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Polling Serial Port Data Through Javascript |WORK|

A good approach for creating user interfaces to serial devices is to build an interactive web page. Using a web-based GUI makes the interface portable to different screens and devices. A micro computer such a Raspberry PI can then act as both a controller for the device and as webserver for providing the interface.

polling serial port data through javascript

The pyserial module will allow us to access the serial port in the python environment. The tornado module contains all code needed to serve webpages and create websockets. The multiprocessing modules has the features needed to write concurrent code in python.

Now let's move to the serial worker. This background task will be interacting with the serial device, by reading data from the serial port and posting it to the incoming queue, and by reading data from the outgoing queue and writing it to the serial port.

As mentioned in the previous sections the key for having i/o on the serial port without blocking the webserver is creating two separate threads running in parallel and communicating via shared queues.

Create a new serial port object. In the case of invalid arguments or invalid options it will throw an error. The port will open automatically by default, which is the equivalent of calling in a process.nextTick. You can disable this by setting the option autoOpen to false in the options.

Listening for the data event puts the port in flowing mode. Data is emitted as soon as it's received. Data is a Buffer object with any amount of data in it. The buffer is only guaranteed to have at least one byte. See the parsers section for more information on how to work with the data, and the Node.js stream documentation for more information on the data event.

Opens the connection of the given serial port. Emits an open event when the port is open, and if provided, calls the callback function after emitting. If the serial port's path does not exist, the callback is invoked with an error message. If no callback handler is provided, the SerialPort object will emit an error event.

updateOptions.baudRate: number The baud rate of the port to be opened. This should match one of the commonly available baud rates, such as 110, 300, 1200, 2400, 4800, 9600, 14400, 19200, 38400, 57600, or 115200. Custom rates are supported best effort per platform. The device connected to the serial port is not guaranteed to support the requested baud rate, even if the port itself supports that baud rate.

Writes data to the given serial port. Buffers written data if the port is not open and writes it after the port opens. The write operation is non-blocking. When it returns, data might still not have been written to the serial port. See drain().

A function to write data and wait until it has finished transmitting to the target serial port before calling the callback. This will wait until the port is open and writes are finished as determined by the operating system.

cgm-pebble is a C and javascript watch face developed usingPebbleSDK. The javascript code runs on a Smartphone maintainingbluetooth connectivity to the Pebble watch. The javascript coderetrieves information from cgm-remote-monitor and sends the lastreading to over bluetooth to the pebble watch. The C code runs on thewatch, receiving messages over bluetooth from a smartphone, andrendering the date, time, value, and trend as reported by a runninginstance of cgm-remote-monitor.

Hello,We have a counter that its output is Modbus RTU (serial) and is connected to a serial-to-Ethernet converter (Moxa nport 5110). A SCADA is reading the data of this counter through the Moxa. Now we should read data of this counter through this moxa in ignition. The question is that can we use Ignition Serial Module to read the data? is it possible that both system (the other SCADA and Ignition) reading simultaneously the data of this counter ?

So there is no way to read the data from this MOXA (either with Modebus RTU over TCP module or Serial Module) if another SCADA is polling the data? the only way is to replace the MOXA to a Modbus gateway (e.g. MGate)?

Just in case, if we replace the current MOXA Nport with MGate MOXA that converts Modbus RTU to Modbus TCP, in this case both Ignition and the other SCADA system can work and read the data of the counter simultaneously without any problem?

The Web Serial API provides a way for websites to read from and write to a serial device with JavaScript. Serial devices are connected either through a serial port on the user's system or through removable USB and Bluetooth devices that emulate a serial port.

The Web Serial API is asynchronous by design. This prevents the website UI from blocking when awaiting input, which is important because serial data can be received at any time, requiring a way to listen to it.

To open a serial port, first access a SerialPort object. For this, you can either prompt the user to select a single serial port by calling navigator.serial.requestPort() in response to a user gesture such as touch or mouse click, or pick one from navigator.serial.getPorts() which returns a list of serial ports the website has been granted access to.

Calling requestPort() prompts the user to select a device and returns a SerialPort object. Once you have a SerialPort object, calling with the desired baud rate will open the serial port. The baudRate dictionary member specifies how fast data is sent over a serial line. It is expressed in units of bits-per-second (bps). Check your device's documentation for the correct value as all the data you send and receive will be gibberish if this is specified incorrectly. For some USB and Bluetooth devices that emulate a serial port this value may be safely set to any value as it is ignored by the emulation.

After the serial port connection is established, the readable and writable properties from the SerialPort object return a ReadableStream and a WritableStream. Those will be used to receive data from and send data to the serial device. Both use Uint8Array instances for data transfer.

When new data arrives from the serial device, port.readable.getReader().read() returns two properties asynchronously: the value and a done boolean. If done is true, the serial port has been closed or there is no more data coming in. Calling port.readable.getReader() creates a reader and locks readable to it. While readable is locked, the serial port can't be closed.

Some non-fatal serial port read errors can happen under some conditions such as buffer overflow, framing errors, or parity errors. Those are thrown as exceptions and can be caught by adding another loop on top of the previous one that checks port.readable. This works because as long as the errors are non-fatal, a new ReadableStream is created automatically. If a fatal error occurs, such as the serial device being removed, then port.readable becomes null.

If the serial device sends text back, you can pipe port.readable through a TextDecoderStream as shown below. A TextDecoderStream is a transform stream that grabs all Uint8Array chunks and converts them to strings.

However, when continuously reading data from a serial device using a loop, port.readable will always be locked until it encounters an error. In this case, calling reader.cancel() will force to resolve immediately with value: undefined, done: true and therefore allowing the loop to call reader.releaseLock().

Closing a serial port is more complicated when using transform streams (like TextDecoderStream and TextEncoderStream). Call reader.cancel() as before. Then call writer.close() and port.close(). This propagates errors through the transform streams to the underlying serial port. Because error propagation doesn't happen immediately, you need to use the readableStreamClosed and writableStreamClosed promises created earlier to detect when port.readable and port.writable have been unlocked. Cancelling the reader causes the stream to be aborted; this is why you must catch and ignore the resulting error.

If a serial port is provided by a USB device then that device may be connected or disconnected from the system. When the website has been granted permission to access a serial port, it should monitor the connect and disconnect events.

After establishing the serial port connection, you can explicitly query and set signals exposed by the serial port for device detection and flow control. These signals are defined as boolean values. For example, some devices such as Arduino will enter a programming mode if the Data Terminal Ready (DTR) signal is toggled.

To deal with this, you can use some built-in transform streams such as TextDecoderStream or create your own transform stream which allows you to parse the incoming stream and return parsed data. The transform stream sits between the serial device and the read loop that is consuming the stream. It can apply an arbitrary transform before the data is consumed. Think of it like an assembly line: as a widget comes down the line, each step in the line modifies the widget, so that by the time it gets to its final destination, it's a fully functioning widget.

To use the transform stream class, you need to pipe an incoming stream through it. In the third code example under Read from a serial port, the original input stream was only piped through a TextDecoderStream, so we need to call pipeThrough() to pipe it through our new LineBreakTransformer.

For debugging serial device communication issues, use the tee() method of port.readable to split the streams going to or from the serial device. The two streams created can be consumed independently and this allows you to print one to the console for inspection. 350c69d7ab


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