SLLS054B - APRIL 1989 - REVISED MAY 1995
LoadThe SN75ALS085 is a monolithic, high-speed, advanced low-power Schottky, dual-channel driver/receiver device designed for use in the AUI of ANSI/IEEE Std 802.3-1988. The two drivers on the device drive a 78- balanced, terminated twisted-pair transmission line up to a maximum length of 50 meters. In the off (idle) state, the drivers maintain minimal differential output voltage on the twisted-pair line and, at the same time, remain within the required output common-mode range.
With the driver enable (TXEN) high, upon receiving the first falling edge into the driver input, the differential outputs will rise to full-amplitude output levels within 25 ns. The output amplitude is maintained for the remainder of the packet. After the last positive packet edge is transmitted into the driver, the driver will maintain a minimum of 70% full differential output for a minimum of 200 ns, then decay to a minimum level for the reset (idle) condition within 8 us. Disabling the driver by taking the driver enable low will also force the output into the idle condition after the normal 8-us timeout. While operating, the drivers are able to withstand a set of fault conditions and not suffer damage due to the faults being applied. The drivers power up in the idle state to ensure that no activity is placed on the twisted-pair cable that could be interpreted as network traffic.
The line receiver squelch function interfaces to a differential twisted-pair line terminated external to the device. The receiver squelch circuit allows differential receive signals to pass through as long as the input amplitude and pulse duration are greater than the minimum squelch threshold. This ensures a good signal-to-noise ratio while the data path is active and prevents system noise from causing false data transitions during line shutdown and line-idle conditions. The RXO outputs default to a high level and the RXEN outputs default to a low level while the squelch function is blocking the data path through the receiver (idle). The line receiver squelch will become active within 50 ns when the input squelch threshold is exceeded. RXEN will be driven high when the squelch circuit is allowing data to pass through the receiver. The receiver squelch circuit can also withstand a set of fault conditions while operating without causing permanent damage to the device.
The purpose of the loop functions is to provide a means by which system data path verification can be done to isolate faulty interfaces and assist in network diagnosis. The LOOP pins are TTL compatible and must be held high for normal operation. When LOOP1\ is taken low, the output of driver 1 (TXO1) immediately goes into the idle state. Also, the input to receiver 1 is ignored and a path from TXI1 to RXO1 is established. When LOOP1\ is taken back high, driver 1 and receiver 1 revert back to their normal operation. When LOOP2\ is taken low, a similar data path is established between TXI1 and RXO2. TXEN1 must be high for the loop functions to operate and TXEN1 can be used to gate the loop function if desired. During loop operation, the respective receiver enable output (RXEN) will reflect the status of TXEN1.
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