SCSI Bus Signals (See ANSI SCSI-2 4.6)Table 3.3 SCSI Bus Signals

Signal

Description

 

 

BSY(BUSY)

An "OR tied" signal that indicates that the bus is

 

being used.

 

 

SEL(SELECT)

An "OR tied" signal used by an initiator to select

 

a target or by a target to reselect an initiator.

 

 

C/D

A signal driven by a target that controls the

(CONTROL/DATA)

direction of data movement on the DATA BUS

 

with respect to an initiator. True indicates input

 

to the initiator. This signal is also used to

 

distiguish between SELECTION and

 

RESELECTION phase.

MSG(MESSAGE)

A signal driven by a target during the MESSAGE

 

phase.

 

 

REQ(REQUEST)

A signal driven by a target on the A cable to

 

indicate a request for an ACK information

 

transfer handshake.

 

 

ACK

A signal driven by an initiator on the A cable to

(ACKNOWLEDGE)

indicate an acknowledgement for a REQ

 

information transfer handshake.

ATN(ATTENTION)

A signal driven by an initiator to indicate the

 

ATTENTION condition.

 

 

RST(RESET)

An "OR tied" signal that indicates the RESET

 

condition.

 

 

DB(7-0,P)

Eight data bit signals, plus a parity bit signal that

(DATA BUS)

from a DATA BUS. DB(7) is most significant bit

 

and has the highest priority during the

 

ATTENTION phase. Bit number, significance

 

and priority decrese downward to DB(0). A data

 

bit is defined as one when the signal value is true

 

and is defined as zero when the signal value is

 

false. Data parity DB(P) shall be odd. Parity is

 

undefined during the ARBITRATION phase.

 

 

3-8

M3099GX/GH OEM Manual