DS2119M
REFERENCE DOCUMENTS
Small Computer Systems Interface (SCSI-3)
Small Computer Systems Interface (SCSI-3)
Small Computer Systems Interface (SCSI-3)
Small Computer Systems Interface (SCSI-3)
SCSI Parallel Interface (SPI)
SCSI Parallel Interface 2 (SPI-2)
SCSI Parallel Interface 3 (SPI-3)
SCSI Parallel Interface 4 (SPI-4)
Project: 0855-M, 1995
Project: 1142-M, 1998
Project: 1302-D, 1999
Project: 1365-D, 200x
Available From:
American National Standards Institute (ANSI) Phone: (212) 642-4900
Global Engineering Documents 15 Inverness Way East; Englewood, CO 80112 Phone: (800) 854-7179
FUNCTIONAL DESCRIPTION
The DS2119 combines LVD and SE termination with DIFFSENS 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 SE, HVD, or LVD, respectively.
The DS2119M’s DIFF_CAP pin monitors the DIFFSENS line to determine the proper operating mode of
the device. The DIFFSENSE pin can also drive the SCSI DIFFSENS line (when MSTR/SLV = 1) to
determine the SCSI bus-operating mode. The DS2119M 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
current sources. 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 and the reference voltage will move less than 100mV. The
terminating resistors maintain their 110Ω value.
HVD Isolation Mode:
The DS2119M identifies that there is an HVD (high-voltage differential)
device on the SCSI bus and isolates the termination pins from the bus.
When ISO = 1, the termination pins are isolated from the SCSI bus, V
REF
remains active. During thermal
shutdown, the termination pins are isolated from the SCSI bus and V
REF
becomes high impedance. The
DIFFSENSE driver is shut down during either of these two events.
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 are recommended between TPWR and ground and placed as close
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ROCHESTER [ Rochester Electronics ]
