Serial Port

1 Serial Port Interface Overview

The GM-3568JHF development board is designed based on the Rockchip RK3568J chip and provides rich serial port interfaces, supporting a variety of serial communication protocols and application scenarios. The serial port is one of the most commonly used communication interfaces in embedded systems, widely applied in device debugging, data transmission, industrial control, and other fields.

1.1 Serial Port Interface Configuration

There are a total of 6 serial ports coming out of RK3568J on the development board:

Serial Port Type	Quantity	Interface Form	Electrical Standard	Application Scenario
RS232	1	DB9 connector	RS232 level	Industrial device communication
RS485	2	Terminal interface	RS485 differential	Industrial bus communication
TTL	2	Pin header interface	3.3V TTL	Debugging, sensor communication
TypeC Debug	1	Type-C interface	USB to TTL	System debugging

1.2 Serial Port Electrical Standards Introduction

RS232 Standard

Voltage Range: ±3V to ±15V
Logic Level: Positive voltage indicates logic 0, negative voltage indicates logic 1
Transmission Distance: Up to 15 meters (9600bps)
Connection Method: Single-ended signal transmission
Anti-interference: General anti-interference ability

RS485 Standard

Voltage Range: ±1.5V to ±6V
Logic Level: Differential signal transmission
Transmission Distance: Up to 1200 meters (9600bps)
Connection Method: Two-wire differential transmission (A+, 😎
Anti-interference: Strong anti-interference ability, suitable for industrial environments

TTL Standard

Voltage Range: 0V to 3.3V
Logic Level: High level 3.3V, low level 0V
Transmission Distance: Generally less than 3 meters
Connection Method: Single-ended signal transmission
Application: Mainly used for board-level communication and debugging
RS232: 1
RS485: 2
TTL: 2 (one of which is on the 40PIN interface; this serial port can be connected to a Raspberry Pi expansion board for RS485 use)

TypeC (Debug): 1 (this interface is a USB-to-TTL interface, which can be used directly after connecting TypeC)

2 Connecting the Serial Port

Different interfaces have different connection methods. Below are the wiring methods for the different serial ports:

2.1 TTL

Taking the USB-to-TTL tool as an example:

TXD -- RXD
RXD -- TXD
GND -- GND

2.2 RS232

Taking the USB-to-232 tool as an example:

Because it is a standard 232 interface, you can connect directly

2.3 RS485

Taking the USB-to-485 tool as an example:

USB-to-485 A -> Development board A

USB-to-485 B -> Development board B

As shown in the figure:

2.4 Serial Communication Test

We use the TTL serial port on the board for the experiment; the corresponding device file is /dev/ttyS4. By directly reading and writing the tty device file, you can control the device to receive or send data through the serial port. Below we use the board together with a serial debugging assistant under Windows.

Tips

The RS485 serial port has automatic direction switching added at the driver layer, so communication is the same as with a normal serial port

Query Serial Port Parameters and Modify the Baud Rate

Use the stty tool to query serial port parameters

# 在开发板的终端执行如下命令
root@linaro-alip:/# stty -F /dev/ttyS4
speed 9600 baud; line = 0;
-brkint -imaxbel

Use the stty tool to modify serial port parameters

# 设置通讯速率，其中 ispeed 为输入速率，ospeed 为输出速率
root@linaro-alip:/# stty -F /dev/ttyS4 ispeed 115200 ospeed 115200
root@linaro-alip:/# stty -F /dev/ttyS4
speed 115200 baud; line = 0;
-brkint -imaxbel

Communicating with the PC

After configuring the serial debugging assistant on the PC, use the following commands on the board to test serial data transmission:

1) Sending data:

# 在板卡上的终端执行如下指令
# 使用 echo 命令向终端设备文件写入字符串 "Hello!"
echo Hello! > /dev/ttyS4

The serial debugging assistant on the PC will receive the content

2) Receiving data:

To test receiving data, you can use the microcom tool:

# 在板卡上的终端执行如下指令
# 使用 microcom 命令读取终端设备文件，-s 参数可以设置波特率
busybox microcom -s 115200 /dev/ttyS4
Hello World! #PC 发送过来的数据
# microcom 命令会等待
# 使用串口调试助手发送字符串
# 板卡的终端会输出接收到的内容

3 Troubleshooting

3.1 Serial Port Device Cannot Be Opened

Symptom: "Permission denied" is reported, or the device does not exist

Solution:

# 检查设备节点是否存在
ls -l /dev/ttyS*

# 检查设备权限
sudo chmod 666 /dev/ttyS4

# 将用户添加到 dialout 组
sudo usermod -a -G dialout $USER

3.2 Data Transmission Errors

Possible Causes:

Baud rate mismatch
Incorrect data bits, stop bits, or parity bits configuration
Hardware connection issues

Troubleshooting Steps:

# 检查串口配置
stty -F /dev/ttyS4 -a

# 设置正确的串口参数
stty -F /dev/ttyS4 115200 cs8 -cstopb -parenb

3.3 RS485 Communication Issues

Notes:

Ensure the A and B lines are connected correctly
Check the termination resistor configuration
Verify the ground wire connection

# RS485 测试命令
echo "test" > /dev/ttyS1  # 发送到 RS485 端口 1
cat /dev/ttyS2           # 从 RS485 端口 2 接收

3.4 Serial Port Conflict

Solution:

# 查看串口使用情况
lsof /dev/ttyS*

# 终止占用串口的进程
sudo fuser -k /dev/ttyS4