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ISL4486 Datasheet(PDF) 7 Page - Intersil Corporation |
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ISL4486 Datasheet(HTML) 7 Page - Intersil Corporation |
7 / 11 page 7 Application Information RS-485 and RS-422 are differential (balanced) data transmission standards for use in long haul or noisy environments. RS-422 is a subset of RS-485, so RS-485 transceivers are also RS-422 compliant. RS-422 is a point- to-multipoint (multidrop) standard, which allows only one driver and up to 10 (assuming one unit load devices) receivers on each bus. RS-485 is a true multipoint standard, which allows up to 32 one unit load devices (any mix of drivers and receivers) on each bus. To allow for multipoint operation, the RS-485 spec requires that drivers must handle bus contention without sustaining any damage. Another important advantage of RS-485 is the extended common mode range (CMR), which specifies that the driver outputs and receiver inputs withstand signals that range from +12V to -7V. RS-422 and RS-485 are intended for runs as long as 4000’ (~1200m), so the wide CMR is necessary to handle ground potential differences, as well as voltages induced in the cable by external fields. Receiver Features These devices utilize a differential input receiver for maximum noise immunity and common mode rejection. Input sensitivity is ±200mV, as required by the RS422 and RS-485 specifications. Receiver inputs function with common mode voltages as great as 7V outside the power supplies (i.e., +12V and -7V), making them ideal for long networks, or industrial environments, where induced voltages are a realistic concern. Receiver input resistance surpasses the RS-422 spec of 4k Ω, and exceeds the RS-485 “Unit Load” requirement of 12k Ω minimum. The 20kΩ input resistance allows at least 50 devices on the RS-485 bus. All the receivers include a “fail- safe if open” function that guarantees a high level receiver output if the receiver inputs are unconnected (floating). Receivers easily meet the data rate supported by the driver, and receiver outputs are three-statable via the active low RE input. Driver Features The RS-485/RS-422 driver is a differential output device that delivers at least 2.3V across a 54 Ω load (RS-485/ PROFIBUS), and at least 2.5V across a 100 Ω load (RS-422) even with VCC = 4.5V. The drivers feature low propagation delay skew to maximize bit width, and to minimize EMI. Outputs of the drivers are not slew rate limited, so faster output transition times allow data rates of at least 40Mbps with the ISL4486, and 30Mbps with the ISL81486. Driver outputs are three-statable via the active high DE input. For parallel applications, bit-to-bit skews between any two ISL4486 transmitter and receiver pairs are guaranteed to be no worse than 15ns (6ns max for any two Tx, 9ns max for any two Rx). Data Rate, Cables, and Terminations Twisted pair is the cable of choice for RS-485, RS-422, and PROFIBUS networks. Twisted pair cables tend to pick up noise and other electromagnetically induced voltages as common mode signals, which are effectively rejected by the differential receivers in these ICs. RS-485/RS-422 are intended for network lengths up to 4000’ (~1200m), but the maximum system data rate decreases as the transmission length increases. According to guidelines in the RS-422 and PROFIBUS specifications, networks operating at data rates in excess of 3Mbps should be limited to cable lengths of 100m (328 feet) or less, and the PROFIBUS specification recommends that the more expensive “Type A” (22AWG) cable be used. Nevertheless, the ISL4486’s large differential output swing, fast transition times, and high drive-current output stages allow operation at 40Mbps over standard “CAT5” cables in excess of 400 feet (121m). Figure 8 details the ISL4486 performance at this condition, with a 120 Ω termination resistor at both the driver and the receiver ends. Note that the differential signal delivered to the receiver at the end of the cable (A-B) still exceeds 1V, so even longer cables could be driven if lower noise margins are acceptable. If more noise margin is desired, shorter cables produce a larger receiver input signal as illustrated in Figure 7. Performance should be even better if the “Type A” cable is utilized. The lower data rate of the ISL81486 allows for driving longer cables. To minimize reflections, proper termination is imperative when using these high data rate transceivers. In point-to-point, or point-to-multipoint (single driver on bus) networks, the main cable should be terminated in its characteristic impedance (typically 120 Ω for “CAT5”, and 220Ω for “Type A”) at the end farthest from the driver. In multi-receiver applications, stubs connecting receivers to the main cable should be kept as short as possible. Multipoint (multi-driver) systems require that the main cable be terminated in its characteristic impedance at both ends. Stubs connecting a transceiver to the main cable should be kept as short as possible. Built-In Driver Overload Protection As stated previously, the RS-485 spec requires that drivers survive worst case bus contentions undamaged. These transmitters meet this requirement via driver output short circuit current limits, and on-chip thermal shutdown circuitry. The driver output stages incorporate short circuit current limiting circuitry which ensures that the output current never exceeds the RS-485 spec, even at the common mode voltage range extremes. In the event of a major short circuit condition, the devices also include a thermal shutdown feature that disables the drivers whenever the die temperature becomes excessive. This eliminates the power dissipation, allowing the die to cool. The drivers automatically reenable after the die temperature drops about 15 degrees. If the contention persists, the thermal shutdown/reenable cycle repeats until the fault is cleared. Receivers stay operational during thermal shutdown. ISL4486, ISL81486 |
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