Websockets
Overview
Websockets provide an excellent way to perform real time communication between a server and client. The following example demonstrates using a websocket to communicate between a PC client connected to a joystick and a RPI connected to an Arduino and display. The PC yoystick client sends the data from the joystick. The PC web client gets data from the Arduino.
Server
The following is an example of the server code that would run on the RPI.
#!/usr/bin/env python
# WS server example
import asyncio
import json
import serial
import time
import websockets
#create serial object
ser = serial.Serial(‘/dev/ttyACM0’, 9600)
#clear input serial buffers
ser.reset_input_buffer()
#create a set to store websocket connections
connected = set()
#Receiver runs continuously
async def hello(websocket, path):
#add each connection to the list
connected.add(websocket)
#use to enable continuous connection to socket
async for message in websocket:
y = json.loads(message)
x = y[“Axes”][0]
x = int(x * 1000)
print(x)
ser.write(‘B{}\n’.format(x).encode())
#Sender is called once and runs in the background
async def world():
while True:
#check if data is available in the websocket
if (ser.in_waiting > 0):
data = ser.read(ser.in_waiting).decode(‘ascii’)
print (data)
#removes unused sockets after they disconnect
for websocket in connected.copy():
try:
await websocket.send(data)
except:
connected.remove(websocket)
await asyncio.sleep(1)
start_server = websockets.serve(hello, “0.0.0.0”, 8765,
ping_interval=None)
asyncio.get_event_loop().run_until_complete(start_server)
asyncio.ensure_future(world())
asyncio.get_event_loop().run_forever()
Client (joystick)
The following is an example of client code that would reside on a PC. This client sends data in real time from a joystick to the server.
#!/usr/bin/env python
import asyncio
import pygame
import websockets
import json
import time
#initialize pygame
pygame.init()
#initialize joystick
pygame.joystick.init()
#create and initialize one joystick object
stick = pygame.joystick.Joystick(0)
stick.init()
uri = “ws://192.168.1.9:8765”
#interval must be set to none or the client dies without a regular
response from server
websocket = websockets.connect(uri, ping_interval=None)
async def joy():
global websocket
async with websocket as websocket:
while True:
#request update of joystick status
pygame.event.pump()
axes = stick.get_numaxes()
axis_values = [0] * axes
for i in range(axes):
axis_values[i] = stick.get_axis(i)
#send all four axis value packaged as json
await websocket.send(json.dumps({“Axes”:
axis_values}))
#delay necessary to prevent overload of receiver,
adjust as necessary
time.sleep(.1)
asyncio.get_event_loop().run_until_complete(joy())
pygame.joystick.quit()
Client (browser)
The following is client code that allows you to use a browser to see data sent from the server.
<!DOCTYPE html>
<html>
<head>
<title>WebSocket demo</title>
</head>
<body>
<script>
var ws = new WebSocket(“ws://192.168.1.9:8765/”),
messages = document.createElement(‘ul’);
ws.onmessage = function (event) {
var messages = document.getElementsByTagName(‘ul’)[0],
message = document.createElement(‘li’),
content = document.createTextNode(event.data);
message.appendChild(content);
messages.appendChild(message);
};
document.body.appendChild(messages);
</script>
</body>
</html>
Arduino
The following is coordinated code that runs on the Arduino.
#include <Adafruit_NeoPixel.h>
#include <Adafruit_LEDBackpack.h>
#include <avr/power.h>
#define PIN 13
#define NUMPIXELS 8
#define WAIT 0
#define READ_VALUE 1
#define BUTTONA 2
#define PRESSED LOW
Adafruit_7segment matrix = Adafruit_7segment();
Adafruit_NeoPixel pixels = Adafruit_NeoPixel(NUMPIXELS, PIN,
NEO_GRB + NEO_KHZ800);
uint32_t thisTime = 0, lastTime = 0;
int state = WAIT;
String inString = “”;
char inCmd;
void set_pixels(uint8_t value) {
for (int x=0; x<8; x++) {
if (value & (1<<x)) {
pixels.setPixelColor(x, pixels.Color(50,0,0));
}
else {
pixels.setPixelColor(x, pixels.Color(0,0,0));
}
}
pixels.show();
}
void run_cmd(char cmd, int value) {
if (cmd == ‘A’)
set_pixels(value);
else if (cmd == ‘B’) {
matrix.print(value, DEC);
matrix.writeDisplay();
}
}
void setup(){
Serial.begin(9600);
matrix.begin(0x70);
matrix.print(0, DEC);
matrixd.writeDisplay();
pixels.begin();
set_pixels(0x00);
pinMode(BUTTONA, INPUT_PULLUP);
}
void loop() {
if (Serial.available()) {
int inChar = Serial.read();
if (state == READ_VALUE) {
if (isDigit(inChar))
inString += (char)inChar;
else if (inChar == ‘\n’) {
int inValue = inString.toInt();
run_cmd(inCmd, inValue);
inString = “”;
state = WAIT;
}
}
else if (state == WAIT) {
if (isUpperCase(inChar)) {
inCmd = (char)inChar;
state = READ_VALUE;
}
}
}
thisTime = millis();
if (thisTime - lastTime >= 1000) {
int aVal = analogRead(3);
if (BUTTONA == PRESSED)
bVal = true;
else
bval = false;
Serial.print(y);
lastTime = thisTime;
}
}