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Parallel SCSI Interface Product Manual, Rev. A ) 19

2.4 Signal sources

Table 3 indicates the type of SCSI device allowed to source each signal. No attempt is made to show if the

source is driving asserted, driving negated, or is released. All SCSI device drivers that are not active sources

shall be in the high-impedance state. The RST signal may be asserted by any SCSI device at any time.

Table 3: Signal sources

Table abbreviations are defined as follows:

All: The signal shall be driven by all SCSI devices that are actively arbitrating.

S ID: A unique data bit (the SCSI ID) shall be driven by each SCSI device that is actively arbitrating; the other

data bits shall be released (i.e., not driven) by this SCSI device. The P_CRCA and DB(P1) bit(s) may be

released or driven to the true state, but shall not be driven to the false state during this phase.

I&T: The signal shall be driven by the initiator, target, or both, as specified in the SELECTION PHASE and

RESELECTION PHASE.

Init: If driven, this signal shall be driven only by the active initiator.

None: The signal shall be released; that is, not driven by any SCSI device. The bias circuitry of the bus termi-

nators pulls the signal to the false state.

Win: The signal shall be driven by the one SCSI device that wins arbitration.

Targ: If the signal is driven, it shall be driven only by the active target.

PT: Target that initiated the QAS arbitration.

16 data bit (P) cable

8 data bit (A) cable

SCSI bus phase BSY SEL

C/D, I/O,

MSG, REQ ACK, ATN DB(7-0) P_CRCA

DB(15-8),

DB(P1)

BUS FREE None None None None None None None

ARBITRATION

(NORMAL)

All Win None None S ID S ID S ID

QAS ARBITRATION PT Win None None S ID S ID S ID

SELECTION I & T Init None Init Init Init Init

RESELECTION I & T Targ Tar g Init Tar g Tar g Ta r g

COMMAND Targ None Tar g Init Init Init None

ST DATA IN Tar g None Tar g Init Ta r g Targ Targ

ST DATA OUT Targ None Tar g Init Init Init Init

DT DATA IN Ta rg Tar g Targ Init Targ Tar g Ta r g

DT DATA OUT Targ Ta r g Targ Init Init Targ Init

STATUS Tar g None Ta rg Init Ta rg Tar g None

MESSAGE IN Targ None Tar g Init Ta r g Targ None

MESSAGE OUT Targ None Ta r g Init Init Init None

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Seagate Ultra 320 User Manual Page 33