How to Connect a PLC to MQTT

Start a New Project in MapleLogic

– For this example, select the PLC-FB
– Click Next to continue to next step

Set Project Details

– Choose the CPU Type: PLC-FB1616
– Name the Project
– Click OK to continue

Start a New Scanned Program

– Right click on Program
– Select Scan under LD program
– Name the program
– Click OK to continue

PLC Parameter Settings

– Double click on PLC Parameter
– Select the Ethernet tab
– Enter the IP address
– Click OK

This IP address is going to bethe target address for Modbus TCP/IP communication. The gateway HMI will be polling the Micro PLC for values and it will need this IP address to do that. The Maple PLC protocol will be used. These steps will be covered in this article.

Modbus Addressing

– Here is where to view the modbus settings
– In this example we’ll be using a Coil Status Register (Y) which is a 0x address.
– Holding Registers (D) will be used as well which are 4x addresses.

PLC Ladder Logic Layout

– Here is the logic that’ll be used in this example.
– 4 rungs of logic for turning Machine 1 on and off, product count, up time, and down time.

Machine 1 On and Off

– Add a Normally Open Contact (F5) on rung 1. Use X00 as an external contact.
– Use Y10 as the Machine1_ON_OFF Coil (F9)
– You can view the device registers in the Device Monitor window on the right side.

When Machine 1 is on, Machine1_ProductCount and Machine1_UpTime will be activated. When Machine 1 is off, Machine1_DownTime will be activated and Machine1_ProductCount and Machine1_UpTime will stop.

Machine 1 Product Count

Add a Normally Open Contact (F5), use X00. Add a Positive Transition – Sensing Contact (F7), use F94 for a 2 second clock timer. Add an Application Instruction, use an increment instruction to be stored in a 16-word register D10. Name it Machine1_ProductCount

This rung will be used to simulate the product count for Machine 1. Every 2 seconds a product will be counted and that count will be displayed in the D10 register. This will only be activated when the Machine1_ON_OFF bit is on.

Machine 1 Up Time

Add a Normally Open Contact (F5), use X00. Add a Positive Transition – Sensing Contact (F7), use F93 for a 1 second clock timer. Add an Application Instruction, use an increment instruction to be stored in a 16-word register D11. Name it Machine1_UpTime

This rung will be used to simulate the up time for Machine 1. The up time will update every one second and that count will be displayed in the D11 register. This will only be activated when the Machine1_ON_OFF bit is on.

Machine 1 Down Time

Add a Normally Closed Contact (F6), use X00. Add a Positive Transition – Sensing Contact (F7), use F93 for a 1 second clock timer. Add an Application Instruction, use an increment instruction to be stored in a 16-word register D12. Name it Machine1_DownTime

This rung will be used to simulate the down time for Machine 1. The down time will update every one second and that count will be displayed in the D12 register. This will only be activated when the Machine1_ON_OFF bit is off.

Variable Editor – Y Register Modbus Address

– Navigate to View > Variable Editor
– Select Y: Bit
– Here is where to view the modbus address for Y10. It’s 000017 or 0x_17

The reason the modus address is 000017 or 0x_17 is because Y10 is mapped to the 17th (or 23rd if hex) coil offset in the Modbus memory table, not simply just coil 10. The PLC address Y10 is a native Maple PLC device address, not a Modbus address. When using Modbus TCP, the PLC internally maps its device memory (X, Y, D, M, etc.) into a separate Modbus address table. That mapping does not always directly equal the device number.

Variable Editor – D Word Modbus Addresses

– Navigate to View > Variable Editor
– Select D: Word
– Here is where to view the modbus addresses for D10, D11, D12. They are 400011, 400012, 400013, or 4x_11, 4x_12, 4x_13.

Due to the Modbus addressing offset by 1 because EBPro displays Modbus Holding Registers (4x) using 1-based addressing, while the Maple PLC’s in MapleLogic internal D registers are effectively 0-based in the Modbus mapping.
So the mapping works like this:
D0 > 4x_1
D1 > 4x_2
D10 > 4x_11
D11 > 4x_12
D12 > 4x_13
EBPro is adding +1 offset when displaying the Modbus 4x address.

