modbus register conversion

Question

I'm using the easy modbus library and reading the ambient temperature with a pt-100 and a plc fl055 0808n v2, when my temperature reaches 30 degrees celsius, it begins to show random negatives numbers in my modbus tcp application but in the plc ide it just do normally.

This is my code:

if (PLC != null && PLC.Connected)
{
    try
    {
        temp = PLC.ReadHoldingRegisters(0, 2);
        double aux = Convert.ToDouble(temp[0]);
        double temperaturaLinearizada = get_lineariza(aux, 65535, 60);

        Console.WriteLine($"Valor lido: {aux}, Temperatura Linearizada: {temperaturaLinearizada}");

        lbl_temperatura.Text = $"Temperatura: {temperaturaLinearizada.ToString("0.00")}, PT100: {aux.ToString("0.00")}";

        temp = PLC.ReadHoldingRegisters(4, 2);
        lbl_temperatura.Text += $", L: {temp[0]}";
    }
    catch (Exception ex)
    {
        Console.WriteLine($"Erro: {ex.Message}");
    }
}

I don't know what to do to fix this

Answer 1

You need to check the data type returned by ReadHoldingRegisters. It looks like get_lineariza expect temp[0] to be ushort (through aux), but temp is short[], in fact. From this "get_lineariza(aux, 65535, 60)" I can conclude what input range should be ushort where 32767 is 30 deg. But if ReadHoldingRegisters returns short[] instead of ushort[], ushort 32768 means short -32768, thus you get negatives when temp is > 30.

If ReadHoldingRegisters is coded by you, you need to modify it to return unshort[]. Otherwise apply

if (aux < 0) aux += 65536;

Answer 2

If in your PLC data type of temperature is the float, then it takes two registers to store data. Thats why you reading two consecutive registers. Looks like you are using Easymodbus library. This library has a method to convert registers to float. You convertion code should be like that:

public static ModbusClient PLC = new ModbusClient();
temp = ModbusClient.ConvertRegistersToFloat(PLC.ReadHoldingRegisters(0, 2));