DS2118M
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FUNCTIONAL DESCRIPTION
The DS2118 combines LVD and SE termination with DIFFSENSE sourcing and detection.
A bandgap reference is fed into two amplifiers, which creates a 1.25V reference voltage and a 2.85V
reference voltage. The control logic determines which of these references will be applied to the
termination resistors. If the SCSI bus is in LVD mode, then the 1.25V reference will be used. If the SCSI
bus is in SE mode, then the 2.85V reference will be used. That same control logic will switch in/out
parallel resistors to change the total termination resistance accordingly. Finally, in SE mode the Rp pins
will be switched to ground.
The DIFFSENSE circuitry decodes trinary logic. There will be one of three voltages on the SCSI control
line called DIFFSENS. Two comparators and a NAND gate determine if the voltage is below 0.6V, above
2.15V, or in between. That indicates the mode of the bus to be HVD, SE, or LVD, respectively.
The DS2118M’s DIFF_CAP pin monitors the DIFFSENS line to determine the proper operating mode of
the device; this mode is indicated by the SE/LVD/HVD outputs. The DIFFSENSE pin can also drive the
SCSI DIFFSENS line (when MSTR/SLV = 1) to determine the SCSI bus operating mode. The DS2118M
switches to the termination mode that is appropriate for the bus based on the value of the DIFFSENS
voltage. These modes are:
LVD Mode: LVD termination is provided by a precision laser-trimmed resistor string with two
amplifiers. This configuration yields a 105Ω differential and 150Ω common-mode impedance. A
fail-safe bias of 112mV is maintained when no drivers are connected to the SCSI bus.
SE Mode: When the external driver for a given signal line turns off, the active terminator will
pull that signal line to 2.85V (quiescent state). When used with an active negation driver, the
power amp can sink 22mA per line while keeping the voltage reference in regulation. The
terminating resistors maintain their 110Ω value.
HVD Isolation Mode: The DS2118M identifies that there is an HVD (high-voltage differential)
device on the SCSI bus and isolates the termination pins from the bus.
When ISO is pulled high, the termination pins are isolated from the SCSI bus, V
REF
remains active, and
the bus mode indicators (SE/LVD/HVD) remain active. During thermal shutdown, the termination pins
are isolated from the SCSI bus, V
REF
becomes high impedance, and the bus mode indicators
(SE/LVD/HVD) remain active. The DIFFSENSE driver is shut down during either of these two events.
An internal pulldown resistor assures that the DS2118M will be terminating the bus if the ISO pin is left
floating.
To ensure proper operation, the TPWR pin should be connected to the SCSI bus TERMPWR line. As
with all analog circuitry, the TERMPWR and V
DD
lines should be bypassed locally. A 2.2µF capacitor
and a 0.01µF high-frequency capacitor is recommended between TPWR and ground and placed as close
as possible to the DS2118M. The DS2118M should be placed as close as possible to the SCSI connector
to minimize signal and power-trace length, thereby resulting in less input capacitance and reflections,
which can degrade the bus signals.
To maintain the specified regulation, a 4.7µF capacitor is required between the V
REF
pin (VREF) and
ground of each DS2118M. A high frequency cap (0.1µF ceramic recommended) can also be placed on the
