AM79C974 datasheet(129/153 Pages) AMD

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P R E L I M I N A R Y

129

Am79C974

Decoupling methods which are NOT recommended

include:

connecting a capacitor only between the VDD and

VSS planes so that it sits on the component side of

the board. This doesn’t allow the capacitor to re-

duce noise directly at the chip, but it does allow for

a reduction of power plane noise and of board

components. (capacitors).

connecting a capacitor only between the VDD and

VSS planes so that it sits on the solder side of the

board. This configuration also doesn’t allow direct

decoupling at the chip. It does save board space,

but it requires an extra manufacturing step.

TERMINATION CONSIDERATIONS

The use of active or regulated termination for termina-

tors on the motherboard is recommended (see Figure

E-6), while external SCSI terminators can be used to

terminate other parts of the SCSI bus. Terminators must

match the impedance seen by a signal at the end of the

SCSI bus to the characteristic impedance of the SCSI

bus. This impedance is typically 84 +/– 12

Ω, but can

vary greatly with PC board characteristics and cabling.

VIN

VOUT

Low

Dropout

Voltage

Regulator

V adj

µf

Alum. or

4.7

µf Tant.

15 V

121

Ω

1/4W

154

Ω

1/4W

150

µf

Alum. or

22 µf Tant.

10 V

µf

Ceramic

25 V

R3-20 are

110

Ω 1%

R3-11

R12

R14

R15

R16

R17

R18

R19

R20

R13

TERMPWR

DB (0-7,P)

ATN

BSY

ACK

RST

MSG

SEL

C/D

REQ

I/O

2.85 V

18681A-68

Figure E-6. Regulated Termination

AM79C974 Datasheet(PDF) 129 Page - Advanced Micro Devices

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