Add a New Project

– Select the cMT-G01X Gateway model
– Click OK

Configuring Communication Protocol

– Click the Settings button

Selecting the Communication Protocol

– Click the New Device/Server button
– Select Modbus > TCP/IP Master
– Choose Ethernet
– Choose Station Number: 0 (this must match the station number in MapleLogic, which is also 0)

IP Address Settings

– Click Settings
– Enter the IP address of the PLC. 192.168.255.76 for this example.
– The default Port number is 502. DO NOT change this.
– In Click OK to save the settings

Port 502 is the official, standardized TCP port number assigned to Modbus TCP. So whenever a device is acting as a Modbus TCP server (like the Maple PLC), it listens on port 502 by default.

Configuring MQTT Settings

– Click MQTT to open these settings.

MQTT Server Properties

– Click Enable to open the server settings
– Click Settings… to open the properties
– Cloud service set to Generic
– Protocol can be left as default as MQTT v3.1
– Enter the IP address of the local MQTT server. 192.168.255.75 for this example. Port number is 1883.
– Click OK to save the settings

You will need to have an MQTT broker server set up. In this example a local Mosquitto server is being used as the MQTT broker server. Therefore the IP address of the PC is being used.

Add Machine 1 On Off Topic

– Select the Topic Publisher tab
– Add a topic named Machine1_OnOff and use a nickname of your choosing, it doesn’t have to match the topic name, but it does in this example.
– Check off Value-trigger based. This means that a message is published only when a tag’s value changes, instead of being sent continuously on a timed interval.
– Click OK to save the topic

All of the topics are being published in this example. If you refer back to the ladder logic, topics will need to be created to correspond with the bit and word registers used in MapleLogic.

Machine 1 On Off Address

– Click the Address tab
– Select Bit
– Select the Modbus TCP/IP Master protocol as a device
– Select 0x_17 as an address.

Y10 is being used as the bit address in the ladder logic in MapleLogic, therefore select the modbuss address for that bit address, which is 0x_17 or 000017.

Machine 1 Product Count Address

– Click the Address tab
– Select Word
– Select the Modbus TCP/IP Master protocol as a device
– Select 4x_11 as an address. 4x is used for holding registers.

D10 is being used as the word address in the ladder logic in MapleLogic, therefore select the modbuss address for that word address, which is 4x_11 or 400011.
The other holding register addresses used in the ladder logic are D11 and D12, the modbus address are 4x_12 and 4x_13.

Downloading Project to Gateway HMI

– Click the Download button on the toolbar
– Enter the IP address of the Gateway HMI
– Click Download to begin the processs

MapleLogic – Online

– Select Online on the toolbar
– Select Connect Option to open the communication setup

Choosing Ethernet

– Select Ethernet ( USB is also an option, ethernet will be used in this example)
– Enter the IP address of the PLC
– Click OK

If you choose USB instead, the Maple Systems PLC’s use a micro usb type cable for uploading and downloading projects to the PLC.

Link+Download+Monitor

– Click the Link+Download+Monitor button on the tool to download and monitor the project online

Transmission to PLC

– Once you click download transmission will begin to the PLC

RUN mode

– A prompt will pop up once the transmisssion completes to put the PLC into RUN mode
– Click YES

PLC Program Online

– Once the download process completes, and selecting RUN mode the logic will be activated and live values can be viewed in MapleLogic
– A PLC indicator will pop up to switch from RUN to STOP mode and display that the PLC is OK
– In the Message Window on the bottom, is the view of the PLC’s status and will indicate everything is running smoothly.

Add MQTT Connection in MQTT Explorer

– Choose the MQTT Broker Server (local MQTT Mosquitto server host url in this example: 192.168.255.75)
– 1883 is the port number
– Save the settings
– Click Connect

In this example, there’s a local MQTT Mosquitto server being used, so the host url is the IP address of the PC being used.

Viewing Topics

– Now view MQTT Explorer after connecting to the local MQTT broker server.
– If the MQTT configuration and mosbus addressing were done correctly, all of the topics can be monitored here.