1001 RPi0 4G CAT1-ETH(RS485 OPT

Keywords

Raspberry Pi, Raspberry Pi Zero, Cat1 4G LTE, USB2.0-A, Ethernet, Expansion Board, USB HUB

Ⅰ、Introduction

The Raspberry Pi Zero series (including Zero, Zero W(H), and Zero 2W) is a highly cost-effective embedded system platform with a compact size, low power consumption, and decent performance, making it suitable for many lightweight application scenarios. The Zero series, despite its compact size, offers many expansion interfaces. The underside of the board features gold-plated test points for USB and power connections. These USB and power test points allow us to connect various types of peripherals for expansion. This expansion board is a USB hub, connecting the expansion board to the USB port of the Zero via pogo pins. It extends four USB ports through the USB connection: one USB port is converted to a 100Mbps wired Ethernet connection, one USB port is connected to a 4G Cat1 module, and two USB2.0-A host interfaces are provided.

4G Cat1 is a cost-effective module designed for medium-speed IoT applications around 10Mbps. The 10Mbps downlink and 5Mbps uplink speeds can meet the vast majority of networking and transmission needs. It supports 4G full network coverage, including China Unicom, China Telecom, and China Mobile networks.

Ⅱ、Hardware Resources

2.1 Use gold-plated pogo pins to connect the Raspberry Pi Zero series, eliminating the need for external cables;

2.2 The two groups of gold-plated pogo pins are used separately for power supply and USB communication.

2.3 The expansion board is equipped with a USB-C power supply interface, which can power the entire system. The Micro USB port on the Zero can also be used for power.

2.4 The onboard USB hub expands the Zero's USB into four ports: one port is used for connecting a 4G LTE Cat1 module, two are USB host interfaces, and one is a USB to 10/100Mbps Ethernet adapter.

2.5 The 4G LTE Cat1 module supports full 4G network coverage, with a downlink speed of up to 10Mbps and an uplink speed of up to 5Mbps.

2.6 The 4G LTE Cat1 module is a plug-and-play in most systems, driver-free and does not require manual dialing, usually recognized as the eth1 network card.

2.7 The 4G LTE Cat module uses a Nano-sized SIM card and offers the option of either an SMA antenna or a first-generation IPEX antenna.

2.8 The expansion board supportsother Pi with USB contacts in the same position, such as the Orange Pi Zero 2W.

2.9 Onboard PWR/MODE/ACT LED.

2.10 Onboard reset button for 4G module.

Note 1: After connecting this expansion board, the Micro USB port on the Zero will no longer be usable.

Note 2: In some systems, it is necessary to disable the OTG function and set the USB mode to Host mode.

Note 3: The expansion board supports all versions of the Raspberry Pi Zero, including the Zero, Zero W, Zero WH, and Zero 2W.

Ⅲ、System flashing

3.1 The hardware in this document is based on the Raspberry Pi Zero 2W. The Raspberry Pi OS used in this document is: 2024-07-04-raspios-bookworm-arm64.img.xz(Raspberry Pi OS with desktop).

(If you are using the Raspberry Pi Zero (1st generation) series boards, they only support 32-bit systems. Please be sure to download the correct version.)

You can download the Raspberry Pi OS in:

https://www.raspberrypi.com/software/operating-systems/#raspberry-pi-os-64-bit

Or ubuntu-24.04-preinstalled-server-arm64+raspi.img.xz (Ubuntu Server 24.04 LTS, The Raspberry Pi Zero 2W cannot use the Desktop version due to performance limitations.).

You can download the Ubuntu OS in:

https://ubuntu.com/download/raspberry-pi

3.2 Use Imager or balenaEtcher to flash the OS image in TF card. Click here to read the instructions for System flashing

Ⅲ、Demonstration of Using the Raspberry Pi OS

4.1 View hardware devices

4.1.1 View USB devices

Open the terminal in Raspberry Pi OS and enter the commandlsusb, as shown in the image below:

The system has detected the USB hub and the 4G module:

Device 002：External USB Hub;

Device 003：4G module;

Device 004：USB to 100Mbps Ethernet adapter;

Device 005：USB2.0-A Interface;

Device 008：USB2.0-A Interface.

If the system does not recognize the USB HUB and 4G module, please carefully check if the pogo pins are aligned with the gold-plated contacts. Also, please open the config.txt file located in the root directory of the TF card's system partition on the PC to check the USB initialization script:

Please verify if both of the red boxed areas in the image are fully configured. If not, please manually complete the configuration and save the file:

# otg_mode=1

dtoverlay=dwc2,dr_mode=host

4.1.2 View network devices

Open the terminal in Raspberry Pi OS and enter the command ifconfig -a, as shown in the image below:

We can see that eth0 is a USB-to-Ethernet adapter, and eth1 is a 4G Cat1 connection (with the IP address in the image above is 192.168.5.8).

