RS-485

Edge 2 has one RS-485 port, a three-pin, 3.81 mm terminal block on the port edge of the board, between HDMI and Ethernet. It is a half-duplex, two-wire interface for industrial devices such as meters, PLCs and sensors that use Modbus RTU or similar protocols.

Figure 1. The RS-485 terminal block

Specifications

• Driven by UART4 of the RK3568.
• Half duplex. The transceiver switches between receiving and transmitting by itself, so software does not need to control the direction.
• Isolated: the RS-485 side has its own power supply and ground, separated from the board by optocouplers.
• No termination resistor is fitted on the board. At the ends of a long bus, add a 120 Ω resistor between A and B.

Pin Assignments

Pin 1 is the terminal next to HDMI.

Pin#	PinName	Signal Description
1	GND	Isolated RS-485 ground
2	485B	RS-485 differential signal, negative
3	485A	RS-485 differential signal, positive

Wiring

Connect 485A to the device's A (sometimes marked D+), 485B to its B (D−), and GND to its ground.

Figure 2. RS-485 wiring

Test the Port with Mixtile OS

This test sends text between Edge 2 and a computer through a USB to RS-485 adapter. In Mixtile OS, the RS-485 port is /dev/ttyS4.

1. On the computer, open the adapter's serial port at 9600 baud, for example with picocom:

   Host PC

   picocom -b 9600 /dev/ttyUSB0

2. On Edge 2, set the same speed and send a line:

   Edge 2

   stty -F /dev/ttyS4 9600 raw -echo
   echo "hello from Edge 2" > /dev/ttyS4

   Expected result: hello from Edge 2 appears in picocom.

3. To receive, run the following on Edge 2 and type in picocom:

   Edge 2

   cat /dev/ttyS4

   Expected result: what you type appears on Edge 2. Press Ctrl+C to stop.

note

This procedure has not yet been verified on Edge 2 hardware with Mixtile OS.