const ModbusRTU = require("modbus-serial");
// create an empty modbus client
const client = new ModbusRTU();
// open connection to a serial port
client.connectRTUBuffered("\\\\.\\COM5", { baudRate: 9600 },true);
// set timeout, if slave did not reply back
client.setTimeout(500);

// list of meter's id
const metersIdList = [1,3,9];

const getMetersValue = async (meters) => {
    try{
        // get value of all meters
        for(let meter of meters) {
            // output value to console
            console.log(await getMeterValue(meter));
            // wait 100ms before get another device
            await sleep(100);
	}
    } catch(e){
        // if error, handle them here (it should not)
        console.log(e)
    } finally {
        // after get all data from salve repeate it again
        setImmediate(() => {
            getMetersValue(metersIdList);
        })
    }
}

const getMeterValue = async (id) => {
    try {
        // set ID of slave
        await client.setID(id);
        // read the 1 registers starting at address 0 (first register)
        let val =  await client.readInputRegisters(0, 1);
        // return the value
        return val.data[0];
    } catch(e){
        // if error return -1
        return -1
    }
}

const sleep = (ms) => new Promise(resolve => setTimeout(resolve, ms));


// start get value
getMetersValue(metersIdList);