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ADM1485ARZ Scheda tecnica(PDF) 8 Page - Analog Devices |
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ADM1485ARZ Scheda tecnica(HTML) 8 Page - Analog Devices |
8 / 11 page REV. –8– ADM1485 As with any transmission line, it is important that reflections are minimized. This can be achieved by terminating the extreme ends of the line using resistors equal to the characteristic 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. RT RT D R DD R R D R Figure 5. Typical RS-485 Network Thermal Shutdown The ADM1485 contains thermal shutdown circuitry that protects the part from excessive power dissipation during fault conditions. Shorting the driver outputs to a low impedance source can result in high driver currents. The thermal sensing circuitry detects the increase in die temperature and disables the driver outputs. The thermal sensing circuitry is designed to disable the driver outputs when a die temperature of 150 °C is reached. As the device cools, the drivers are re-enabled at 140 °C. Propagation Delay The ADM1485 features very low propagation delay, ensuring maximum baud rate operation. The driver is well balanced, ensuring distortion free transmission. Another important specification is a measure of the skew between the complementary outputs. Excessive skew impairs the noise immunity of the system and increases the amount of electro- magnetic interference (EMI). Receiver Open-Circuit Fail-Safe The receiver input includes a fail-safe feature that guarantees a logic high on the receiver when the inputs are open circuit or floating. APPLICATION INFORMATION 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) that specify the electrical charac- teristics 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. In order to cater to true multipoint communications, the RS-485 standard was defined. This standard meets or exceeds all the requirements of the RS-422 but also allows for up to 32 drivers and 32 receivers to be connected to a single bus. An extended com- mon-mode range of –7 V to +12 V is defined. The most significant difference between the RS-422 and the RS-485 is the fact that the drivers may be disabled, thereby allowing more than one (32 in fact) 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 III. 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 k Ω min 12 k Ω min Receiver Input Sensitivity ±200 mV ±200 mV Receiver Input Voltage Range –7 V to +7 V –7 V to +12 V No. of Drivers/Receivers per Line 1/10 32/32 Cable and Data Rate The transmission line of choice for RS-485 communications is a twisted pair. Twisted pair cable tends to cancel common-mode noise and also causes cancellation of the magnetic fields generated by the current flowing through each wire, thereby reducing the effective inductance of the pair. The ADM1485 is designed for bidirectional data communications on multipoint transmission lines. A typical application showing a multipoint transmission network is illustrated in Figure 5. 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 may be enabled simultaneously. |
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