The DS2120 Ultra3 LVD SCSI terminator is a low-voltage differential (LVD) terminator. If the device is connected in an LVD-only bus, the DS2120 uses LVD termination. If any single-ended (SE) or high-voltage differential (HVD) devices are connected to the bus, the DS2120 disconnects from the bus. This is accomplished inside the part automatically by sensing the voltage on the SCSI bus DIFFSENS line.
For the LVD termination, the DS2120 integrates two current sources with nine precision resistor strings.
Three DS2120 terminators are needed for a wide SCSI bus.
The DS2120 combines LVD termination with DIFFSENSE sourcing and detection.
LVD termination is provided by a laser-trimmed resistor biased with two current sources and a common-mode voltage source, generated from a bandgap reference of 1.25V. The configuration is a y-type terminator with a 105Ω differential and 150Ω common-mode resistance. A fail-safe bias of 112mV is maintained when no drivers are connected to the SCSI bus. In non-LVD mode, the resistors are isolated from the bus.
The DIFF_CAP pin of DS2120 monitors the DIFFSENS line to determine the proper operating mode of the device. If the voltage on the DIFF_CAP is between 0.7V and 1.9V, the device enters LVD mode after the mode-change delay. If the voltage at the DIFF_CAP later crosses one of the thresholds, the DS2120 again changes modes after the mode-change delay. The mode-change delay is the same when changing in or out of LVD mode. A new mode change can start anytime after a previous mode change has been detected.
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 Isolation Mode: The DS2120 identifies that there is a SE (single-ended) device on the SCSI bus and isolates the termination pins from the bus. HVD Isolation Mode: The DS2120 identifies that there is an HVD 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 and VREF remains active.
The mode-change delay/filter is still active and the LVD pin continues to indicate the correct bus mode.
During thermal shutdown, the termination pins are isolated from the SCSI bus and VREF becomes high impedance. The DIFFSENS driver is shut down during either of these two events. The DIFF_CAP receiver is disabled and the LVD goes low, indicating a non-LVD condition.
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 2.2μF capacitor and a 0.01μF high-frequency capacitor are recommended between TPWR and ground and placed as close as possible to the DS2120. The DS2120 should be placed as close as possible to the SCSI connector to minimize signal and power trace length, thereby lessening input capacitance and reflections that can degrade the bus signals.
To maintain the specified regulation, a 4.7μF capacitor is required between the VREF pin and ground of each DS2120. A high-frequency cap [0.1 μF ceramic recommended] can also be placed on the VREF pin in applications that use fast rise/fall-time drivers.
DIFFSENS Noise Filtering: The DS2120 incorporates a digital filter to remove high-frequency transients on the DIFFSENS control line, thereby eliminating erroneous switching between modes. This filter eliminates the need for the external capacitor and resistor, which previously performed this function. The external filter can be used in addition to the digital filter if the DS2120 and DS2118M or DS2119M are to be used interchangeably.
Dallas Logic DS2120E/T&R/C08