DS21T09S+ Datasheet DS21T09S+ | P4-P6

DS21T09

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DETAILED PIN DESCRIPTION Table 1

PIN SYMBOL FCN DESCRIPTION

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…R18

T Signal Termination. 110! 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.

PDO

O Power-Down Output. Connect to DS21T07 for Wide SCSI

applications; 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 V

REF

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

µF capacitor; see Figures 4 and 6.

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! termination. Connect to SCSI bus signal

lines. 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 25 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 and V

ref

will move less than

60 mv. To maintain the specified regulation, a 4.7 µF capacitor is required between the V

REF

pin and

ground. A high frequency capacitor (0.1 µF ceramic recommended) can also be placed on the V

ref

pin in

applications that use fast rise/fall time drivers. The power down capacitance on terminating resistors R1-

R13 and R15-R18 is <3 pF; R14 is slightly higher due to the bus current sensing circuitry.

The DS21T09 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

recommended between TPWR and ground. It should be placed as close as possible to the DS21T09. The

DS21T09 should be placed as close as possible to the connector to minimize signal and power trace

length, thereby resulting in less input capacitance and reflections which can degrade the bus signals.

DS21T09

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BUS GROUND SENSING

If internal and external SCSI devices are connected to the SCSI bus, the DS21T09 will disconnect from

the SCSI bus according to the circuit in Figure 3. To utilize this automatic disconnect method of bus

sensing, configure the DS21T09 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 connector.

BUS GROUND SENSING CIRCUIT Figure 3

BUS GROUND DETECT CONFIGURATION Figure 4

DS21T09

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BUS CURRENT SENSING

The DS21T09 has the capability to use current sensing to determine if the SCSI bus is over- or under-

terminated. A 1Ω=resistor in series with the SCSI bus (preferable the RST line) is used to monitor the bus

current when that line pulls low (Active or “asserted” state). The resistor is between pad R14 and TCS on

the die. Based on the current measured, the DS21T09 will disconnect or connect from the SCSI bus. The

configuration for this automatic isolation technique is shown in Figure 6.

Figure 5 shows a simplified diagram of the sensing circuit. The voltage across the 1Ω=sense resistor is

differentially amplified and converted into a single-ended voltage with respect to ground. This is fed into

a bank of comparators and measured against a reference voltage. The circuit takes a measurement each

time TCS is driven below a 0.8 volts threshold and the outputs are latched on the rising edge of TCS. If

BUS

is greater than 32 mA, the DS21T09 will be isolated from the SCSI bus. It is recommended that the

signal on TCS be asserted for at least 25 µsec to allow the signal (and comparator outputs) to settle into a

known state. A timing diagram of the sensing and latching operation is shown in Figure 7.

It is preferred that the -RST line be used for monitoring the bus termination status because -RST is only

asserted during power up or during a major change in bus configuration. Note that R14 will have a higher

input capacitance than the other lines because of the additional circuitry required for bus sensing.

The DS21T09 will be isolated from the SCSI bus as shown in Table 2.

BUS CURRENT SENSING CIRCUITRY Figure 5

P1-P3 P4-P6 P7-P9 P10-P10

DS21T09S+

Mfr. #:

Buy DS21T09S+

Manufacturer:

Maxim Integrated

Description:

SCSI Interface IC

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DS21T09S+