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LTC489CN-PBF Datasheet(PDF) 7 Page - Linear Technology |
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LTC489CN-PBF Datasheet(HTML) 7 Page - Linear Technology |
7 / 12 page LTC488/LTC489 7 4889fb Typical Application A typical connection of the LTC488/LTC489 is shown in Figure 5. Two twisted-pair wires connect up to 32 driver/ receiver pairs for half-duplex data transmission. There are no restrictions on where the chips are connected to the wires, and it isn’t necessary to have the chips connected at the ends. However, the wires must be terminated only at the ends with a resistor equal to their characteristic impedance, typically 120Ω. The input impedance of a receiver is typically 20k to GND, or 0.5 unit RS485 load, so in practice 50 to 60 transceivers can be connected to the same wires. The optional shields around the twisted- pair help reduce unwanted noise, and are connected to GND at one end. Cables and Data Rate The transmission line of choice for RS485 applications is a twisted-pair. There are coaxial cables (twinaxial) made for this purpose that contain straight-pairs, but these are less flexible, more bulky, and more costly than twisted-pairs. Many cable manufacturers offer a broad range of 120Ω cables designed for RS485 applications. Losses in a transmission line are a complex combination of DC conductor loss, AC losses (skin effect), leakage, and AC losses in the dielectric. In good polyethylene cable such as the Belden 9841, the conductor losses and dielectric losses are of the same order of magnitude, leading to relatively low overall loss (Figure 6). When using low loss cables, Figure 7 can be used as a guideline for choosing the maximum line length for a given APPLICATIONS INFORMATION Figure 5. Typical Connection Figure 7. Cable Length vs Data Rate Figure 6. Attenuation vs Frequency for Belden 9841 120Ω 120Ω 3 RX 2 1 4889 F05 DX 1 3 SHIELD RX EN DX 1/4 LTC486 12 SHIELD 4 12 3 2 DX DX 1/4 LTC486 1 EN 1/4 LTC488 OR 1/4 LTC489 RX 3 1 2 4 RX 1/4 LTC488 OR 1/4 LTC489 EN EN FREQUENCY (MHz) 0.1 0.1 1 10 1 10 100 4889 F06 DATA RATE (bps) 10k 10 100 1k 10k 100k 1M 10M 4889 F07 2.5M |
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