The SN75LBC968 is a nine-channel transceiver with active termination that drives and receives the signals from the single-ended, parallel data buses such as the Small Computer-Systems Interface (SCSI) bus. The features of the line drivers, receivers, and active-termination circuits provide the optimum signal-to-noise ratios for reliable data transmission. Integration of the termination and transceivers in the LinBiCMOS process provides the necessary analog-circuit performance, has low quiescent power, and reduces the capacitance presented to the bus over separate termination and I/O circuits.
The transceivers of the SN75LBC968 can be enabled to function as totem-pole or open-drain outputs. The open-drain mode drives the wired-OR lines of SCSI (BSY, SEL, and RST) by inputting the data to the direction control input DE/RE instead of the A input. When driving the data through the A input, the outputs become totem poles and provide active signal negation for a higher voltage level on low-to-high signal transitions on heavily loaded buses. In either mode, the turnon and turnoff output transition times are limited to minimize crosstalk through capacitive coupling to adjacent lines and RF emissions from the cable. The receivers are also designed for optimum analog performance by precisely controlling the input-voltage thresholds, providing wide input-voltage hysteresis and including an input-noise filter. These features significantly increase the likelihood of detecting only the desired data signal and rejecting noise.
The communication between the SN75LBC968 and the controller can be accomplished at 3.3-V logic levels provided that the VCC1 input connects to the same supply rail as the controller. This provides a bridge from the lower-voltage circuit and the 5-V SCSI bus. The SN75LBC968 also removes the need for special I/O buffers (and associated power dissipation) on the controller itself. The SN75LBC968 must be used with a SCSI controller with support for Differential SCSI.
The integrated, current-mode, active termination supplies a constant 24 mA of current (TERMPWR) to the bus when the bus voltage falls below 2.5 V. This makes the next low-to-high (negation) signal transition independent of the low-level (asserted) bus voltage, unlike voltage-mode terminators. The termination current is provided through the TE input and from TERMPWR and can be disabled by letting the TE input float or by connecting it to ground. The termination circuitry is independent from the line drivers and receivers and VCC or VCC1. Operational termination is present as long as TERMPWR is applied.
The switching speeds of the SN75LBC968 are sufficient to transfer data over the data bus at ten million transfers per second (Fast-SCSI). The specification, tsk(lim), is for system skew budgeting and maintenance of bus set-up and hold times. The device is available in the space-efficient shrink-small-outline package (SSOP) with 25-mil lead pitch. The SN75LBC968 meets or exceeds the requirements of ANSI X3.1311994 (SCSI-2) and the proposed SPI (SCSI-3) standards, and is characterized for operation from 0°C to 70°C.
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