The PT100Ω is a temperature sensor widely used in the industrial field because it can measure a wide range of temperatures from -100 to +500 degrees Celsius. The ML-THRT1 module converts the resistance value of the PT100Ω sensor to a digital temperature value. Simply read the digital value via Modbus RTU over RS-485.

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A termination resistor may be necessary if a ML-THRT1 is the last node in an RS-485 network.

The ML-THRT1 returns the digital temperature multiplied by 10 in 2 bytes. For example, a value of 20.5°C will be returned as 205 (0x00DC). Negative values have the most significant bit set to 1. For example, a value of -20.5°C will be returned as 32988 (0X80DC). For negative values, subtract 32768 to obtain the absolute value.

• If the temperature read is greater than 500°C, the value 5555 is returned.
• If the temperature read is lower than -100°C, the value -1111 is returned.
• If the sensor is not connected, the value 8888 is returned.

The Modbus slave address can be set using the rotary dial (0 ~ 10) on the side the ML-THRT1.

To read the digital temperature value, send a Modbus query with Function code 3 and data length 1 to start address 400 (0x0190).

The following example illustrates how to use the ML-THRT1 with the MOACON.

#include "moacon500.h"
void cmain(void)
{
    openCom(0, 57600, com8N1);
    comPower(1);    
    int Temp;
    char res;    
    while(1)
    {
        res = RTU_readRegs(0, 1, &Temp, 400, 1);        
        if(res == -1)
        {
            printf("Temp = %d", Temp);
        }
        else
        {
            printf("Temp = Error");
        }        
        delay(1000);
    }
}

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