RS-485 interface

The RS485 is commonly used to connect DMX512, Modbus or generic RS485 serial devices.

Two RS485 lines are available on CM-Home. This lines are opto-isolated from the CPU and available on screw terminals as shown below.

Using the RS485 ports

The ports are visible by Linux on /dev/ttyUSB0 (left port) and /dev/ttyUSB2 (right port) and can be managed by software like any other serial line.
The DE signal required to get the bus is managed in hardware by the FTDI FT4232 chip.

Python example

Send and receive chars using the left RS485 port. To do the same on the right port change /dev/ttyUSB0 in /dev/ttyUSB1.

#!/usr/bin/python
import serial
import time

ser = serial.Serial(
    port='/dev/ttyUSB0',
    baudrate=115200,
    timeout=1,
    parity=serial.PARITY_NONE,
    stopbits=serial.STOPBITS_ONE,
    bytesize=serial.EIGHTBITS
)
ser.flushOutput()
ser.flushInput()

rx_counter=0
tx_counter=0
while True:
    tx_counter=tx_counter+1
    ser.write("Tx counter = %d | Rx counter = %d\n\r".encode('utf-8') % (tx_counter,rx_counter))
    print("Tx counter = %d | Rx counter = %d" % (tx_counter,rx_counter))

    a=ser.read(1)

    if a:
        rx_counter=rx_counter+1
        ser.write("Tx counter = %d | Rx counter = %d | Received: %s %02x\n\r".encode('utf-8') % (tx_counter,rx_counter,a,ord(a)))
        print("Tx counter = %d | Rx counter = %d | Received: %s %02x" % (tx_counter,rx_counter,a,ord(a)))