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SN75LBC971ADL Datasheet(PDF) 2 Page - Texas Instruments |
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SN75LBC971ADL Datasheet(HTML) 2 Page - Texas Instruments |
2 / 15 page SN75LBC971A SCSI DIFFERENTIAL CONVERTER-DATA SLLS186B – OCTOBER 1994 – REVISED JULY 1997 2 POST OFFICE BOX 655303 • DALLAS, TEXAS 75265 description (continued) The SCSI, differential, converter-data chip operates in two modes depending on the state of the DRVBUS input. With DRVBUS low, a bidirectional latch circuit sets the direction of data transfer. Each data bit has its own latch, and each bit’s direction is independent of all other bits. When neither the single-ended nor the differential sides are asserted, the latch disables both A- and B-side output drivers. When the input to either side is asserted, the latch enables the opposite side’s driver and sets data flow from the asserted input to the opposite side of the device. When the input deasserts, the latch maintains the direction until the receiver on the enabled driver detects a deassertion. The latch then returns to the initial state. No parity checking is done by this device; the parity signal passes through the device like other data signals do. When DRVBUS is high, direction is determined by the SDB signal. However, a change in SDB does not always immediately change the direction. When DRVBUS first asserts, the direction indicated by SDB is latched and takes effect immediately. When SDB changes while DRVBUS is high, the drivers that were on immediately turn off. However, the other driver set does not turn on until the receivers sense a deasserted state on all nine data lines. This is done to prevent the active drivers from turning on until all other drivers are off and the terminators pull the lines to a deasserted state. The single-ended SCSI bus interface consists of CMOS, bidirectional inputs and outputs. The drivers are rated to ±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 American National Standard of Information Systems (ANSI) X3.131-1994 Small Computer System Interface-2 (SCSI-2). The SN75LBC971A is characterized for operation over the temperature range of 0 °C to 70°C. TERMINAL FUNCTION TERMINAL I/O DESCRIPTION NAME NO. I/O DESCRIPTION ADBn–, where n = {0,1,2,3,4,5,6,7,P} 4, 6, 8, 10, 19, 21, 23, 25, 27 I/O, Single-ended SCSI voltage levels, Strong pullup Bidirectional I/O for data and parity bits to and from the single-ended SCSI controller. As outputs, these terminals can source or sink 16 mA. As inputs, they are pulled up with about 4-mA to eliminate external resistors. BDBn+, where n = {0,1,2,3,4,5,6,7,P} 29, 31, 33, 35, 37, 46, 48, 50, 52 I/O, RS-485, Weak pulldown Bidirectional I/O for data and parity to and from the differential SCSI bus. BDBn–, where n = {0,1,2,3,4,5,6,7,P} 30, 32, 34, 36, 38,47, 49, 51, 53 I/O, RS-485, Weak pulldown Bidirectional I/O for the complement of data and parity to and from the differential SCSI bus. DRVBUS 2 Input, TTL levels, Weak pulldown A high-level logic signal from the control transceiver enables either the single-ended or differential drivers as directed by SDB. DSENS 56 Input, TTL levels, Weak pullup A low-level input initializes the internal latches and disables all drivers. RESET 55 Input, TTL levels, Weak pullup A low-level input initializes the internal latches and disables all drivers. SDB 1 Input, TTL levels, Weak pulldown A high-level logic signal from the control transceiver sends data from the differential bus to the single-ended bus. A low-level signal reverses the flow. |
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