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CRCW0603010RJNEAHP Datasheet(PDF) 11 Page - Texas Instruments |
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CRCW0603010RJNEAHP Datasheet(HTML) 11 Page - Texas Instruments |
11 / 22 page Vcc GND B A D DE R RE a) Independent driver and receiver enable signals b) Combined enable signals for use as directional control pin c) Receiver always on R D Vcc GND B A D DE R RE R D Vcc GND B A D DE R RE R D SN888C www.ti.com SLLSEI4 – SEPTEMBER 2013 APPLICATION INFORMATION Device Configuration The SN888C device is a half-duplex RS-485 transceiver operating from a single 5-V ±10% supply. The driver and receiver enable pins that allow for the configuration of different operating modes. Figure 13. Transceiver Configurations Using independent enable lines provides the most flexible control as the lines allow for the driver and the receiver to be turned on and off individually. While this configuration requires two control lines, it allows for selective listening to the bus traffic, whether the driver is transmitting data or not. Only this configuration allows the SN888C device to enter low-power standby mode because it allows both the driver and receiver to be disabled simultaneously. Combining the enable signals simplifies the interface to the controller by forming a single direction-control signal. Thus, when the direction-control line is high, the transceiver is configured as a driver, while when low, the device operates as a receiver. Tying the receiver enable to ground and controlling only the driver-enable input also uses only one control line. In this configuration, a node not only receives the data on the bus sent by other nodes, but also receives the data sent on the bus, enabling the node to verify the correct data has been transmitted. Bus Design An RS-485 bus consists of multiple transceivers connected in parallel to a bus cable. To eliminate line reflections, each cable end is terminated with a termination resistor, RT, whose value matches the characteristic impedance, Z0, of the cable. This method, known as parallel termination, allows for relatively high data rates over long cable length. Common cables used are unshielded twisted pair (UTP), such as low-cost CAT-5 cable with Z0 = 100 Ω, and RS-485 cable with Z0 = 120 Ω. Typical cable sizes are AWG 22 and AWG 24. The maximum bus length is typically given as 4000 ft or 1200 m, and represents the length of an AWG 24 cable whose cable resistance approaches the value of the termination resistance, thus reducing the bus signal by half or 6 dB. Actual maximum usable cable length depends on the signaling rate, cable characteristics, and environmental conditions. Table 1. VID With a Failsafe Network and Bus Termination VCC RL Differential RFS Pull-Up RFS Pull-Down VID Termination 560 Ω 560 Ω 230 mV 1 K Ω 1 K Ω 131 mV 5 V 54 Ω 4.7 K Ω 4.7 K Ω 29 mV 10 K Ω 10 K Ω 13 mV An external failsafe-resistor network must be used to ensure failsafe operation during an idle bus state. When the bus is not actively driven, the differential receiver inputs could float allowing the receiver output to assume a random output. A proper failsafe network forces the receiver inputs to exceed the VIT threshold, thus forcing the SN888C receiver output into the failsafe (high) state. Table 1 shows the differential input voltage (VID) for various failsafe networks with a 54- Ω differential bus termination. Copyright © 2013, Texas Instruments Incorporated Submit Documentation Feedback 11 Product Folder Links: SN888C |
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