Q: We used a MAX485EESA+, when there was no terminating resistor Rt between A,B it is possible to conduct normal RS485 communication. However, when Rt was connected some of the chips could not communicate normally, what is the possible reason?
A: The fail-safe feature of MAX485 is limited to the A/B open circuit condition (shown below). When A,B has a 120-ohm resistor connected in between, if the bus has no drive signal, A/B is equivalent to being short circuited; the signal ranges from-0.2V to +0.2V and the receiver output can be a high voltage level or low voltage level at this time.
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To ensure output is a high voltage level, it is possible to connect a 1k pull up, pull down resistor
- between A and Vcc, B and GND; or:
- choose components with true fail-safe feature, such as MAX13458EESA. When A/B is under open circuit or short circuit circumstances, it will be able to ensure the receiver outputs a high voltage level:
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To ensure output is a high voltage level, it is possible to connect a 1k pull up, pull down resistor
- between A and Vcc, B and GND; or:
- choose components with true fail-safe feature, such as MAX13458EESA. When A/B is under open circuit or short circuit circumstances, it will be able to ensure the receiver outputs a high voltage level: