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DS2109

Plug and Play SCSI Terminator

Dallas Semiconductor -> Maxim Integrated 

DS2109

BUS CURRENT SENSING

The DS2109 has the capability to use current sensing to

determine if the SCSI bus is over– or under–terminated.

A series 1 ohm resistor between pads R14 and TCS is

inserted into the SCSI bus (preferably the –RST line)

and used to monitor the current when that line pulls low

(active, or “asserted” state). Based on the current mea-

sured, the DS2109 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 cir-

cuit. The voltage across the 1 ohm sense resistor is dif-

ferentially 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 volt-

age. 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 IBUS is greater than

32 mA, the DS2109 will be isolated from the SCSI bus. It

is recommended that the signal on TCS be asserted for

at least 25 msec to allow the signal (and comparator out-

puts) 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 addi-

tional circuitry required for bus sensing.

The DS2109 will be isolated from the SCSI bus as

shown in Table 2.

BUS CURRENT SENSING CIRCUITRY Figure 5

TO SCSI CONTROLLER

FROM SCSI BUS (–RST)

R14

110 OHM

1 OHM

IBUS

TCS

–+

REF

DECODE

AND

LATCH

022698 6/10

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