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ADM485 Datasheet(PDF) 12 Page - Analog Devices |
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ADM485 Datasheet(HTML) 12 Page - Analog Devices |
12 / 16 page ADM485 Rev. F | Page 12 of 16 APPLICATIONS 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 EIA that 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 cater to true multipoint communications, the RS-485 standard was defined. This standard meets or exceeds all the requirements of RS-422 but also allows for 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 the RS-422 standard and the RS-485 standard is the fact that the drivers can 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 7. 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 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 causes cancellation of the magnetic fields generated by the current flowing through each wire, thereby reducing the effective inductance of the pair. The ADM485 is designed for bidirectional data communications on multipoint transmission lines. A typical application showing a multipoint transmission network is illustrated in Figure 30. 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 simultaneously. RT RT D R DD R R D R Figure 30. Typical RS-485 Network As with any transmission line, it is important that reflections be 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. THERMAL SHUTDOWN The ADM485 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 ADM485 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 electromagnetic 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. |
Similar Part No. - ADM485_15 |
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