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I cannot understand the principle that the transceiver uses. Why do we need to connect it to RS-485, and how is the data transmitted? I mean how does it transmit the data and why? It is like I cannot understand the concept.

These are the datasheets, but I am more interested in what they do rather than the technical characteristics.

https://datasheets.maximintegrated.com/en/ds/MAX1487-MAX491.pdf

https://www.st.com/resource/en/datasheet/st485b.pdf

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    \$\begingroup\$ it's not clear what you're actually mean with "why"; do you mean, "why does someone design a RS 485 transceiver", or do you mean "why does the output behave like it does considering the way the transceiver is built", or "why build RS485 transceiver that way (and not some other)". \$\endgroup\$ Oct 20 '20 at 19:38
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    \$\begingroup\$ Is the confusion about the level shifting, the 485 protocol, or serial communications in general? \$\endgroup\$ Oct 20 '20 at 19:39
  • \$\begingroup\$ @CristobolPolychronopolis,about serial communications in general,does the transceiver get the data and puts it out?It is like I had made the same question about amplificators and I did not understand that what they do is that they valure the voltage difference and then give a voltage at the Output \$\endgroup\$
    – velectron
    Oct 20 '20 at 19:41
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It's almost never a good practice to connect the two MCUs' RX-TX pairs directly for relatively long-range (e.g. >2m) asynchronous serial communication. The possible problems are mostly solved by RS-485.

RS-485 has a differential line (consisting of A and B lines) for relatively long-range asynchronous serial communication. But your MCU does not have the capability of direct communicating through a differential line.

What that xceiver does is to make the bidirectional conversion between the differential line and your MCU's single-ended signaling:

  • The xceiver takes the data from the differential line and converts it to a single-ended signal so that your MCU can read it directly (through RXD pin),
  • Tthe xceiver takes the single-ended data from your MCU (through TXD pin) and puts it on the differential line so that any other device on the line can access the data.
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That part in itself converts single-ended transmit and receive signals to a differential, higher current bidirectional signal. This is part of the RS485 spec (and others) to improve the effective distance the signal can be sent, and reduce the number of pairs.

What this part does not implement is the serial conversion itself. This requires some form of shift register, so that (e.g.) an 8-bit value can be sent over a single line or pair, by presenting one bit on that line or pair at a time in a predetermined order. Also included are synchronization mechanisms so that the receiver always knows which bit is being sent at any particular time. The receiver shift register--again, on a different part, probably a uC--then collects them and presents them as a complete byte to the processor.

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