DS2117M

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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

Small Computer Systems Interface (SCSI-3) SCSI Parallel Interface 3 (SPI-3) Project: 1302-D, 1999

Small Computer Systems Interface (SCSI-3) SCSI Parallel Interface 4 (SPI-4) Project: 1365-D, xxxx

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 DS2117 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 are applied to the termination

resistors. If the SCSI bus is in LVD mode, the 1.25V reference is used. If the SCSI bus is in SE mode, the

2.85V reference is used. That same control logic switches in/out parallel resistors to change the total

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 DS2117M’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 DS2117M

switches to the termination mode that is appropriate for the bus based on the value of the DIFFSENS

voltage. These modes are LVD mode, SE mode, and HVD isolation mode.

LVD Mode

LVD termination is provided by a precision laser-trimmed resistor string with two current sources. This

configuration yields a 105

Ω 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 pulls 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 110

Ω value.

HVD Isolation Mode

The DS2117M identifies that there is an HVD (high voltage differential) device on the SCSI bus and

isolates the termination pins from the bus.

bus mode indicators (SE/LVD/HVD) remain active. During thermal shutdown, the termination pins are