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LTC488CN Datasheet(PDF) 9 Page - Linear Technology |
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LTC488CN Datasheet(HTML) 9 Page - Linear Technology |
9 / 12 page LTC488/LTC489 9 4889fb APPLICATIONS INFORMATION 4889 F10 110Ω RX 130Ω 130Ω 110Ω 5V RX RECEIVER 1.5k 120Ω 5V 1.5k RX 120Ω 5V C 100k RECEIVER RECEIVER 4889 F11 120Ω DRIVER Y Z Receiver Open-Circuit Fail-Safe Some data encoding schemes require that the output of the receiver maintains a known state (usually a logic 1) when the data is finished transmitting and all drivers on the line are forced in three-state. The receiver of the LTC488/LTC489 has a fail-safe feature which guarantees the output to be in a logic 1 state when the receiver inputs are left floating (open-circuit). When the input is terminated with 120Ω and the receiver output must be forced to a known state, the circuits of Figure 10 can be used. The termination resistors are used to generate a DC bias which forces the receiver output to a known state, in this case a logic 0. The first method consumes about 208mW and the second about 8mW. The lowest power solution is to use an AC termination with a pullup resistor. Simply swap the receiver inputs for data protocols ending in logic 1. Fault Protection All of LTC’s RS485 products are protected against ESD tran- sients up to 2kV using the human body model (100pF, 1.5k). However, some applications need more protection. The best protection method is to connect a bidirectional TransZorb® from each line side pin to ground (Figure 11). A TransZorb is a silicon transient voltage suppressor that has exceptional surge handling capabilities, fast response time, and low series resistance. They are available from General instruments, GSI, and come in a variety of break- down voltages and prices. Be sure to pick a breakdown voltage higher than the common mode voltage required for your application (typically 12V). Also, don’t forget to check how much the added parasitic capacitance will load down the bus. Figure 11. ESD Protection with TransZorbs Figure 10. Forcing “0” When All Drivers Are Off |
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