, shall be calculated from signal conditions existing at that SCSI device's own SCSI bus connec-
tion. Thus, these measurements (except skew delay) can be made without considering delays in the cable.
See Section 2.3 for Fast synchronous transfer option timing.
Table 1: Signal sources
Bus phase BSY
Bus Free None
Arbitration All
Selection I&T
Reselection I&T
Information transfer phases
Command Target
Data In Target
Data Out Target
Status Target
Message In Target
Message Out Target
ALL: The signal shall be driven by all actively arbitrating SCSI devices.
Signals C/D, I/O,
SEL MSG, REQ
None None
Winner None
Init. None
Target Target
None Target
None Target
None Target
None Target
None Target
None Target
ACK/ATN DB(7-0,P) (15-8,P1)
None None
None SCSI ID
Init. Init.
Init. Target
Init. Init.
Init. Target
Init. Init.
Init. Target
Init. Target
Init. Init.
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