Main Differences Between Modbus Protocol and USS Protocol – How to Choose

Modbus Protocol Modbus was developed by Modicon (now Schneider Electric) in 1979 as a serialcommunication protocol, originally used for data exchange between PLCs and has become the earliest and most widely adopted serial communication standard in industrial automation. It has several variants, including Modbus RTU, Modbus ASCII, and Modbus TCP, supporting communication through RS-232/RS-485 serial communication and Ethernet transmission. Modbus uses a master-slave structure, with the master station initiating requests and multiple slave stations responding to them. It employs a linear bus topology, connecting devices through the physical layer of RS-232 or RS-485. The Modbus protocol is simple and stable, widely used in various industrial automation systems, and is still evolving, supporting new variants such as Modbus/TCP. USS Protocol USS (Universal Serial Interface) protocol is a serial communication protocol developed by Siemens, specifically designed for controlling its AC and DC drive series, supporting read/write process data, monitoring registers, and modifying drive parameters via the bus. The USS protocol also employs a master-slave structure, with the master station typically being a PLC and the slave station being the drive, using the RS-485 physical layer. It allows for precise control and monitoring of the drives, and is widely used in Siemens’ SINAMICS and MICROMASTER variable frequency drive series. The USS protocol has evolved into the Profibus-DP drive configuration file, suitable for a wider range of devices, such as Danfoss, Eurotherm, and others. Compared to Modbus, the USS protocol is customized specifically for Siemens’ drives and controllers, and offers higher customizability and precision in data exchange and control between devices. Three, the main differences between the USS protocol and the MODBUS protocol Communication Architecture: Master-Slave Mode and Data Transmission Modbus and USS protocols both use a master-slave mode, where the Modbus master can be any device (such as PLCs, computers, etc.), and the slave is external devices like sensors, actuators. The master transmits data through requests and responses, with the master sending requests and the slave returning data. The USS protocol’s master is typically a Siemens PLC (such as the S7 series), and the slave is a Siemens controller. USS not only transmits control parameters but also supports retrieving and modifying operating parameters, focusing on drive control and monitoring. Data Transmission: Information Structure Morphology Coding In data transmission, the Modbus protocol transmits data based on memory (registers), with each memory capable of carrying different types of data information, such as discrete input, discrete output, input registers, holding registers, etc. This type of memory plays a crucial role in data transmission, and access to this memory is achieved using different function codes. USS protocol is more focused on the operational control supervision of the driver. It has a simple message format, including key transmission control codes (such as run, stop, speed adjustment, etc.) and device configuration information. Because the application of the USS protocol is more specialized, its functions are much simpler than those of the Modbus protocol. (3) Main Uses and Equipment Applicability Modbus protocol is widely used for communication between various machine devices, such as PLCs, sensors, variable frequency drives, actuators, etc. The advantage of Modbus is its openness, allowing equipment manufacturers to complete Modbus applicability according to actual conditions, making it the required communication protocol for cross-brand equipment rooms. USS protocol is specifically used for Siemens machinery and equipment, particularly in the communication between its variable frequency drives, controllers, and PLCs. Although the USS protocol is limited to machines and equipment from special manufacturers, it has obvious advantages in terms of compatibility modes and efficiency between such devices. Performance and Transmission Speed The performance of the Modbus protocol may be limited by its data file structure and transmission speed in certain cases. Particularly in serial communication mode, the transmission speed of Modbus RTU is generally low, although it can be increased through Modbus TCP, but the communication latency and network bandwidth still remain a limiting factor. USS protocol is more efficient in reducing controller performance. Due to its specialized design for Siemens drives and PLCs, USS can provide faster communication speeds and lower latency between various devices. It is dedicated to transmitting data for a few actual operations rather than complex machine communication, and therefore, its performance is significantly better than Modbus in certain applications. Stability and Scalability The stability of the Modbus protocol is relatively high, especially in industrial production environments, where it has been proven to be safe and reliable during long-term operation in various equipment rooms. However, Modbus’ scalability is somewhat limited, particularly when it is necessary to connect many devices and when complex algorithm designs are required, as Modbus may not be able to meet more complex requirements. USS protocol demonstrates relatively high stability and reliability in the control of Siemens machines, but its scalability is relatively weak, as its design is crucial for meeting the needs of special machine equipment (such as Siemens variable frequency drives). If it involves communication across machines of different well-known brands, the applicability range of the USS protocol will be very limited. Four, Reference Standards for Protocol Selection When choosing between Modbus or USS protocols, customers should make decisions based on the model of the device, business requirements, and their communication natural environment. If interoperability between multiple brands of equipment needs to be achieved, Modbus can be a more suitable choice. Its openness and cross-brand compatibility mode are widely applied in various industrial automation. However, if the application field involves the control and supervision of many Siemens machines, the USS protocol has more advantages due to its high efficiency and professional design.