This section discusses the remote applications for Modbus TCP/IP. Modbus TCP/IP was developed mainly from another protocol, namely Modbus RTU (remote terminal unit).
This was used until companies started to converge towards the use of Ethernets and the birth of the Internet. The Modbus/TCP protocol is basically the command/response Modbus RTU protocol wrapped up in a TCP packet, meaning limitations of the protocol in advanced applications still exist. The Modbus/TCP protocol is an application-layer protocol, and as such, resides on top of the TCP/IP and Ethernet layers.
The protocol is most commonly used by large Multi-national corporations who work in the primary sector of industry i.e. oil companies, gas power stations e.t.c. It is often used in place of mechanical relays to perform actions on remote devices. It can also be used for monitoring purposes.
Applications of Modbus
A typical application would be Tank level measurement from a reservoir or water tower back to a pump or lift station for the purpose of pump control and would look something like this:
In the above diagram the level transmitter (LT) is used to monitor the level of the tank and it is connected to a PLC, which we will name PLC1, which is subsequently connected to manual controls for the separate pumps. PLC1 is then connected through phone lines or more typically the companies own dedicated lines to a second PLC, which is used to control the pumps.
Modbus TCP/IP would be used between the PLCs to allow commands at one end to communicate the appropriate response to the relevant part.
The modbus system would also allow users to perform maintenance and repair on remote devices from the office using a PC and browser thereby reducing support costs and improving customer service.
Furthermore the ability to log onto a plant’s control system from home allows the maintenance engineer to maximize his plant’s uptime and reduce the number of times that he is called out from home. In addition, managing geographically distributed systems becomes easy using commercially available internet/intranet technologies.
Another aspect of Modbus TCP/IP is the fact that some systems allow for a computer generated log to be viewed showing all past actions. This would allow for operators and instrumentation engineers to easily trace the source of a problem and remedy it or put in place to controls to prevent it happening again.
This trace-ability would not be easily used in a system of mechanical relays. Furthermore, problems can often be scouted and action taken before they become effective and have a negative effect on the businesses operation.