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ISL4485EIBZ Datasheet(PDF) 7 Page - Intersil Corporation |
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ISL4485EIBZ 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 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 The ISL4485E utilizes a differential input receiver for maximum noise immunity and common mode rejection. Input sensitivity is ±200mV, as required by the RS-422 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 ±7V outside the power supplies (i.e., +12V and -7V), making them ideal for long networks where induced voltages are a realistic concern. The receiver includes a “fail-safe if open” function that guarantees a high level receiver output if the receiver inputs are unconnected (floating). The output is three-statable via the active low RE input, and the receiver easily meets the 20Mbps data rate. Driver Features The RS-485/422 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). The ISL4485E driver features low propagation delay skew to maximize bit width, and to minimize EMI, and the outputs are three-statable via the active high DE input. Outputs of ISL4485E drivers are not slew rate limited, so faster output transition times allow data rates up to 20Mbps. Data Rate, Cables, and Terminations Twisted pair is the cable of choice for RS-485/422 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. RS-485/422 are intended for network lengths up to 4000', but the maximum transmission length decreases as the data rate increases. According to guidelines in the RS-422 specification, a 20Mbps network should be limited to less than 50' of 24 AWG twisted pair. Nevertheless, the ISL4485E's large differential voltage swing, fast transition times, and high drive-current output stages allow operation at 20Mbps in RS-485/422 networks as long as 450'. Figure 7 details ISL4485E operation at 20Mbps driving 300' of CAT 5 cable terminated in 120 Ω at the driver and the receiver (i.e., double terminated). The acceptance criteria for this test was the ability of the driver to deliver a 1.5V differential signal to the receiver at the end of the cable (i.e., |A-B| ≥ 1.5V). If a more liberal acceptance criteria is used, the distance can be further extended. For example, Figure 8 illustrates the performance in the same configuration, but with a cable length of 450', and an acceptance criteria of no more than 6dB attenuation across the cable (i.e., |A-B| = |Y-Z|/2). Driver differential output voltage decreases with increasing differential load capacitance, so maintaining a 1.5V differential output requires a data rate reduction, as shown in Figure 9. To minimize reflections, proper termination is imperative when using this 20Mbps device. 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 (preferably less than 12 inches). Multipoint (multi-driver) systems require that the main cable be terminated in its characteristic impedance at both ends. Again, 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 ISL4485E device meets 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, this device also includes 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 ISL4485E |
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