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ADM483AR-REEL Datasheet(PDF) 11 Page - Analog Devices |
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ADM483AR-REEL Datasheet(HTML) 11 Page - Analog Devices |
11 / 16 page ADM483 Rev. 0 | Page 11 of 16 APPLICATIONS DIFFERENTIAL DATA TRANSMISSION Differential data transmission is used to reliably transmit data at high rates over long distances and through noisy environments. Differential transmission nullifies the effects of ground shifts and noise signals that appear as common- mode voltages on the line. There are two main standards approved by the Electronics Industries Association (EIA), which specify the electrical characteristics of transceivers used in differential data transmission. The RS-422 standard specifies data rates up to 10 Mbaud and line lengths up to 4000 ft. A single driver can drive a transmission line with up to 10 receivers. To achieve true multipoint communications, the RS-485 standard was defined. This standard meets or exceeds all the requirements of RS-422, but also allows up to 32 drivers and 32 receivers to be connected to a single bus. An extended common- mode range of –7 V to +12 V is defined. The most significant difference between RS-422 and RS-485 is that the drivers may be disabled, allowing up to 32 to be connected to a single line. Only one driver should be enabled at a time, but the RS-485 standard contains additional specifications to guarantee device safety in the event of line contention. Table 5. Comparison of RS-422 and RS-485 Interface Standards Specification RS-422 RS-485 Transmission Type Differential Differential Maximum Cable Length 4000 ft. 4000 ft. Minimum Driver Output Voltage ±2 V ±1.5 V Driver Load Impedance 100 Ω 54 Ω Receiver Input Resistance 4 kΩ min 12 kΩ min Receiver Input Sensitivity ±200 mV ±200 mV Receiver Input Voltage Range –7 V to +7 V –7 V to +12 V Drivers/Receivers per Line 1/10 32/32 CABLE AND DATA RATE The preferred transmission line for RS-485 communications is a twisted pair. Twisted pair cable tends to cancel common- mode noise and the magnetic fields generated by the current flowing through each wire, thereby reducing the effective inductance of the pair. The ADM483 is designed for bidirectional data communica- tions on multipoint transmission lines. A typical application showing a multipoint transmission network is shown in Figure 24 . An RS-485 transmission line can have as many as 32 transceivers on the bus. Only one driver can transmit at a particular time, but multiple receivers can be enabled simul- taneously. As with any transmission line, it is important to minimize reflections. This can be done by terminating the extreme ends of the line by using resistors equal to the char- acteristic impedance of the line. Stub lengths of the main line should also be kept as short as possible. A properly terminated transmission line appears purely resistive to the driver. RO RE DE DI D R A B MAXIMUM NUMBER OF TRANSCEIVERS ON BUS: 32 ADM483 RO RE DE DI D R A B ADM483 R ADM483 D RO RE DE DI AB R ADM483 D RO RE DE DI AB Figure 24. Typical Half-Duplex RS-485 Network Topology |
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