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I am using MAX485 chip for serial communication. But a weird behavior I found which I can not explain. Can any one help me in understanding why LED is ON in Receiver pin (RO) when I have not given any serial data from pin A and B? As per my understating LED should be always OFF as there is no data connection.

schematic

simulate this circuit – Schematic created using CircuitLab

In above circuit, I have just switched on my MAX485 board by connecting source on Vcc and GND no data pins A and B are connected. And RE is connected with GND to turn board in receiver mode. So ideally RO should always be off due to no data.

Thanks

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2 Answers 2

Reset to default
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The part works exactly as described in the datasheet.

When A and B pins are open, and the receiver is enabled, the receiver output is high.

However, since the DI and DE pins are floating, you actually don't know if the transmitter is enabled or not, and if it is enabled, you don't know what data it is sending, so there can be multiple reasons why the LED is turned on.

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  • \$\begingroup\$ sorry for my mistake. receiver mode is not enabled. if you look closely RE is connected to GND (LOW) which means receiver mode is disabled as per datasheet. then why LED is blinking? \$\endgroup\$
    – Ravi
    Oct 31, 2020 at 15:51
  • \$\begingroup\$ If you look closely, RE low means it is enabled. That's why it is blinking. \$\endgroup\$
    – Justme
    Oct 31, 2020 at 16:20
  • \$\begingroup\$ thanks i got it. BTW what does this High impedance here ? 4th case in the truth table High-Z ? \$\endgroup\$
    – Ravi
    Oct 31, 2020 at 17:05
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The general message is: set all inputs to valid logic levels

A and B inputs are not connected to anything so any state can be assumed on the A and B lines and that will produce either high or low on the RO output (given that you have nRE set low i.e. you are enabling that output). If high is produced on RO then it will turn on the LED. Digging deeper this explains what you see more clearly: -

enter image description here

Try setting A and B to known valid levels.

The RS485 chip knows nothing about data or protocols - it just acts like a buffer.

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  • \$\begingroup\$ In that setup, the chip would also not know if it is transmitting or not, as DE and DI are floating, and the chip does not provide a default state for them via internal biasing. \$\endgroup\$
    – Justme
    Oct 30, 2020 at 12:56
  • \$\begingroup\$ <= -0.2V does it mean i have to connect A-B to GND ? which means in your second case all RE,DE,A,B should be connected to GND to get 0 on RO ? what about DI ? can we keep it floating ? \$\endgroup\$
    – Ravi
    Oct 30, 2020 at 17:46
  • \$\begingroup\$ No, you connect them to a compatible interface like that from another RS485 chip. \$\endgroup\$
    – Andy aka
    Oct 30, 2020 at 17:49
  • \$\begingroup\$ i am a newbie in electronics finding hard time to understand this table ...any beginner's pointer will be really helpful. \$\endgroup\$
    – Ravi
    Oct 30, 2020 at 17:52
  • \$\begingroup\$ This is a question and answer site and not a forum. If you don't understand the table then you need to say specifically what you don't understand about it. \$\endgroup\$
    – Andy aka
    Oct 30, 2020 at 17:54

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