Modbus is a serial communication standard and has become a de facto standard communication protocol and is now a commonly available means of connecting industrial electronic devices. In Modbus RTU and Modbus ASCII RS485 is used as the physical layer. Its possible to use a Raspberry Pi as Master or Slave in Modbus applications, but a RS485 interface is needed. Our RS422 / RS485 Serial HAT is a fully galvanic isolated serial communication HAT designed for use with the Raspberry Pi and the perfect choice for such kind of applications.
The objective of this document is to show how to configure the HAT and the Raspberry Pi to be used for Modbus serial communication.
Check out our instructable about MODBUS & Arduino too!
Step 1: Bill of Material
- Raspberry Pi A+, B+, B2 or B3
- RS422/RS485 serial HAT
Step 2: Wiring
The wiring is very simple. You have to connect only the A and B terminals of the HAT with the A and B line of the Modbus system. Y and Z terminals are not used for this kind of application. For long distances it is recommend to use twisted pairs for A and B.
Step 3: DIP Switch Setting
Our RS422/RS485 HAT comes with 3 DIP switch banks. You have to set these DIP switches for Modbus as shown in the picture above.
Switch 1: 1-OFF 2-ON 3-ON 4-OFF
Switch 2: 1-OFF 2-OFF 3-ON 4-ON
Switch 3: 1-OFF or ON* 2-OFF 3-OFF 4-OFF
*Depending of the position of the RS422/RS485 HAT in the Modbus line you have to switch the terminating resistor ON or OFF. Please switch the resistor to ON position only if the HAT is on one end of the bus line. In all other cases switch the terminating resistor OFF:
Step 4: Free Up Serial Line and Enable UART
First we have to config the Raspberry Pi:
Goto -> 5 Interfacing Options -> P6 Serial -> choose YES
Additionally remove any references to console=serial0,115200and ttyAMA0 from /boot/cmdline.txt.
Our file looks like this:
dwc_otg.lpm_enable=0 console=tty1 root=/dev/mmcblk0p2 rootfstype=ext4 elevator=deadline rootwait
Update the Raspbian Jessie operating system now:
sudo apt-get update
sudo apt-get upgrade
On the Raspberry Pi 3 /dev/ttyAMA0 is used by the Wifi and Blue-
tooth module. There is a “mini UART” available on /dev/ttyS0 by default. It is better though, to use the hardware UART and switch the Wifi/Bluetooth module to mini UART. To do that, add this line at the end of /boot/cmdline.txt
Reboot your Raspberry Pi
Step 5: Software
You will find an easy-to-use and tested Modbus RTU and Modbus
ASCII implementation for Python on: