modbusmaster/readme.md

modbusmaster

Disable Bluetooth on RPi3

Add at the end of /boot/config:

dtoverlay=pi3-disable-bt

Remove mentions of serial0 from /boot/cmdline.

Enable rs485 mode

Use the submodule rpirtscts to enable to alternate functions of the related pins at the RPi MCU. It is submoduled here, can be found directly at https://github.com/mholling/rpirtscts

cd rpirtscts
gcc -o rpirtscts rpirtscts.c
sudo ./rpirtscts on

This needs to be done at every boot.

Kudos to danjperron, cmp. https://www.raspberrypi.org/forums/viewtopic.php?f=98&t=224533&hilit=rs+485#p1383709

Pinout

Find a good pinout diagram at https://raw.githubusercontent.com/ppelleti/hs-wiringPi/master/pin-diagram.png.

TX is at GPIO14, RX is at GPIO15 and RTS (control line for transmitter enable) is at GPIO17.

Python snippet to test

import serial.rs485
ser=serial.rs485.RS485(port='/dev/ttyAMA0',baudrate=2400)
ser.rs485_mode = serial.rs485.RS485Settings(False,True)
ser.write('a test'.encode('utf-8'))

Find an signal screenshot here:

Channel 1 in yellow has the TX line, channel 3 in purple has the RTS (transmitter enable line).