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LTC490 Fiches technique(PDF) 6 Page - Linear Technology |
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LTC490 Fiches technique(HTML) 6 Page - Linear Technology |
6 / 8 page LTC490 6 Figure 8. RS485 Cable Length Specification. Applies for 24 Gauge, Polyethylene Dielectric Twisted Pair. S APPLICATI I FOR ATIO one driver can be connected on the bus, the cable can be terminated only at the receiving end. The optional shields around the twisted pair help reduce unwanted noise, and are connected to GND at one end. The LTC490 can also be used as a line repeater as shown in Figure 6. If the cable length is longer than 4000 feet, the LTC490 is inserted in the middle of the cable with the receiver output connected back to the driver input. LTC490 • TA07 120 Ω LTC490 DRIVER DX RX 2 3 5 6 7 8 RECEIVER DATA IN DATA OUT Figure 6. Line Repeater Thermal Shutdown The LTC490 has a thermal shutdown feature which pro- tects the part from excessive power dissipation. If the outputs of the driver are accidently shorted to a power supply or low impedance, source, up to 250mA can flow through the part. The thermal shutdown circuit disables the driver outputs when the internal temperature reaches 150 °C and turns them back on when the temperature cools to 130 °C. If the outputs of two or more LTC490 drivers are shorted directly, the driver outputs can not supply enough current to activate the thermal shutdown. Thus, the thermal shutdown circuit will not prevent con- tention faults when two drivers are active on the bus at the same time. 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 120 Ω 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 cables 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 7). FREQUENCY (MHz) 0.1 0.1 1.0 10 1.0 10 100 LTC490 • TA08 Figure 7. Attenuation vs Frequency for Belden 9841 When using low loss cables, Figure 8 can be used as a guideline for choosing the maximum line length for a given data rate. With lower quality PVC cables, the dielectric loss factor can be 1000 times worse. PVC twisted pairs have terrible losses at high data rates (>100kbs), and greatly reduce the maximum cable length. At low data rates however, they are acceptable and much more economical. DATA RATE (bps) 10k 10 100 1k 10k 100k 1M 10M LTC490 • TA09 2.5M |
Numéro de pièce similaire - LTC490 |
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Description similaire - LTC490 |
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