Common PLC Interfaces and Communication Protocols Worth Learning

4. Support for Multi-point CommunicationRS232: Only one transceiver is allowed to be connected on the bus, and it does not support multi-station transceiver capability, so it can only communicate point-to-point and does not support multi-point communication.

RS485: The RS485 interface allows up to 128 transceivers to be connected on the bus. It has multi-station communication capability, allowing users to easily establish a device network using a single RS485 interface.

5. Difference in Communication LinesRS232: Can use three-core twisted pair cables, three-core shielded cables, etc. RS485: Can use two-core twisted pair cables, two-core shielded cables, etc. In low-speed, short-distance, and interference-free situations, ordinary twisted pairs can be used; conversely, in high-speed, long-distance transmissions, dedicated RS485 cables (STP-120Ω for RS485 & CAN) with impedance matching (generally 120Ω) must be used, and in environments with severe interference, armored twisted shielded cables (ASTP-120Ω for RS485 & CAN) should be used.

Supplement: Since the transmission distance of RS232 is only 15 meters, what is its use?In fact, its application is very wide; it can connect various devices, such as monitoring, upgrading, or debugging other devices, etc. Its function is quite similar to USB. With the increasing prevalence of USB ports, more devices that convert USB to RS-232 or other interfaces will emerge. Through the USB interface, more RS-232 devices can be connected, not only achieving higher transmission speeds and true plug-and-play but also solving the problem of USB interfaces not being able to transmit over long distances (the USB communication distance is within 5 meters).

Difference Between RS485 and MODBUS:RS485 is a physical interface, simply put, it is hardware. MODBUS is an internationally standardized communication protocol used for data exchange between devices from different manufacturers (usually for industrial purposes); the so-called protocol can also be understood as the “language” mentioned by someone above, simply put, it is software.

Generally, two devices transmit data through the MODBUS protocol: initially, RS232 was used as the hardware interface (that is, the serial communication port on ordinary computers); there are also RS422 and the commonly used RS485. This interface has a long transmission distance and is used more in general industrial sites. The MODBUS protocol is divided into three modes: MODBUS RTU, MODBUS ASCII, and the later developed MODBUS TCP. The first two (MODBUS RTU, MODBUS ASCII) use serial communication (RS232, RS422, RS485) as their physical hardware interface. MODBUS TCP, on the other hand, is designed to keep pace with today’s world development trend, where everything can be connected via Ethernet or the Internet to transmit data. Thus, the MODBUS TCP mode emerged, and the hardware interface for this mode is the Ethernet port, which is the network port commonly used on our computers.

Then we can understand through an image the communication protocols supported by S7-200 PLC.

Modbus Communication

Modbus was invented by Modicon (now a brand of Schneider Electric) in 1979 and is the world’s first real bus protocol used in industrial sites. To better popularize and promote Modbus in distributed applications based on Ethernet, Schneider Electric has transferred ownership of the Modbus protocol to IDA (Interface for Distributed Automation) and established the Modbus-IDA organization, laying the foundation for the future development of Modbus. In China, Modbus has become the national standard GB/T19582-2008. According to incomplete statistics, by 2007, the number of Modbus node installations had exceeded 10 million.

The Modbus protocol is a universal language applied to electronic controllers. Through this protocol, controllers can communicate with each other, and controllers can communicate with other devices over a network (such as Ethernet). It has become a general industrial standard. With it, control devices produced by different manufacturers can be connected to form an industrial network for centralized monitoring. This protocol defines the message structure that a controller can recognize, regardless of the network used for communication. It describes the process of a controller requesting access to other devices, how to respond to requests from other devices, and how to detect and log errors. It establishes a common format for the message domain layout and content. Modbus is a single-master master/slave communication mode. There can only be one master on a Modbus network, while the number of slaves can be several.

Modbus has the following features:

1. Standard and open, users can use the Modbus protocol freely and confidently without paying license fees or infringing on intellectual property rights. Currently, more than 400 manufacturers support Modbus, and more than 600 types of products support Modbus.

2. Modbus can support multiple electrical interfaces, such as RS-232, RS-485, and can also be transmitted over various media, such as twisted pairs, optical fibers, and wireless.

3. Modbus’s frame format is simple, compact, and easy to understand. It is easy for users to use and simple for manufacturers to develop.

Note: The S7-200 only supports the Modbus RTU protocol and does not support the Modbus ASCII protocol;

ProfiBus Communication

As one of the many fieldbus family members, ProfiBus is the most widely used fieldbus standard in the European industrial sector and is currently one of the internationally accepted fieldbus standards. ProfiBus belongs to the unit-level and field-level SIMATIC network, suitable for transmitting small to medium amounts of data. Its openness allows many manufacturers to develop their products that comply with the ProfiBus protocol, which can be connected to the same ProfiBus network.

ProfiBus is an electrical network, and the physical transmission medium can be shielded twisted pairs, optical fibers, or wireless transmission. It officially became an international standard for fieldbus in 1989. PROFIBUS is an international, open, device-independent fieldbus standard, with transmission speeds selectable in the range of 9.6kbaud to 12Mbaud. When the bus system starts, all devices connected to the bus should be set to the same speed. PROFIBUS is widely used in manufacturing automation, process industry automation, and automation in other fields such as buildings and transportation power. PROFIBUS is also a fieldbus technology used for workshop-level monitoring and data communication and control at the field device level. It can achieve decentralized digital control and field communication networks from the field device level to workshop-level monitoring, providing feasible solutions for comprehensive factory automation and intelligent field devices.

USS Communication

USS (Universal Serial Interface) is a communication protocol developed by Siemens specifically for drive devices, which has undergone continuous development and improvement over the years. Initially, USS was used for parameterization of drive devices, focusing more on parameter settings. It is widely used in connections between drive devices and operator panels, as well as debugging software (such as DriveES/STARTER).

Recently, due to its simple protocol and low hardware requirements, USS is increasingly used for communication with controllers (such as PLCs) to achieve general level communication control. (Note: USS provides a low-cost and relatively simple communication control method; due to its design, USS cannot be used in situations requiring high communication speeds and data transmission volumes. In these high-demand communication scenarios, a more real-time communication method, such as PROFIBUS-DP, should be selected. When designing the system, this limitation of USS must be considered.

For example, if USS communication control is used for dozens of frequency converters in applications requiring high-speed synchronization (such as paper production lines), the effect can be imagined.

All Siemens frequency converters come with an RS485 communication port, and the PLC, as the master, allows a maximum of 31 frequency converters as slaves in the communication link. Based on the address of each frequency converter or using broadcast methods, it can access the frequency converters that need to communicate. Only the master can issue communication request messages, and the address characters in the message specify the slave to which data is to be transmitted. The slave can only send data to the master after receiving the master’s request message; slaves cannot directly exchange data with each other.

Before using the USS protocol, the Siemens instruction library needs to be installed first. The USS protocol instructions are in the library folder of the STEP7-MICRO/WIN32 instruction tree, which provides 14 subprograms, 3 interrupt programs, and 8 instructions to support the USS protocol. When calling an instruction, one or several subprograms will be automatically added.

Basic Features of the USS Protocol:

The working mechanism of USS is that communication is always initiated by the master. The USS master continuously polls each slave, and the slave decides whether and how to respond based on the received instructions. The slave will never actively send data.

Slaves should respond under the following conditions:

1. The received master message has no errors;

2. The slave is addressed in the received master message.

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