PROFIBUS cable length

At PROFIBUS DP the baud rate and the maximum cable length are irrevocably connected to each other. The higher the transmission speed, the less cable length can be installed per segment. These cable lengths per baud rate are defined in the PROFIBUS standard. A PROFIBUS user has to fully grasp these guidelines, as changes in cable length or baud rate have consequences for each other.

The following three aspects are important for the design of a PROFIBUS DP network:

• Cable length
• Baud rate
• Repeater

Installation guidelines

The user can select a baud rate that lies between 9.6 Kbps and 12 Mbps. The corresponding maximum cable length allowed per segment is established in the PROFIBUS standard:

• 9.6 kbps = 1200 meter
• 19.2 kbps = 1200 meter
• 45.45 kbps = 1200 meter
• 93.75 kbps = 1200 meter
• 187.5 kbps = 1000 meter
• 500 kbps = 400 meter
• 1.5 Mbps = 200 meter
• 3 Mbps = 100 meter
• 6 Mbps = 100 meter
• 12 Mbps = 100 meter

The above guidelines only apply if you change a PROFIBUS cable within the IEC 61158 norm. If an alternative cable is changed, then the above table does not apply and the long term consequences with regards to reliability cannot be guaranteed.

Whatever the cable length and the baud rate, the number of devices per segment remains at 32. It is recommended to maintain a distance of one meter between two devices with baud rates of 1.5 Mpbs.

Exceptions

There are exceptions to the PROFIBUS installation guidelines: cables that need to comply with special environmental conditions are not always able to reach the maximum cable length and deliver reliable communication. This is indicated in the data sheets of cable manufacturers.

Solution to increase the cable length

The relatively limited cable lengths (at high baud rates) can be increased by applying PROCENTEC repeaters, ProfiHubs and ComBricks. The repeaters functionality makes it possible to start a new segment in which the cable length can be applied, including the 31 devices (number 32 is the repeater itself).

The PROCENTEC training courses are a reliable starting point before beginning with PROFIBUS. With the training, you can overcome frequently made architectural mistakes. PROCENTEC training courses are also a great opportunity to discuss your personal situation with experts and receive advice. See starting training dates.