Hello. I'm hoping to some insight to my problem. I'm working with an Arduino DUE card and have it connected to a Max485 chip to communicate via Modbus to a slave device.
byte message[] = {0x01,0x04,0x75,0x32,0x00,0x01,0x8a,0x09}; // Query input register 0x7532
byte inBound[sizeof(message)];
void setup() {
Serial.begin(38400);
Serial2.begin(9600);
}
void loop() {
digitalWrite(2,HIGH); //Tx Enabled
Serial2.write(message,sizeof(message)); //Send message
Serial2.flush(); //Be sure evrything is out of the buffer
digitalWrite(2,LOW); //Tx Disabled, Rx Enabled
delay(2); //Give modbus slave some time to respond
if (Serial2.available()>=0){
for(int j =0; j <= sizeof(inBound); j++){
inBound[j] = Serial2.read(); //Read data
Serial.print("j="); Serial.print(j); Serial.print(" ->"); Serial.print(inBound[j],HEX);
Serial.println();}
}
delay(2000); //Wait 2 seconds to query again
}
On the max485:
I've got 2&3 tied together controlled via PIN2 on the Arduino
Tx and Rx going to Serial 2 on the DUE
5v and GND
7 and 6 Going to the modbus slave A and B.
When looking at the signals on my oscilloscope, the output on channels A & B is correct (both request and response).However, I get nothing on the Rx line. What I do get appears to look like the request that is inverted but no response portion - this tells me something is up, it shouldn't be inverted. The output from the print statements are FF's. What am I missing? The scope trace shows the modbus device working, getting a request and sending back the correct response. I've tried various delays in getting the max485 to play nice no luck. To sum it up, sending the request to the modbus slave works fine, the slave gets the requests and sends back the correct result. The Rx part of this little project is getting all messed up. I don't think I'm using the chip wrong. Seems others have it wired up the same way I do and getting results. I've tried terminating the bus with 120ohm resisters, tried both pull down and pull up resisters on the A and B lines. I've also tired decoupling capacitors across power and ground, even tried a new chip (thinking i might have destroyed the one i was using - no luck on any approach). Any help would be appreciated.
