How to setup a Raspberry Pi 3 for Real-time Control: The easiest step-by-step guide
Introduction
Here, I am going to explain how to setup Raspberry Pi 3 for development of a RT control application.

Get SD Card Image of RT-Linux for RPI
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Get the pre-compiled SD card image in the link below
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In this tutorial, I am using an image located in the folder of this document
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https://drive.google.com/open?id=1NVvlsqfjDoWI2gM42Lavmmj2-H9tWXM5
Write the image
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Prepare an SD card. Here, I am using an 32GB micro SD card.
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With an SD card writing tool (here, Etcher), write the image in the SD card.
Setup a static IP address of Raspberry Pi
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Open /etc/dhcpcd.conf in the SD card file system and edit like this
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interface eth0
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static ip_address=192.168.1.105/24
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static routers=192.168.1.1
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static domain_name_servers=192.168.1.1
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In the address, 192.168.1.105/24, you may use different ip address e.g., 192.168.1.xxx/24. In this example I will use 192.168.1.105
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If your router gateway is using different IP, (e.g., 10.1.10.1), then change the value 192.168.1 to these numbers. For details see this link below https://caffinc.github.io/2016/12/raspberry-pi-3-headless/
Insert the SD card in RPI and power on the RPI
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Now, remove the SD card from your host PC, and insert it into your RPI.
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HERE, DO NOT CONNECT ANY USB DEVICE DURING THE BOOTING TIME. IT COULD FREEZE THE RPI.
Search the IP address of Raspberry Pi
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Execute the following commands in your host computer to search the ip address. If you did correctly for the upper part, the IP should be 192.168.1.205
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apt-get install nmap
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sudo apt-get install nmap
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hostname -I
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nmap -sn 192.168.1.0/24
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If everything works correctly, you can SSH the raspberry pi in your host PC.​
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ssh pi@192.168.1.105​
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​If everything is good, you should be able to access the RPI w with SSH.

References
[1] https://www.raspberrypi.org/documentation/configuration/wireless/wireless-cli.md
[2] https://caffinc.github.io/2016/12/raspberry-pi-3-headless/
[3] https://guysoft.wordpress.com/2017/10/09/realtimepi/