RS485 Repeater in Profibus Network

The devices in the Profibus network can be divided into four types: Class 1 Master, Class 2 Master, Slave and network connector. The task of the first-class master station is to control the operation of the entire network and exchange data with the slave stations, which is usually performed by a PLC with a DP interface; the second-class master station is used to configure and diagnose the network, usually from the host computer. Responsible; the slave station is connected to the underlying equipment on site, collects on-site signals such as sensors and drivers, and transmits them to the master station and executes the commands sent by the master station; the network connector is used to connect the master station and the slave station, and at different levels to the network to expand. Including: repeaters, couplers, cables, connectors, etc.

First of all you need to know: What is the repeater used for? Why emphasize the RS485 repeater? When should repeaters be used?

What is the repeater?

A repeater is a device that works at the physical layer (OSI reference model), and its function is to re-amplify the received, attenuated signal and send it out. Since the physical layer of Profibus-DP uses RS485, optical fiber and wireless technology (Profibus-PA uses MBP technology, which will be discussed later), and usually it is expanding the Profibus-DP network based on RS485, so RS485 is used. repeater. Of course, if you’re extending a wireless network, use a wireless repeater. This article only discusses the RS485 repeater under the Profibus-DP network.

When should repeaters be used?

In the following two cases, you need to use an RS485 repeater.

1) The length of the wiring cable exceeds the maximum network segment length at the communication rate;

2) The number of devices in the network segment exceeds 32;

The Profibus network can use different communication rates. Under different communication rates, the supported maximum network segment length is different; if the length of the wiring cable exceeds the maximum network segment length, a repeater needs to be used to amplify the signal.

RS485 stipulates that the number of devices in the same network segment cannot exceed 32. The physical layer of Profibus-DP adopts RS485 (except optical fiber), so it also follows the same rules. When the number of devices in the same network segment exceeds 32, a repeater is required.

The RS485 repeater can divide the network into two network segments, the two are completely isolated, and each network segment can reach the maximum communication distance. For example, at a communication rate of 1.5M bit/s, the maximum network segment length is 200 meters. Then both network segment 1 and network segment 2 can reach 200 meters, and the total can reach 400 meters.
The RS485 repeater itself also belongs to the slave device of Profibus-DP (although the DP address does not need to be set). As mentioned earlier, the RS485 network supports up to 32 devices. Therefore, if the RS485 repeater is used, both network segments can be used separately. Connect 31 more devices.

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