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ADM488A Datasheet(PDF) 11 Page - Analog Devices |
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ADM488A Datasheet(HTML) 11 Page - Analog Devices |
11 / 16 page ADM488A/ADM489A Rev. 0 | Page 11 of 16 THEORY OF OPERATION The ADM488A/ADM489A are ruggedized RS-485 transceivers that operate from a single 5 V supply. They contain protection against radiated and conducted interference and are ideally suited for operation in electrically harsh environments or where cables can be plugged/unplugged. They are also immune to high RF field strengths without special shielding precautions. The ADM488A/ADM489A are intended for balanced data transmission and comply with both EIA RS-485 and RS-422 standards. They contain a differential line driver and a differen- tial line receiver, and are suitable for full-duplex data transmission. The input impedance on the ADM488A/ADM489A is 12 kΩ, allowing up to 32 transceivers on the differential bus. The ADM488A/ADM489A operate from a single 5 V ± 10% power supply. A thermal shutdown circuit prevents excessive power dissipation caused by bus contention or by output shorting. This feature forces the driver output into a high impedance state if, during fault conditions, a significant temperature increase is detected in the internal driver circuitry. The receiver contains a fail-safe feature that results in a logic high output state if the inputs are unconnected (floating). The ADM488A/ADM489A can transmit at data rates up to 250 kbps. Figure 20 shows a typical application for the ADM488A/ADM489A, a full-duplex link where data transfers at rates of up to 250 kbps. A terminating resistor is shown at both ends of the link. This termination is not critical because the slew rate is controlled by the ADM488A/ADM489A and reflections are minimized. The communications network can be extended to include multipoint connections, as shown in Figure 22. As many as 32 transceivers can be connected to the bus. Table 6 and Table 7 show the truth tables for transmitting and receiving. Table 6. Transmitting Truth Table Inputs Outputs RE DE DI Z Y X1 1 1 0 1 X1 1 0 1 0 0 0 X1 High-Z High-Z 1 0 X1 Hgh-Z High-Z 1 X is don’t care. Table 7. Receiving Truth Table Inputs Output RE DE A to B RO 0 0 ≥ +0.2 V 1 0 0 ≤ −0.2 V 0 0 0 Inputs open circuit 1 1 0 X1 High-Z 1 X is don’t care. R D RO DI ADM488A A B Z Y R D RO DI ADM488A A B Z Y NOTES 1. MAXIMUM NUMBER OF NODES = 32. RT VCC VCC VCC GND GND RT VCC Figure 20. ADM488A/ADM489A Full-Duplex Data Link |
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