The G218B Multi-mode LVD/SE SCSI terminator provides a smooth transition into the next generation of the SCSI Parallel Interface (SPI-3). It automatically senses the bus, via DIFFSENS, and switches the termination to either single-ended (SE) or low voltage differential (LVD) SCSI, dependent on which type of devices are connected to the bus. If the G218B detects a HVD SCSI device, it switches to a high impedance state. For a 16-bit Wide SCSI bus to be operational, three G218B terminators are needed. The Multi-mode terminator contains all functions required to terminate and auto detect and switch modes for SCSI Parallel Interface (SPI) bus architectures. For the SE termination, one regulator and nine precision 110ohm resistors are used. For the LVD termination, the G218B integrates 18 regulated supplies with nine precision resistor strings.

Auto-selectable multi-mode Single-Ended or LVD Termination Meets SCSI-2, SCSI-3 SPI Ultra (Fast-20), Ultra 2(SPI-2 LVD), Ultra160 (SPI-3 LVD) standards. Supports active negation 3pF channel capacitance Thermal protection Hot-Swap compatible 5% tolerance on SE and LVD termination resistance 28 and 36 pin SSOP package 28 Pin SSOP: Pin Compatible with DS2119 36 Pin SSOP: Pin Compatible with DS2118, LX5241 and UCC5630

DISCNCT does not disable VREF DISCNCT does not disable VREF DISCNCT does not disable VREF DISCNCT does not disable VREF DISCNCT does not disable VREF DISCNCT does not disable VREF

SSOP-28 / Tube SSOP-28 / Tape & Reel SSOP-36 / Tube SSOP-36 / Tape & Reel TSSOP-28 / Tube TSSOP-28 / Tape & Reel

Note: DIFFSENSRefers to the SCSI bus signal. DIFFSENSERefers to the G218B pin name and internal circuitry relating to differential sensing. DIFFBRefers to the G218B pin name and internal circuitry relating to monitoring the DIFFSENS line.

The G218B is used in multi-mode active termination applications, where single ended (SE) and low voltage differential (LVD) devices might coexist. The LVD termination section consists of 18 source/sink amplifiers (VTOP, VBOT), biasing circuitry and nine precision resistor strings (RTOP, RMID, RBOT). The SE termination section consists a 2.85V source/sink regulator with 9 precision 110ohm resistor. The DIFFSENSE section consists 1.3V, 5mA driver and a sensing circuit (Figure 1) DIFFSENS is used to identify which types of SCSI devices are present on the bus. If the voltage on DIFFSENS is between 0-0.5V the bus is single-ended; it is between 0.7-1.9V the bus is LVD; and it is greater than 2.4V, the bus is HVD. The G218B DIFFB pin monitors the DIFFSENS line to determine the proper operating mode of the device; this mode is indicated by the SE/LVD/HVD outputs. HVD Isolation Mode The G218B identifies that there is an HVD (high voltage differential) device on the SCSI bus and isolates the termination pins from the bus. When DISCNCT is pulled high, the termination pins are isolated from the SCSI bus, and the bus mode indicators (SE/LVD/HVD) as well as VREF remains active. During thermal shutdown, the termination pins are isolated from the SCSI bus and VREF 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 lines should be bypassed locally. A 2.2�F capacitor and a 0.01�F high frequency capacitor is recommended between TPWR

and ground and placed as close as possible to the G218B. The G218B 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. The DIFFSENSE pin can drive the SCSI DIFFSENS line (when M/S is pulled high) to determine the SCSI bus operating mode. The G218B 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 resistor string with two amplifiers. This configuration yields a 105 differential and 150 common mode impedance. A fail safe bias 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 2.85 volts (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 value. To maintain the specified regulation, a 4.7�F capacitor is required between the VREF pin and ground of each G218B. A high frequency cap (0.1�F ceramic recommended) can also be placed on the VREF pin in applications that use fast rise/fall time drivers. A typical SCSI bus configuration is shown Figure2. An internal pull down resistor assures that the G218B will be terminating the bus if the DISCNCT pin is left floating.