In the PROFITech Certified PROFIBUS Engineer classes one of the more frequently asked questions is “How many devices can be on a PROFIBUS network?” The short answer is that up to 126 Masters and slaves can be on a single PROFIBUS network.
To find out the number of PROFIBUS devices 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 (things that listen but do not participate in communication on the bus) or bus infrastructure devices. Examples of infrastructure devices are RS-485 repeaters and Fiber Optical Repeaters. All of these devices have RS-485 drivers.
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 that can be 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 (entities with RS-485 drivers) on a segment is 32. So, we need to start a new segment for 2 reasons, either we have gone the max length for the given Baud rate or we have more than 32 devices with RS-485 drivers. When either of these occurs, we need to add a repeater.
And what are the allowable addresses?
Devices that have PROFIBUS addresses are Masters and Slaves. These devices also have RS-485 drivers. The maximum number of devices with addresses is 126. The available addresses for PROFIBUS devices are 0-126.
|0||Reserved for Class 2 master/Engineering tool|
|1||Sometimes used for a programming tool, but available as a master or slave address.|
|2-125||Available for master or slave addressing|
|126||Reserved for use of slaves without address switches. The slave 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 master and slave devices on the bus including an Engineering tool. To achieve that number we have to have a minimum of 4 RS-485 repeaters.
So the long answer is that to talk to 126 master and slave devices, we have to have 130 PROFIBUS devices total.
PROFIBUS Consulting Engineer
PROFI Interface Center