4.2 Test network devices

4.2.1 ping tests

When testing, there is a priority order: if eth0 is connected, it will be used first. If there are specific requirements for internal and external networks, please adjust the metric values of each network and the DNS server settings.

Use the -I parameter to specify which network interface to start the ping packet from, as shown below:

ping www.mcuzone.com -I eth0

ping www.mcuzone.com -I eth1

Priority can be checked by executing the route command; the network card with the smaller metric value will be preferred for communication.

We can also force communication through another network by disabling a specific network card. For example, to disable eth0, you can execute the following command:

sudo ifconfig eth0 down

And enable eth0 by the following command:

sudo ifconfig eth0 up

4.2.2 Set network adapter priority

To set the priority of network cards, you need to modify the metric value of the network card. First, we need to install the ifmetric software.

sudo apt install ifmetric

After the installation is complete, you can modify the metric value of the network card. If you need to change the metric value of eth0 to 102, you can execute:

sudo ifmetric eth0 102

After making the changes, use the route command to check, and we can see that the metric value of eth0 has been updated to 102:

However, this modification is only temporary; if the system is restarted, it will revert to its original state. To permanently change the metric value of eth0 to 102, you need to edit the startup file to achieve this.

Open rc.local by the following command:

sudo nano /etc/rc.local

Add the command you want to execute at startup above exit 0, such as sudo ifmetric eth0 102, then save the file. You can modify the metric of eth0 every time the system starts by this way.

After modifying the metric values, it is possible that the system's nameserver (i.e., DNS server) is still using the previous higher-priority network card's nameserver. Since the default network card has now changed, this can lead to domain name resolution issues. So we also need to update the nameserver.

Open resolv.conf by the following command:

sudo nano /etc/resolv.conf

Check if the current nameserver is correct. If it is not, please change it to the nameserver of the network card with higher priority or to some common nameserver addresses (such as 8.8.8.8, etc.).

4.2.3 Using udhcpc to specify DNS servers

In the previous section, we discussed specifying DNS servers by modifying the nameserver. In fact, we can also achieve this by using udhcpc.

Install the udhcpc software:

sudo apt install udhcpc

In this section, in the example we are testing, eth0 is a USB-to-Ethernet adapter, and usb0 is a 4G Cat1 (which corresponds to eth1 in the previous section). If we need to use the network DNS of the USB-to-Ethernet adapter (eth0), please use the following command:

sudo udhcpc -i eth0

If we need to use the network DNS of the 4G Cat1 (usb0), please use the following command:

sudo udhcpc -i usb0

4.2.4 Examples of how to use udhcpc

Example 1: The 100Mbps Ethernet port is connected to the Internet, and the 4G Cat1 is connected normally. In this case, if we ping a certain domain name, it will succeed:

At this point, unplugging the 100M network port cable will make the ping fail:

Run sudo udhcpc -i usb0 and switch the DNS server to 4G Cat1, then you should be able to ping normally:

Example 2: The 100Mbps Ethernet port is connected to the internal network (not connected to the Internet), 4G Cat1 is connected normally. In this case, if we ping a certain domain name, it will failed:

Execute sudo udhcpc -i usb0, switch the DNS server to the 4G Cat1, and then you should be able to ping normally, and access internal network devices via the IP addresses:

4.2.5 Test speed by iperf3

You can download iperf3 (Windows version) in:

http://www.mcuzone.com/down/Software.asp?ID=10000634

Install iperf3 (Linux version) by using the following command:

sudo apt-get install iperf3

100M Ethernet speed test results: It is about 70Mbps in client mode and it is about 90Mbps in server mode:

Note: The USB to 100M Ethernet adapter may not reach full speed due to the performance limitations of the Zero 2W, the USB hub, and the bandwidth usage of the 4G Cat1.

4G CAT1 speed test results:

Note: Network speed tests are affected by network conditions and testing methods. The actual speed may vary; this test is for reference only.

Ⅴ、Demonstration of AT Command for 4G Cat1 Module on the Raspberry Pi OS

5.1 Method to open AT Command serial port

To operate 4G Cat1 Module using AT commands on a Raspberry Pi, you first need to open the AT command serial port. The method to open it is as follows:

5.1.1 Open ttyUSB serial port

Input the following command: lsusb

Record the ID value of the 4G module: 1782 4e00

Use the following command to open the ttyUSB serial port, where the value after echo is the ID recorded above:

sudo modprobe option

sudo sh -c 'echo 1782 4e00 > /sys/bus/usb-serial/drivers/option1/new_id'

After execution is complete, the system should have three additional devices: ttyUSB0, ttyUSB1, and ttyUSB2.

