Modbus-TCP connections offer many advantages over RTU-RS-485 alternatives. Image: PV-Tech.
In recent years the market has seen some commercial and industrial (C&I) inverter models and associated data loggers without RS-485 connection ports. This leads to the question, will the Modbus RTU RS-485 link be phased out?
What is the difference between Modbus RTU RS-485 and Modbus TCP?
Modbus RTU RS-485 is a Remote Terminal Unit (RTU) Modbus over an RS-485 serial connection, while Modbus TCP is Modbus over an Ethernet connection.
With Modbus TCP, the Modbus data is encapsulated in a TCP / IP packet and uses IP addresses to communicate with slave devices. This means that Modbus TCP devices can be easily connected and communicated via existing Ethernet and fiber optic networks. Therefore, any Ethernet network that supports TCP / IP should support Modbus TCP directly. In addition, Modbus TCP allows many more addresses than Modbus RTU RS-485, including multiple masters and speeds in the gigabit range.
While Modbus RTU is limited to 247 nodes per network, Modbus TCP networks can have as many slaves as the physical layer can handle. Often times that number is somewhere around 1,024.
The following table contains the basic differences with regard to the application in C&I PV Solar Equipment.
|Modbus RTU RS-485||Modbus-TCP|
|RTU has been around for a long time and is supported by many inverter, power meter, and measurement station / sensor manufacturers.||Modbus TCP / IP can be easily integrated into a new or existing Ethernet network (intranet or Internet).|
|The maximum number is 247 nodes per network||Up to 1,024 devices per network|
|The maximum distance of RS-485 without using a repeater is 1.2 km at baud rates of up to 90 Kbps.||This depends on the capacity of the communication cable (ie 100 m for a Cat6) and can use the existing intranet / internet network.|
|Can reach 10 Mbit / s over 10 m.
(Operating speed depends on the cable length)
|Modbus TCP data is sent at the speed of your Ethernet connection.
(Most Modbus / TCP hardware these days is 100 Mbit / s)
Data logger compatibility
Inverter and PV monitoring companies offer many data logger devices with the RS-485-Modbus-RTU and the Ethernet-Modbus-TCP ports on the device. The data logger is the communication bridge between the inverter and the monitoring peripheral devices such as power meters and weather stations. The inverter often communicates with the data logger via Modbus TCP Ethernet, in contrast to electricity meters and weather stations, which normally only have Modbus RTU RS-485 connections.
A PV monitoring weather station with a Modbus-TCP-Ethernet connection would offer a significant integration advantage. A Modbus TCP connection at the station enables even faster and easier setup with configuration options activated via an embedded web server, web browser software program, or mobile app.
Remote monitoring and communication
Modbus TCP is an open protocol that is often chosen to harmonize communication and to enable the connection of devices from industrial and energy sectors over longer distances via TCP / IP.
To enable real-time monitoring, larger PV power plants require reliable two-way communication with low latency. This monitoring level provides accurate information on electrical and environmental parameters and enables the system operator to evaluate overall performance and identify abnormal conditions and faults.
The simple and efficient Ethernet communication of Modbus TCP with the local SCADA system or other data acquisition platforms enables remote monitoring. Command communication between the “direct marketer” (direct marketer who normally trades PV-generated electricity) and remotely controllable PV systems is also possible in a TCP / IP inverter network.
A Modbus-TCP weather station
The upcoming PVMet 200 TCP from RainWise is the first compact PV monitoring weather station with a native Modbus TCP Ethernet connection. In the past, the only way to provide TCP-Ethernet connectivity to Modbus RTU devices was through expensive stand-alone Modbus RTU to Modbus TCP converter gateways.
O&M service companies can benefit from easy setup via an embedded web server and remote access for trouble-free firmware upgrades. Future PVmet-TCP models will also allow remote calibration updates of thermopile sensitivity. Irradiation sensors have a number of sensitivity calibrations that can vary from sensor to sensor. In most cases, especially with thermopile sensors, each sensor is supplied with a sensitivity constant; this is typically expressed as microvolts per wattmeter squared (µV / Wm-2).
The main advantage of this direct TCP connection is that you can monitor the device from anywhere. The disadvantage is that your inverter becomes a target for hackers. In some cases, your solar inverter presents itself directly as an internet-enabled device, so the use of VPN security and firewall devices in a TCP / IP network with Modbus TCP devices is a must.
The rapid adoption of Ethernet in the process control and automation industry has enabled Modbus TCP to become the most widely used, fastest growing and supported industrial protocol. In contrast to Modbus RTU, Modbus TCP enables several masters to query the same slave device at the same time. This is allowed because TCP / IP allows various messages to be sent, buffered, and delivered over Ethernet without the need for token passing or full bus control, which is often the case with the RS-485 protocol.
We are confident that the Modbus TCP Ethernet connection will become increasingly popular for most devices in an inverter environment. RainWise has direct and timely examples of Modbus TCP demand for weather stations, especially from leading inverter companies such as SMA and FIMER / ABB.
But we can also assume that Modbus RTU with the serial RS-485 connection will exist in some form for a long time to come.