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 I
BUS
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
1 OHM
110 OHM
I
BUS
TCS
– +
REF
DECODE
AND
LATCH
022698 6/10
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