Node.js Serialport synchronous write-read

Question

Does anyone have any example code to use the node.js serialport module in a blocking/synchronous way?

What I am trying to do is send a command to a micro-controller and wait for the response before sending the next command.

I have the sending/receiving working but the data just comes in with the listener

serial.on( "data", function( data) {
        console.log(data);
    }); 

Is there a way to wait for the returned data after doing a

serial.write("Send Command");

Should I be setting a global flag or something?

I am still new to the async programming style of node.js

Thanks

Answer 1

There is no such option and it's actually not necessary. One way of doing this is to maintain a queue of commands. Something like this:

function Device (serial) {
    this._serial = serial;
    this._queue = queue;
    this._busy = false;
    this._current = null;
    var device = this;
    serial.on('data', function (data) {
        if (!device._current) return;
        device._current[1](null, data);
        device.processQueue();
    });
}

Device.prototype.send = function (data, callback) {
    this._queue.push([data, callback]);
    if (this._busy) return;
    this._busy = true;
    this.processQueue();
};

Device.prototype.processQueue = function () {
    var next = this._queue.shift();

    if (!next) {
        this._busy = false;
        return;
    }

    this._current = next;
    this._serial.write(next[0]);
};

Answer 2

This can now be done using the serialport-synchronous library in npm.

Consider the following serial port flow:

1. << READY
2. >> getTemp
3. << Received: getTemp
4. << Temp: 23.11

We can get the temperature value with the following code:

import { SerialPortController } from 'serialport-synchronous'

const TEMP_REGEX = /^Temp: (\d+\.\d+)$/
const ERROR_REGEX = /^ERROR$/
const READY_REGEX = /^READY$/

const controller = new SerialPortController({
  path: '/dev/ttyUSB0',
  baudRate: 19200,
  handlers: [{
    pattern: READY_REGEX,
    callback: main // call the main() function when READY_REGEX has matched.
  }]
})

// push the log events from the library to the console
controller.on('log', (log) => console[log.level.toLowerCase()](`${log.datetime.toISOString()} [${log.level.toUpperCase()}] ${log.message}`))

// open the serial port connection
controller.open()

async function main () {
  try {
    // send the getTemp text to the serialport
    const result = await controller.execute({
      description: 'Querying current temperature', // optional, used for logging purposes
      text: 'getTemp',                             // mandatory, the text to send
      successRegex: TEMP_REGEX,                    // mandatory, the regex required to resolve the promise
      bufferRegex: TEMP_REGEX,                     // optional, the regex match required to buffer the response
      errorRegex: ERROR_REGEX,                     // optional, the regex match required to reject the promise
      timeoutMs: 1000                              // mandatory, the maximum time to wait before rejecting the promise
    })
    
    // parse the response to extract the temp value
    const temp = result.match(TEMP_REGEX)[1]
    console.log(`\nThe temperature reading was ${temp}c`)
  } catch (error) {
    console.error('Error occured querying temperature')
    console.error(error)
  }
}

Output looks something like this:

2022-07-20T01:33:56.855Z [INFO] Connection to serial port '/dev/ttyUSB0' has been opened
2022-07-20T01:33:58.391Z [INFO] << READY
2022-07-20T01:33:58.392Z [INFO] Inbound message matched unsolicited handler pattern: /^READY$/. Calling custom handler function
2022-07-20T01:33:58.396Z [INFO] Querying current temperature
2022-07-20T01:33:58.397Z [INFO] >> [TEXT] getTemp
2022-07-20T01:33:58.415Z [INFO] << Received: getTemp
2022-07-20T01:33:58.423Z [INFO] << Temp: 23.11
2022-07-20T01:33:58.423Z [DEBUG] Received expected response, calling resolve handler

The temperature reading was 23.11c