DS2118M
REFERENCE DOCUMENTS
Small Computer Systems Interface (SCSI-3) SCSI Parallel Interface (SPI)
Project: 0855-M, 1995
Small Computer Systems Interface (SCSI-3) SCSI Parallel Interface 2 (SPI-2) Project: 1142-M, 1998
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 DS2118M combines LVD and SE termination with DIFFSENS sourcing and detection. The LVD
termination section consists of two source/sink amplifiers (VTOP, VBOT), biasing circuitry and nine
precision resistor strings (RTOP, RMID, RBOT). The SE termination section consists of a 2.85V
source/sink regulator with 9 precision 110 Ohm resistors. The DIFFSENSE section consists of a 1.3V, 5
mA driver and a sensing circuit (Figure 1).
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 112
mV 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 2.85 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.
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, Vref remains active, and
the bus mode indicators (SE/LVD/HVD) remain active. During thermal shutdown, the termination pins
are isolated from the SCSI bus, Vref 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 pull-down 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 VDD lines should be bypassed locally. A 2.2
µF
capacitor
and a 0.01
µF
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
2 of 8
DALLAS [ DALLAS SEMICONDUCTOR ]
