Hello and in this tutorial I will show you how to implement a MQTT Client to subscribe to the topics by TwinCAT TF6701.

Reference Link:

MQTT#Broker Setup

Beckhoff#Using TwinCAT TF6701 to Publish message inAWS IOT Core

MQTT#using Python Library as a Subscriber and Publisher

Beckhoff#Play with EK9160 Iot Coupler

Function Blocks

Here the function blocks that are used will be explained.

FB_IotMqttClient.Subscribe

This method is used by the client to Subscribe a topic to the broker that it wants to receive.Multiple topics are supported also.

VAR_INPUT

Name	Type	Description
sTopic	STRING	Topic of the MQTT message
eQoS	TcIotMqttQos	Quality of Service

Return value

Name	Type	Description
Subscribe	BOOL	True=Call was successful.

FB_IotMqttMessage

Its function block is used with the message queue(FB_IotMqttMessageQueue, i will explain it in the next session), and the MQTT incoming messages are collected by the message queue.

Binary Information or string are sent in here, the maximum size of an MQTT message in PLC, is dependent on the hardware and should be less than few MB.

We can adjust the max mqtt message size by changing the cMaxSizeOfMqttMessage parameter,and default=100kb.

Finally, UTF-8 format is used and it is a common type for MQTT just like JSON.

CompareTopic

A method to compare the topic received from Broker,Ture=Topic is identical .

VAR_INPUT

Name	Type	Description
sTopic	STRING	Topic that would like to compare

GetTopic

VAR_INPUT

Name	Type	Description
pTopic	POINTER TO STRING	topic buffer
nTopicSize	UINT	maximum size of topic buffer in bytes

GetPayload

A method to get the message that received from the broker.

VAR_INPUT

Name	Type	Description
pPayload	PVOID	The memory address for the buffer into which the payload is to becopied is specified here
nPayloadSize	UDINT	The maximum available buffer size (in bytes) is specified here.
bSetNullTermination	BOOL	If the payload type requires zero termination (string), this can beimplemented during the copy process

FB_IotMqttMessageQueue

A Function Block offers a message queue for MQTT messages, which can be used with the block FB_IotMqttClient.It operates by FIFO – first in first out.

The max size of the MQTT message queue in default is 1000, and the by default the maximum size of a message is limited to 100 kB, the size of the MQTT message queue is limited to 1000 kB

Dequeue

The method returns the return value TRUE if the removal of an MQTT message from the queue was

successful. The number of MQTT messages currently in the queue (nQueuedMessages property) is reduced

by one through the removal of a message.

VAR_INPUT

Name	Type	Description
fbMessage	REFERENCE TO FB_IotMqttMess	For the message itself the reference is transferred to an instance of type FB_IotMqttMessage
nPayloadSize	UDINT	The maximum available buffer size (in bytes) is specified here.
bSetNullTermination	BOOL	If the payload type requires zero termination (string), this can beimplemented during the copy process

ResetQueue

all accumulated MQTT messages are deleted from the queue when this method is called.

Configration

Here is the configuration in this tutorial. The publisher is implemented by the python library, and the Subscriber is done by TF6701.Broker is built in Raspberry-pi3.

Setup a Broker

Some Useful Link while using Raspberry

Raspberrypi#Using VNC Viewer

RaspberryPI#Set the static ip without starting the OS

RaspberryPI#Enable the SSH Interface without start the OS

Here is the tutorial to build a broker in the raspberry pi/ubuntu.

MQTT#Broker Setup

If the connection is refused:

MQTT#Connection is refuesd solution

Python MQTT Client Implementation

Code

Python Publicher

import paho.mqtt.client as mqtt
import time

def on_connect(client,userdata,flag,rc):
print(‘Connected with result code ‘+str(rc))
client.subscribe(‘RoboDK/Station/s1/Lamp/Red’)

def on_disconnect(client,userdata,rc):
if rc !=0:
print(‘Something Wrong..’)

def on_publish(client,userdata,mid):
print(‘publish: {0}’.format(mid))

BROCKER_URL=’192.168.1.50′
BROCKET_PORT=1883

client=mqtt.Client()
client.on_connect=on_connect
client.on_disconnect=on_disconnect
client.on_publish=on_publish

client.connect(BROCKER_URL,BROCKET_PORT,60)
count=0

while count<1000:
ret=client.publish(‘RoboDK/Station/s1/Lamp/Yellow’,str(count))
ret=client.publish(‘RoboDK/Station/s1/Lamp/Red’,str(count+200))
ret=client.publish(‘RoboDK/Station/s1/Lamp/Green’,str(count+100))
count=count+1
time.sleep(1)
client.disconnect()

TwinCAT

VAR

PROGRAM MAIN
VAR
MQTTClient:FB_IotMqttClient;
payload:STRING(255);
payload_r,payload_y,payload_g:STRING(20);
fbMessageQueue : FB_IotMqttMessageQueue;
fbMessage : FB_IotMqttMessage;
sTopic :STRING;
bSetNullTermination :BOOL;
bScbscrible:BOOL;
sTopicReceived:STRING(255);
Value_r,Value_g,Value_y:DINT;
END_VAR

PROGRAM

MQTTClient.sHostName:=’192.168.1.50′;
MQTTClient.nKeepAlive:=60;
MQTTClient.ipMessageQueue:=fbMessageQueue;

MQTTClient.Execute(bConnect:=TRUE);

IF MQTTClient.bConnected THEN
IF NOT bScbscrible THEN
MQTTClient.Subscribe(
sTopic:=sTopic
,eQoS:=0
);
bScbscrible:=TRUE;
END_IF
END_IF

sTopic:=’RoboDK/Station/s1/#’;
IF fbMessageQueue.nQueuedMessages > 0 THEN
IF fbMessageQueue.Dequeue(fbMessage:=fbMessage) THEN
fbMessage.GetTopic(pTopic:=ADR(sTopicReceived), nTopicSize:=SIZEOF(sTopicReceived));
IF fbMessage.CompareTopic(sTopic:=’RoboDK/Station/s1/Lamp/Red’) THEN
payload_r:=’ ‘;
fbMessage.GetPayload(pPayload:=ADR(payload_r), nPayloadSize:=SIZEOF(payload_r), bSetNullTermination:=FALSE);
ELSIF fbMessage.CompareTopic(sTopic:=’RoboDK/Station/s1/Lamp/Yellow’) THEN
payload_y:=’ ‘;
fbMessage.GetPayload(pPayload:=ADR(payload_y), nPayloadSize:=SIZEOF(payload_y), bSetNullTermination:=FALSE);
ELSIF fbMessage.CompareTopic(sTopic:=’RoboDK/Station/s1/Lamp/Green’) THEN
payload_g:=’ ‘;
fbMessage.GetPayload(pPayload:=ADR(payload_g), nPayloadSize:=SIZEOF(payload_g), bSetNullTermination:=FALSE);
END_IF
END_IF
END_IF

Value_r:=STRING_TO_DINT(payload_r);
Value_g:=STRING_TO_DINT(payload_g);
Value_y:=STRING_TO_DINT(payload_y);