What is Modbus protocol?
MODBUS, developed by Modicon (now Schneider Electric) in 1979, is a means of communication with multiple devices through a single twisted pair. At first, it worked on RS232, but then it used for both RS485 to get higher speed, longer distance, and multi-drop network. MODBUS has quickly become a common standard in the automation industry, and Modicon has launched to the public as a free protocol.
MODBUS is a Master-Slave system, Master is connected to one or more Slave. The master is usually a PLC, PC, DCS, or RTU. Slave MODBUS RTU is usually field devices, all connected to the network in multi-drop configuration. When a MODBUS RTU owner wants information from the device, he will send a message about the data needed, summarizing error detection to the device address. All other devices on the network will receive this message but only the specified device will respond.
What modbus standards are in common use?
Currently, there are 03 modbus standards that are commonly used in industry – automation: Modbus RTU, Modbus ASCII, Modbus TCP
All data are sent in the same format. The only difference between the 3 MODBUS types is how the data is encrypted. Detail:
- Modbus ASCII:
All data is encrypted using hexadecimal, using 4-bit ASCII properties. For every byte of information, two communication bytes are needed, double that of MODBUS RTU or MODBUS/TCP. However, MODBUS ASC II is the slowest of the three types of protocols, but it is suitable when the telephone modem or the connection uses radio waves because ASC II uses message identification features. Because of this delineation feature, any trouble in the transmission medium will not cause the receiver to misinterpret information. This is important when it comes to slow modems, cell phones, noisy connections or other fastidious media.
- Modbus RTU:
Data is encoded in binary, and only needs one communication byte per data byte. It is ideal for RS 232 or RS485 multi-point networks, ranging from 1200 to 115 baud rates. The most popular speeds are 9600 to 19200 baud. MODBUS RTU is the most widely used industrial protocol, so most of this article will only focus on its basis and application. More details on the Modbus RTU protocol can be found here.
- Modbus TCP:
MODBUS / TCP is simply MODBUS over Ethernet. Instead of using this device for connecting to slave devices, so the IP addresses are used. With MODBUS / TCP, MODBUS data is simply summarized in a TCP / IP packet. Therefore, any Ethernet network that supports MODBUS / IP will immediately support MODBUS / TCP.