SN75971B

SCSI DIFFERENTIAL CONVERTER-DATA

SLLS322A – NOVEMBER 1999 – REVISED JANUARY 2000

2

POST OFFICE BOX 655303

description (continued)

The SCSI, differential, converter-data chip operates in two modes depending on the state of the DRVBUS input.

With DRVBUS low, a bidirectional latch circuit sets the direction of data transfer. Each data bit has its own latch,

and each bit’s direction is independent of all other bits. When neither the single-ended nor the differential sides

are asserted, the latch disables both A- and B-side output drivers. When the input to either side is asserted, the

latch enables the opposite side’s driver and sets data flow from the asserted input to the opposite side of the

device. When the input deasserts, the latch maintains the direction until the receiver on the enabled driver

detects a deassertion. The latch then returns to the initial state. No parity checking is done by this device; the

parity signal passes through the device like other data signals do.

When DRVBUS is high, direction is determined by the SDB signal. However, a change in SDB does not always

immediately change the direction. When DRVBUS first asserts, the direction indicated by SDB is latched and

takes effect immediately. When SDB changes while DRVBUS is high, the drivers that were on immediately turn

off. However, the other driver set does not turn on until the receivers sense a deasserted state on all nine data

lines. This is done to prevent the active drivers from turning on until all other drivers are off and the terminators

pull the lines to a deasserted state.

The single-ended SCSI bus interface consists of CMOS, bidirectional inputs and outputs. The drivers are rated

to

±16 mA of output current. The receiver inputs are pulled high with approximately 4 mA to eliminate the need

for external pullup resistors for the open-drain outputs of most single-ended SCSI controllers. The single-ended

side of the device is not intended to drive the SCSI bus directly.

The differential SCSI bus interface consists of bipolar, bidirectional inputs and outputs that meet or exceed the

requirements of EIA-485 and ISO 8482-1982/TIA TR30.2 referenced by American National Standard of

Information Systems (ANSI) X3.131-1994 Small Computer System Interface-2 (SCSI-2) and SCSI-3 Fast-20

Parallel Interface (Fast-20) X3.277:1996.

The SN75971B is characterized for operation over the temperature range of 0

°C to 70°C.

Terminal Functions

TERMINAL

I/O

DESCRIPTION

NAME

NO.

I/O

DESCRIPTION

ADBn–, where

n = {0,1,2,3,4,5,6,7,P}

4, 6, 8, 10, 19,

21, 23, 25, 27

I/O, Single-ended

SCSI voltage levels,

Strong pullup

Bidirectional I/O for data and parity bits to and from the single-ended SCSI

controller. As outputs, these terminals can source or sink 16 mA. As inputs,

they are pulled up with about 4-mA to eliminate external resistors.

BDBn+, where

n = {0,1,2,3,4,5,6,7,P}

29, 31, 33, 35,

37, 46, 48, 50, 52

I/O, RS-485,

Weak pulldown

Bidirectional I/O for data and parity to and from the differential SCSI bus.

BDBn–, where

n = {0,1,2,3,4,5,6,7,P}

30, 32, 34, 36,

38,47, 49, 51, 53

I/O, RS-485,

Weak pulldown

Bidirectional I/O for the complement of data and parity to and from the

differential SCSI bus.

DRVBUS

2

Input, TTL levels,

Weak pulldown

A high-level logic signal from the control transceiver enables either the

single-ended or differential drivers as directed by SDB.

DSENS

56

Input, TTL levels,

Weak pullup

A low-level input initializes the internal latches and disables all drivers.

RESET

55

Input, TTL levels,

Weak pullup

A low-level input initializes the internal latches and disables all drivers.

SDB

1

Input, TTL levels,

Weak pulldown

A high-level logic signal from the control transceiver sends data from the

differential bus to the single-ended bus. A low-level signal reverses the

flow.