How to connect an A/D-converter via RS485?

[chrueschiii]

Greetings,

is there a possibility to connect an analog-digital converter to the Due's serial ports via rs485?
I tried already a RS485-TTL-converter. but it's not working.

[ard_newbie]

Why would you need an RS485 connection to an A/D converter ?

BTW, AFAIK RS485 is simply a physical layer mostly used when long distance wires are needed to avoid EMI thanks to a differential bus, and you have to code your own protocol. 

You can have this type of differential bus PLUS a protocol with a CAN bus. A CAN transceiver (3.3V CAN transceiver) is needed (unless your wires are shorter than 3 to 4 meters long) and a CAN library (see collin80 CAN library for the DUE on github).

[chrueschiii]

The converter is the LDU 68.1 by Soemer.
Technical data of the converter.
The wires are very short (<0.5m)

[ard_newbie]

You will find page 801 of Sam3x datasheet how you can set USART peripheral in RS485 Mode. Connect RX/TX/RTS to an RS485 transceiver (3.3V). Page 773 provides RTS pins locations (plus see Graynomad pinout diagram).

A basic sketch that I tried (without transceivers, only RX1 to TX2 and RX2 to TX1):

Code: [Select]

/********************************************************************************/
/*                          USART RS485 Mode                                    */
/*                    Hook RX1 to TX2  and RX2 to TX1                           */
/********************************************************************************/

char c = 0;
void setup() {

  Serial.begin(250000);

  Serial1.begin(115200); // Serial1 is for USART0
  Serial2.begin(115200); // Serial2 is for USART1

  USART0->US_MR |= US_MR_USART_MODE_RS485;

  USART1->US_MR |= US_MR_USART_MODE_RS485;

  Serial2.print("Hello");
}


void loop() {
  String s;
  s.reserve(10);
  s = "";

  while (Serial1.available() > 0) {
    c = Serial1.read();
    s += c;

  }
  if (s.length() > 0) {
    Serial.println(s);
    Serial2.print(s);
  }
  delay(1000);
}




[chrueschiii]

Thanks for the sketch. But why do i need Serial2? For which purpose?

[ard_newbie]

You don't need it, that's only an example to show how you select RS485 feature for USART and see the result with Serial by connecting USART0 to USART1 on a single board. 

[chrueschiii]

Thanks. But the sketch is only receiving. Do I send commands just with Serial1.write?
A command consists out of one or two capital letters followed by a carriage return.
For example I want to accomplish the following communication:
Sending: OP1\r (Opening channel to converter one)
Receiving: OK 
Sending: GN\r (Get Data)
Receiving: N+12345

Another Question:
How do I connect the converter to the DUE? RX+,Rx-,Tx+,Tx- to just one RX Pin and one TX Pin