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LTC488I Datasheet(PDF) 9 Page - Linear Technology |
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LTC488I Datasheet(HTML) 9 Page - Linear Technology |
9 / 12 page 9 LTC488/LTC489 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 out- put to be in a logic 1 state when the receiver inputs are left floating (open-circuit). When the input is terminated with 120 Ω and the receiver output must be forced to a known state, the circuits of Figure 10 can be used. S APPLICATI I FOR ATIO 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 transients 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. TransZorb is a registered trademark of General Instruments, GSI 488/9 F11 120 Ω DRIVER Y Z Figure 11. ESD Protection with TransZorbs® 488/9 F10 110 Ω RX 130 Ω 130 Ω 110 Ω 5V RX RECEIVER 1.5k 120 Ω 5V 1.5k RX 120 Ω 5V C 100k RECEIVER RECEIVER Figure 10. Forcing “0” When All Drivers Are Off |
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