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ISL83488IB Datasheet(PDF) 10 Page - Intersil Corporation |
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ISL83488IB Datasheet(HTML) 10 Page - Intersil Corporation |
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10 / 17 page  10 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 combination 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’, 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 input impedance surpasses the RS-422 spec of 4k Ω, and meets the RS-485 “Unit Load” requirement of 12k Ω minimum. Receiver inputs function with common mode voltages as great as +9V/-7V outside the power supplies (i.e., +12V and -7V), making them ideal for long networks where induced voltages are a realistic concern. 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 rates supported by the corresponding driver. ISL83483, ISL83485, ISL83491 receiver outputs are tri- statable via the active low RE input. Driver Features The RS-485, RS-422 driver is a differential output device that delivers at least 1.5V across a 54 Ω load (RS-485), and at least 2V across a 100 Ω load (RS-422) even with VCC = 3V. The drivers feature low propagation delay skew to maximize bit width, and to minimize EMI. Drivers of the ISL83483, ISL83485, ISL83491 are tri-statable via the active high DE input. ISL83483/88 driver outputs are slew rate limited to minimize EMI, and to minimize reflections in unterminated or improperly terminated networks. Data rate on these slew rate limited versions is a maximum of 250kbps. Outputs of ISL83485, ISL83490, ISL83491 drivers are not limited, so faster output transition times allow data rates of at least 10Mbps. Data Rate, Cables, and Terminations RS-485, RS-422 are intended for network lengths up to 4000’, but the maximum system data rate decreases as the transmission length increases. Devices operating at 10Mbps are limited to lengths of a few hundred feet, while the 250kbps versions can operate at full data rates with lengths in excess of 1000’. Twisted pair is the cable of choice for RS-485, RS-422 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. Proper termination is imperative, when using the 10Mbps devices, to minimize reflections. Short networks using the 250kbps versions need not be terminated, but, terminations are recommended unless power dissipation is an overriding concern. 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) 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. The ISL834XX devices 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. Additionally, these devices utilize a foldback circuit which reduces the short circuit current, and thus the power dissipation, whenever the contending voltage exceeds either supply. In the event of a major short circuit condition, ISL834XX 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. ISL83483, ISL83485, ISL83488, ISL83490, ISL83491 |
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