Arduino Modbus RTU Response over serial? - arduino

Question

I am developing a project where I have to read the holding registers data. I check everything using this http://www.freemodbus.com/ and it is working and get a proper response. While trying with the developed program I am not getting the proper response.
Software Response: 0x01 0x03 0x04 0x1a 0xa0 0x42 0x48 0xcd 0x9f
Arduino response: 0x01 0x04 0x83 0x43 0xff 0xff 0xff 0xff 0xff
note that in Arduino response there is no 0x03 after 0x01 I don't know why it is happening can anyone please help me with this.
please find attached Arduino code below.
static union
{
unsigned long a;
byte b[4];
float f;
}vr;
void readregister(unsigned int address)
{
byte rxbuf[]={0,0,0,0,0,0,0,0,0,0,0};
byte data[] = {0x01,0x03,0x00,0xab,0x00,0x02,0xb5,0xeb};
Serial3.flush();
for(int i=0;i<8;i++)
{
Serial3.write(data[i]);
}
delay(250);
while(Serial3.available()>0)
{
for(int v=0; v<=10;v++)
{
rxbuf[v]=Serial3.read();
Serial.println(rxbuf[v],HEX);
}
}
Serial3.flush();
vr.b[3]=rxbuf[3];
vr.b[2]=rxbuf[2];
vr.b[1]=rxbuf[5];
vr.b[0]=rxbuf[4];
}
void setup() {
// put your setup code here, to run once:
Serial.begin(9600);
Serial3.begin(9600,SERIAL_8E1);
}
void loop() {
// put your main code here, to run repeatedly:
readregister(99);
Serial.print("\n");
delay(3000);
}

Answer 1

Yes, I think you're getting an Error code 4 back from the device. See http://www.simplymodbus.ca/exceptions.htm
On this line, where you prep your request:
byte data[] = {0x01,0x03,0x00,0xab,0x00,0x02,0xb5,0xeb};
You're saying, device 1, function 3, 0x00 is space, then register address 0xAB. Not sure about the extra zero then, then length 2, and then CRC. So I vote that either the CRC is wrong, or the extra zero is wrong.
Also, note that modbus fails on ocassion for no reason, even when you do everything right, so retry after a few seconds and see what happens