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ISL3150EIBZ Arkusz danych(PDF) 11 Page - Intersil Corporation |
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ISL3150EIBZ Arkusz danych(HTML) 11 Page - Intersil Corporation |
11 / 20 page 11 FN6363.1 January 18, 2008 Application Information RS-485 and RS-422 are differential (balanced) data transmission standards used for 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 specification 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 (Rx) Features These devices utilize a differential input receiver for maximum noise immunity and common mode rejection. Input sensitivity is better than ±200mV, as required by the RS-422 and RS-485 specifications. Rx outputs feature high drive levels (typically 28mA @ VOL = 1V to ease the design of optically coupled isolated interfaces). Receiver input resistance of 96k Ω surpasses the RS-422 specification of 4k Ω, and is eight times the RS-485 “Unit Load (UL)” requirement of 12k Ω minimum. Thus, these products are known as “one-eighth UL” transceivers, and there can be up to 256 of these devices on a network while still complying with the RS-485 loading specification. Rx 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. All the receivers include a “full fail-safe” function that guarantees a high level receiver output if the receiver inputs are unconnected (floating), shorted together, or connected to a terminated bus with all the transmitters disabled. Receivers easily meet the data rates supported by the corresponding driver, and all receiver outputs are three-statable via the active low RE input. Driver (Tx) Features The RS-485/RS-422 driver is a differential output device that delivers at least 2.4V across a 54 Ω load (RS-485), and at least 2.8V across a 100 Ω load (RS-422). The drivers feature low propagation delay skew to maximize bit width, and to minimize EMI, and all drivers are three-statable via the active high DE input. The 115kbps and 1Mbps driver outputs are slew rate limited to minimize EMI, and to minimize reflections in unterminated or improperly terminated networks. Outputs of the ISL3156Eand ISL3158E drivers are not limited, so faster output transition times allow data rates of at least 20Mbps High VOD Improves Noise Immunity and Flexibility The ISL315xE driver design delivers larger differential output voltages (VOD) than the RS-485 standard requires, or than most RS-485 transmitters can deliver. The minimum ±2.4V VOD guarantees at least ±900mV more noise immunity than networks built using standard 1.5V VOD transmitters. Another advantage of the large VOD is the ability to drive more than two bus terminations, which allows for utilizing the ISL315xE in “star” and other multi-terminated, “non-standard” network topologies. Figure 8, details the transmitter’s VOD vs IOUT characteristic, and includes load lines for six (20 Ω) and eight (15Ω) 120Ω terminations. The figure shows that the driver typically delivers 1.65/1.5V into 6/8 terminations, even at the worst case temperature of +85°C.The RS-485 standard requires a minimum 1.5V VOD into two terminations, but the ISL315xE delivers RS-485 voltage levels with 3x to 4x the number of terminations. Hot Plug Function When a piece of equipment powers up, there is a period of time where the processor or ASIC driving the RS-485 control lines (DE, RE) is unable to ensure that the RS-485 Tx and Rx outputs are kept disabled. If the equipment is connected to the bus, a driver activating prematurely during power-up may crash the bus. To avoid this scenario, the ISL315xE devices incorporate a “Hot Plug” function. Circuitry monitoring VCC ensures that, during power-up and power-down, the Tx and Rx outputs remain disabled, regardless of the state of DE and RE, if VCC is less than ~3.4V. This gives the processor/ASIC a chance to stabilize and drive the RS-485 control lines to the proper states. FIGURE 7. HOT PLUG PERFORMANCE (ISL315xE) vs ISL83088E WITHOUT HOT PLUG CIRCUITRY TIME (40µs/DIV) VCC 2.5 5.0 2.5 5.0 RL = 1kΩ RO 0 2.5 5.0 0 0 A /Y RL = 1kΩ 3.3V 3.5V ISL315xE ISL315xE RE = GND DE, DI = VCC ISL3150E, ISL3152E, ISL3153E, ISL3155E, ISL3156E, ISL3158E |
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