5.1.2 Open serial port by serial port tool

Install minicom:

sudo apt-get install minicom

Open AT Command serial port by minicom:

sudo minicom -D /dev/ttyUSB0

Or

sudo minicom -D /dev/ttyUSB1

Then directly type the AT command and press Enter to see the result. If you need to view the echo, please type the command: ate1.

Note: The model of the 4G module may change, but the process remains the same. You only need to use lsusb to check the actual USB ID and replace it with the actual value in subsequent commands. Additionally, some 4G Cat1 modules's chipset IDs have already been included in the kernel's supported list, so these Cat1 modules can be automatically recognized as ttyAMAx without the need to add a USB ID.

5.2 Common AT commands

1) Check if the SIM card is detected:

at+cpin?

Return ready to indicate the card has been recognized, if return error, you need to check the hardware.

2) Check antenna signal quality:

at+csq

Return values between 26 and 31 indicate a good signal, with 31 representing a full signal strength; return values between 20 and 25 indicate a barely acceptable signal; return values below 20 indicate a poor signal or that the antenna might not be connected.

3) Check network registration status:

at+cops?

Normally, it should return the network supporter's code: 7, where 7 represents 4G.

Note: The above command at+csq should not include a question mark, while the other two commands require a question mark.

4) View the SIM card's IMEI code:

at+cgsn

5) Reset 4G module (Sometimes, if you reinsert the SIM card, hot swapping may not work; in such cases, you can use this reset command to reset the module.):

at+reset

6) Disable radio frequency:

at+cfun=0

Enable radio frequency:

at+cfun=1

The two commands mentioned above can be used in pairs to allow the module to re-register with the network without restarting the 4G module.

六、Ubuntu系统下使用演示

6.1 编辑初始化脚本

Ubuntu Server烧写完毕后，请在PC端打开TF卡系统分区根目录下的config.txt检查USB的初始化脚本，Ubuntu Server对USB初始化了两遍，前面一遍没有配置成host模式，需要确认下图中的两处红框的位置是否都配置成完全，如果没有，请手动添加完整：

dtoverlay=dwc2,dr_mode=host

6.2 配置用户名和密码

将TF卡插入Zero 2W，启动系统。第一次启动后会要求登录，用户名和密码均为ubuntu，登录成功后会要求修改密码。

修改完毕后就自动进入系统。

6.3 配置网络

系统启动后并无网络可用，需要进行后续设置后才能使用。

注意，系统默认并没有集成ifconfig工具，只有ip命令可用。

执行ip addr查看并记录下网卡名称：

其中2号网卡（enx00e0996df1cd）为USB转百兆网卡，3号网卡（enxac4bb3b9ebe5）为4G模组。

然后运行下面的命令，打开网卡配置文件：

sudo nano /etc/netplan/50-cloud-init.yaml

按照下图编辑网卡配置文件：

保存退出，然后重启。

重启后即可联网，安装net-tools工具以便于使用：

sudo apt install net-tools

6.4 网络及4G模组测试

执行命令ifconfig -a，查看网卡信息：

可见第一块网卡为USB转百兆网卡，第二块网卡为4G模组。

通过使用-I参数，在ping包时加上网卡名称，可指定哪块网卡进行ping包。通过这两块网卡对腾讯的网站的ping包测试可知，第二块网卡即4G模组支持ipv6：

6.5 AT命令交互测试

安装minicom工具：

sudo apt-get install minicom

然后和树莓派OS一样，通过minicom打开AT命令串口：

sudo minicom -D /dev/ttyUSB0

或

sudo minicom -D /dev/ttyUSB1

即可使用常见AT命令进行交互：

七、OpenWrt系统操作演示

7.1 概述

此扩展板搭配树莓派Zero 2W，在OpenWrt系统下可配置为一进一出的交换机模式，扩展板上的4G可作为WAN口（直接4G上网），网口配置为LAN口，用于连接PC。

7.2 准备工作

本文使用的OpenWrt系统为：openwrt-bcm27xx-bcm2709-rpi-2-squashfs-sysupgrade-linux-6.1.98-20240723.img.gz

烧写OpenWrt系统并上电启动后，我们通过树莓派自带网口连接网线至PC网口，待PC的网卡与树莓派的网口连接成功后，我们在Windows设置中找到网络和Internet，在以太网中打开连接的网络查看默认网关的IP地址，这个地址就是OpenWrt系统的后台配置页面地址，如图所示，本文测试的地址为192.168.198.1：