RS232 and RS485

As an engineer at Yeaplink, I have recently conducted an in-depth study of the principles behind RS232 and RS485 interface data communication in industrial 4G routers. This study note records my research findings and reflections, which I hope will help me better apply these technologies in future industrial IoT applications.
In the field of industrial automation, RS232 and RS485 interfaces remain crucial means of connecting various devices. As a core function of industrial routers, understanding the working principles of these two interfaces is essential for optimizing communication performance and solving practical problems.
Let’s first delve into the working principle of the RS232 interface. RS232 is a point-to-point serial communication standard, initially designed for communication between computer terminals and modems. In our industrial routers, RS232 is mainly used for short-distance, low-rate data transmission.
RS232 uses an unbalanced transmission method, representing logical states through voltage differences between the data signal and ground. Specifically, -3V to -15V represents logic “1” (mark state), while +3V to +15V represents logic “0” (idle state). This negative logic design was intended to maintain signal integrity in long-distance transmission, as negative voltages attenuate less during transmission.
In the signal transmission process, RS232 adopts asynchronous communication. Each data byte has a start bit, 5 to 8 data bits, an optional parity bit, and 1 to 2 stop bits. The start bit is always logic “0”, used to synchronize the receiver’s clock. This self-synchronizing mechanism eliminates the need for a separate clock line in RS232, simplifying the interface design.
However,

RS232

has its limitations. Due to its use of single-ended signals, it has relatively weak anti-interference ability, and transmission distance is typically limited to within 15 meters. Moreover, while its maximum transmission rate can theoretically reach 115.2kbps, in actual industrial environments, we usually limit it to 9600bps or 19200bps to ensure communication stability.

In contrast, the RS485 interface adopts a completely different design philosophy, more suited to industrial environment needs. RS485 uses balanced differential signal transmission, greatly improving anti-interference ability and transmission distance. In our industrial routers, the RS485 interface can support communication distances up to 1200 meters and connect up to 32 devices on a single bus, making it very suitable for building complex industrial networks.
The core of RS485 lies in its differential signal transmission mechanism. It uses two signal lines (usually called A and B lines) to transmit data. The logical state is determined by the voltage difference between these two lines: when the A line voltage is 2V higher than the B line, it represents logic “1”, and vice versa for logic “0”. This differential signal design allows RS485 to effectively cancel out common-mode interference, as external interference typically affects both signal lines simultaneously, while the receiver only cares about the voltage difference between the two lines.
RS485 also adopts asynchronous communication, with a data frame structure similar to RS232. However, due to its excellent signal transmission characteristics, RS485 can support higher baud rates, reaching up to 10Mbps over short distances. In practical applications, though, we usually choose appropriate baud rates based on transmission distance and environmental conditions to balance speed and reliability.
In terms of multi-point communication, RS485 adopts a half-duplex mode. This means that at any given moment, only one device on the bus can send data, while other devices are in receiving state. To coordinate communication between multiple devices, we typically implement a master-slave protocol (such as Modbus RTU) at the application layer, with the master device (usually our industrial router) controlling the communication process and polling each slave device.
Now, let’s discuss how these two interfaces are implemented in our industrial routers. From a hardware perspective, both RS232 and RS485 interfaces require specialized transceiver chips to handle signal conversion. For RS232, we use chips like MAX232 to convert TTL levels to RS232 levels. For RS485, we use transceiver chips like MAX485 to convert between TTL and RS485 differential signals.

Components of RS485 Interface

In circuit design, we pay special attention to several key points: First is power isolation. Considering the potential presence of large equipment and electromagnetic interference in industrial environments, we add optocoupler isolation between the RS232/RS485 interface and the router’s internal circuits, effectively preventing ground loops and common-mode interference. Second is overvoltage protection; we add TVS diodes to the signal lines to prevent damage to chips from electrostatic discharge and transient overvoltage.
From a software perspective, the key to implementing these two interfaces lies in flexible drivers and efficient protocol stacks. At the driver layer, we need precise control of UART (Universal Asynchronous Receiver/Transmitter) parameters, including baud rate, data bits, stop bits, and parity mode. Especially for RS485, we also need to implement direction control, quickly switching between sending and receiving.
At the protocol layer, we implement common industrial communication protocols, such as Modbus RTU. This involves complex frame parsing, verification, and assembly processes. For example, in Modbus RTU, each data frame contains device address, function code, data, and CRC check. Our protocol stack needs to be able to quickly parse these fields, execute corresponding operations, and generate response frames.
A particularly noteworthy point is timing control. In RS485 networks, precise timing control is crucial as multiple devices share the same bus. For example, after sending a data frame, we need to ensure that the last byte has been completely sent before switching to receiving mode. This usually requires microsecond-level timing control.
Another challenge is handling communication errors. In harsh industrial environments, signal interference and data errors are inevitable. Our software needs to be able to detect these errors (through checksums or CRC) and implement appropriate error recovery mechanisms, such as retransmission or alarms.
In practical applications, we often need to forward RS232 or RS485 data to Ethernet or cellular networks. This involves protocol conversion and data encapsulation. For example, we may need to encapsulate Modbus RTU data into Modbus TCP packets, or encapsulate serial data into custom TCP packets for transmission. This process not only needs to consider data format conversion but also handle issues like network latency and packet loss.
In terms of optimizing communication performance, we have taken several key measures. First, we implemented a data buffering mechanism. For RS485 multi-point networks, we reduce the frequency of bus switching by batch reading data from multiple devices, improving overall communication efficiency. Second, we implemented an adaptive baud rate detection function. This allows our routers to automatically adapt to different devices’ communication parameters, greatly improving system flexibility and ease of use.
Security is another important consideration. Although RS232 and RS485 themselves do not provide encryption functions, we have implemented data encryption and device authentication mechanisms at the application layer. This is crucial for protecting sensitive industrial data, especially when this data needs to be transmitted over public networks.
In actual projects, we often encounter tricky problems. For example, in a long-distance RS485 network, we found that some devices occasionally experienced communication errors. Through careful analysis, we discovered this was due to signal reflection. The solution was to add appropriate termination resistors at both ends of the bus and optimize the wiring method, ultimately significantly improving communication reliability.
Another interesting case was in a complex industrial environment where the client needed to communicate with different devices using both RS232 and RS485 simultaneously. We implemented a flexible port mapping mechanism through software, allowing users to dynamically configure the working mode and protocol of each serial port. This greatly improved system flexibility, enabling a single router to adapt to various complex communication needs.