SLLS215A -MAY 1995 - REVISED JULY 1996
The SN75LBC970A SCSI differential converter-control is an adaptation of the industry's first 9-channel RS-485 transceiver, the SN75LBC976. When used in conjunction with one or more of its companion data transceiver(s), SN75LBC971A, the chip set provides the superior electrical performance of differential SCSI from a single-ended SCSI bus controller. A 16-bit, Fast-SCSI bus can be implemented with just three devices (two for data and one for control) in the space-efficient, 56-pin, shrink small-outline package (SSOP) and a few external components.
In a typical differential SCSI node, the SCSI controller provides the enables for each external RS-485 transceiver. This could require as many as 27 additional terminals for a 16-bit differential bus controller or relegate a 16-bit single-ended controller to only an 8-bit differential bus. Using the standard nine SCSI control signals, the SN75LBC970A control transceiver decodes the state of the bus and enables the SN75LBC971A data transceiver(s) to transmit the single-ended, SCSI input signals differentially to the cable or receive the differential cable signals and drive the single-ended outputs to the controller.
The single-ended SCSI bus interface consists of CMOS bidirectional inputs and outputs. The drivers are rated at ±16 mA of output current. The receiver inputs are pulled high with approximately 4 mA to eliminate the need for external pullup resistors for the open-drain outputs of most single-ended SCSI controllers. The single-ended side of the device is not intended to drive the SCSI bus directly.
The differential SCSI bus interface consists of bipolar bidirectional inputs and outputs that meet or exceed the requirements of EIA-485 and ISO 8482-1982/TIA TR30.2 referenced by the American National Standard of Information Systems (ANSI) X3.131-1994 Small Computer System Interface-2 (SCSI-2).
The SN75LBC970A is characterized for operation over the temperature range of 0°C to 70°C.
The SN75LBC970A consists of nine RS-485 differential transceivers, nine TTL- or CMOS-level compatible transceivers, a state machine and control logic block, a 20-MHz crystal-controlled oscillator, a timer, a power-up/-down glitch protection circuit, and a thermal-shutdown protection circuit.
The single-ended or controller interface is designated as the A side and the differential port is the B side. Since the device uses the SCSI control signals to decode the state of the bus and data flow direction, the terminal assignments must be matched to the corresponding signal on the SCSI bus. The signal name followed by a a minus sign (-) indicates an active-low signal while a plus sign (+) indicates an active-high signal.
A reset function, which disables all outputs and clears internal latches, can be accomplished from two external inputs and two internally generated signals. RESET \(Reset) and DSENS (differential sense) are available to external circuits for a bus reset or to disable all outputs should a single-ended cable be inadvertently connected to a differential connector. The power-up and thermal-shutdown are internally generated signals that have the same effect when the supply voltage is below 3.5 V or the junction temperature exceeds approximately 175°C.
This data sheet contains descriptions of the SN75LBC970A input and output signals followed by the electrical characteristics. The parameter measurement information is followed by the theory of operation, a state flow chart, and a typical circuit in the application information section.
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