Tech Tip: Installing PROFIBUS Terminators
Training, installation and design guru Andy Verwer provides more essential information for installing PROFIBUS networks correctly and efficiently. Good training can really help, he says.
It is widely known that PROFIBUS cabling must be properly “terminated” in order to stop reflections on the cable that can corrupt telegrams and give bad communication, or even worse, give intermittent problems.
The terms “terminated” and “terminator” refer to resistor networks that are placed at the ends of the cable to absorb the energy of the signal and so avoid any reflections.
Termination is not unique to PROFIBUS
Any high speed or long cabling is subject to reflections. However, it is only really a problem with high frequency signals and/or longer cable lengths.
On PROFIBUS DP (RS485 wiring) we use a simple network of three resistors that is powered by a 5V supply. The resistors are normally in the connector, but they can be in the device itself.
The resistor values are chosen to match the characteristic impedance of the PROFIBUS cable (nominally 150Ω) and the 5V power produces a bias across the wires of about 1V which gives immunity to up to 0.5V of interference or pickup on each wire.
Don’t succumb to “general ignorance”
There are many problems that can occur caused by general ignorance of the rules for where and how to apply terminations.
The rules for termination are very simple:
- Each segment should be terminated at the two ends and nowhere else.
- The terminations must be powered at all times.
A segment here means a section of cable that is isolated from other sections by repeaters, fiber optics or other such devices.
We will have a quick look at one or two problems that are commonly seen. Firstly, it is not unusual to see resistors incorrectly switched on in the devices. This will give additional termination, which could be in the middle of the segment or even give double termination at the end. Either will cause significant reflections.
Another more subtle misunderstanding involves what are called spur lines or stub lines. This is where a Tee junction is used to branch off the main segment line. Spur lines are generally to be avoided in high-speed networks, but short spurs can be used at lower speeds.
Unfortunately each Tee junction that is introduced in a segment gives us another cable end.
The rule says terminate at the “two” ends, but now we have more than two. These spur lines, if used must not be terminated.
Never have more than two terminations on a segment. Better still, avoid using spurs!
A more subtle problem occurs with the power to the terminators. If the last device on our segment is powered down or unplugged (for example when isolating a drive, or replacing a device), the missing power on the termination network will cause reflections and also the 1V bias giving noise immunity will disappear.
An example of this last problem was recently found on a wind turbine installation. The turbine employed a PROFIBUS network to communicate with sensors and actuators in the turbine head where the blades are mounted. This turbine was showing occasional control system errors which caused the generator to fail fairly regularly.
To locate the problem the engineers lowered the mast and did a check on the system. The checks were carried out by connecting an engineer’s panel to the end of the PROFIBUS cable. Upon checking the system each time no problems were found.
So the engineer’s panel was unplugged and the turbine raised again to operational position. Unfortunately the problems continued. What was going on?
Well the engineer’s panel was supplying the power to the terminator at the segment end, but of course only when it was connected. As soon as the panel was unplugged they had an unpowered terminator.
This was actually easily solved by introducing a separate active terminator which was permanently powered and independent of the engineer’s panel.
Good training helps
There are many ways that the simple installation rules can be misunderstood or incorrectly applied. Certified installation training is available from accredited PI training centers across the world and this one-day course helps to avoid problems caused by incorrect design, layout and installation. Further details can be found HERE for North America.
Andy Verwer is director of Verwer Training & Consultancy Ltd, Technical Officer for the UK PROFIBUS Group and a leading member of the PROFIBUS International working groups for training, installation and design. Andy has over 40 years of experience in automation and control systems across a wide range of industries, and he delivers training worldwide. Further details HERE.