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DS2109

Plug and Play SCSI Terminator

Dallas Semiconductor -> Maxim Integrated 

DS2109

DETAILED PIN DESCRIPTION Table 1

PIN

SYMBOL

FCN DESCRIPTION

1

PDI

I

Power Down Internal. For Bus Ground Sensing Circuit connect to pin 22

of internal SCSI connector, see Figure 4.

2–6,

10–13,

16–19,

24–27

R1...R13

R15...18

T Signal Termination. 110 ohm termination. Connect to SCSI bus signal

lines. For Bus Current Sensing Circuit connect to data, parity, and control

lines, except SCSI–RST line, see Figure 6.

7, 8, 21 HS–GND

P Heat Sink Ground. Internally connected to the mounting pad. Should be

either grounded or electronically isolated from other circuitry.

9

PDO

O Power Down Output. Connect to DS21S07A for Wide SCSI applica-

tions, see Figure 8.

14

TPWR

P Termination Power. Connect to the SCSI TERMPWR line. Bypass with

a 2.2 µF capacitor, see Figures 4 and 6.

15

VREF

O Reference Voltage. 2.85 volt reference; must be decoupled with a 4.7 µF

capacitor, see Figures 4 and 6.

20

PDE

I

Power Down External. For Bus Ground Sensing Circuit, connect to pin

36 of external SCSI connector, see Figure 4.

22

TCS

I

Termination Current Sense. Used to sense current on the SCSI bus.

For Bus Current Sensing Circuit, connect to SCSI signal line –RST, see

Figure 6.

23

R14

T Signal Termination. 110 ohm termination. Connect to SCSI bus signal

line. For Bus Current Sensing Circuit connect to SCSI controller chip, see

Figure 6.

28

GND

P Ground. Signal ground; 0.0 volts.

ACTIVE TERMINATION

The voltage regulator circuitry (bandgap reference and

class AB power amplifier) produces a precise laser–

trimmed 2.85 volt level and is capable of sourcing 24 mA

into each of the terminating resistors when the signal

line is low (active). When the external driver for a given

signal line turns off, the active terminator will pull that

signal line to 2.85 volts (quiescent state). When used

with an active negation driver, the power amp can sink

22 mA per line while keeping the voltage reference in

regulation; the terminating resistors maintain their 110Ω

value over the entire voltage range. To maintain the spe-

cified regulation, a 4.7 µF capacitor is required between

the VREF pin and ground. A high frequency cap (0.1 µF

ceramic recommended) can also be placed on the VREF

pin in applications that use fast rise/fall time drivers. The

power down capacitance on terminating resistors

R1–R13 and R15–R18 is <4 pF; R14 is slightly higher

due to the bus current sensing circuitry.

The DS2109 can be removed from the SCSI bus by

using either of two automatic methods, Bus Ground

Sensing or Bus Current Sensing.

As with all analog circuitry, the TERMPWR lines should

be bypassed locally. A 2.2 µF capacitor is recom-

mended between TPWR and ground and placed as

close as possible to the DS2109. The DS2109 should

be placed as close as possible to the connector to mini-

mize signal and power trace length, thereby resulting in

less input capacitance and reflections which can

degrade the bus signals.

BUS GROUND SENSING

If internal and external SCSI devices are connected to

the SCSI bus, the DS2109 will disconnect from the SCSI

bus according to the circuit in Figure 3. To utilize this

automatic disconnect method of bus sensing, configure

the DS2109 as shown in Figure 4. The PDI (Power

Down Internal) pin should be connected to pin 22 of the

internal SCSI connector, and the PDE (Power Down

External) connected to pin 36 of the external SCSI con-

nector.

022698 4/10

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