“How many devices or nodes can be on a PROFIBUS network?” is a common question for PROFIBUS beginners. This information is critical when designing a PROFIBUS network.
In PROFIBUS networks, each segment can have a maximum of 32 nodes, counting master and devices. The maximum number of nodes with addresses in the overall network is 126. The available addresses for PROFIBUS devices are 0-126. The maximum number of devices can only be achieved by employing four RS-485 repeaters.
In this article, you will learn exactly where the maximum node count numbers come from and how to implement it.
Number of Nodes in a PROFIBUS Network
If RS485 transmission technology is used, all field devices are typically connected in a line structure with up to 32 nodes (master and devices) in one segment, as shown in the diagram below. The beginning and end of each segment are provided with active bus termination and supplied with power continuously. The maximum number of nodes within one segment is 32, but if more than 32 nodes are required, or the network span is being extended, repeaters must be used to link multiple segments.
To find out the maximum number of PROFIBUS devices in a network, we need to look a little deeper. PROFIBUS devices must be split into 2 categories, devices without addresses, and devices with addresses. Devices without addresses are either Bus Monitors (components that listen but do not participate in communication on the bus) or bus infrastructures devices, such as RS-485 and fiber optic repeaters.
PROFIBUS is an RS-485 network that conforms to RS-485 rules. RS-485 uses the concept of segmentation on the bus. A segment can be thought of as the maximum length of cable used at a given baud rate before the signal has to be refreshed back to the standard voltage levels and reformed into a square wave. RS-485 stipulates that the maximum number of devices on a segment is 32. A new segment can be started for two reasons, either we have gone the max length for the given Baud rate or have more than 32 devices with RS-485 drivers. When either of these occurs, we need to add a repeater.
What Are the Allowable Addresses for PROFIBUS?
Devices that have PROFIBUS addresses are Masters and Devices. These components have RS-485 drivers.
The maximum number of nodes with addresses in a PROFIBUS network is 126. The available addresses for PROFIBUS nodes are 0-126.
The table below shows the PROFIBUS address numbers and their use.
|0||Reserved for Class 2 master/Engineering tool|
|1||Sometimes used for a programming tool, but available as a master or device address.|
|2-125||Available for master or device addressing|
|126||Reserved for the use of devices without address switches. The device must be set to an address <126 before it can be used|
|127||Reserved as a broadcast address|
We can have a total of 126 masters and devices on a PROFIBUS network, including an Engineering tool. To achieve that number, the network must have a minimum of four RS-485 repeaters. With 126 masters and devices, the network has a total of 130 PROFIBUS devices, counting four repeaters.
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