Profibus Cable, Connector And Termination Tips

Profibus or Process Field Bus is a standard within field bus communications in the automation industry. A network for industrial computers made to withstand high amounts of noise. The two wire Profibus cable makes different network topologies possible, like the star, the tree and the line or a combination.

Profibus comes in four variants, each with a different purpose:

  • PROFIBUS DP (Decentralized Peripherals) used to drive sensors and actuators via a central controller. Dataspeed up to 12 Mbit/s with twisted pair cables and fiber optic cables are an option.
  • PROFIBUS PA (Process Automation) is used to monitor measuring equipment via a process control system. This Profibus variant is ideal for use in explosive areas (Ex-zone 0 and 1). In the cables flows namely a weak current through the bus lines in an intrinsically safe circuit so that sparks do not occur, even at fault. The con. about this variant is the slow dataspeed at 31,25 kbit/s.
  • PROFIsafe used for safety applications, usually with safety PLC’s.
  • PROFIdrive used in motion control.

 

The standards for the Profibus networking are IEC 61158 (Field Busses) and IEC 61784-1 and -2 (previously EN 50170).

In this article I will be focusing on the Profibus DP network, which is the most used bus network. In fact, the Profibus DP is one of the fastest and most wide spread industrial network.

Using a popular network like Profibus DP is a great option for networks. You will not only have a network used for many industrial applications. You will also have a lot of people with Profibus experience and a lot of resurces. The network is so widely spread out and is taught at most schools and educational institutions. Another advantage of the Profibus networks are the Profibus organization and the firmly stated standards about the networks. This means that the documentation for Profibus is easy to find. In my experience – a quick search on the Internet for most Profibus issue will provide you with tons of great material. The standardization also gives us a big assortment of components, which only makes the components cheaper.

 

The Profibus Cable: How To Make Your Own

Making Profibus cables is done with very little effort. The connections are fairly simple since the cable only has two wires and a shield. But you should be careful. Even though it may look simple, connecting the wires correct is not only important. No mistakes can be made here. Wrong connection will not break any of the components. But I can guarantee that the network will not function if the wires are connected wrong.

 

The Cable

For Profibus networks a shielded AWG 22 or 0.34 mm² twisted pair cable is used. The standard isolation color of a Profibus DP cable is purple, and the two wires in the twisted pair are usually green and red. But the colors of the cable are not important. At least not as important as the specifications of the Profibus cable. It is essential to use the right cables when installing a Profibus network, and to help you with that, the official Profibus organization has published a set of Profibus cable specifications:

  • Twisted pair with shielding braid.
  • Wire gauge: 0.34 mm² or AWG 22.
  • Resistance at max 110 Ω/km.
  • Capacity at least 30 pF per meter.
  • Impedance from 35 to 165 Ω at frequencies from 3 to 20 Mhz.

 

You can read the full specification on the Profibus website. Further specifications of the Profibus cable are in the IEC 61784 (Previously EN 50170 chapter 8-2).

 

The Connectors

Usually, 9-pin D-sub connectors are used in the end of the cables to connect to the devices (recommended in EN 50170). The 9-pin D-sub connectors are the default Profibus connectors on most components. And personally, I have never seen any other connectors than the 9-pin D-sub and in rare cases the  Profibus M12 connector. Here is an overview of the 9 pins in a 9-pin D-sub connector:

 

9-pin D-sub connector male and female

 

Making your own Profibus cables are surprisingly easy. With just a few tools you can cut down the price of your Profibus cables with a significant amount, because pre-made Profibus cables are quite expensive. You can make your own Profibus cable in the exact length you need. But when you are making your own cables, you have to be aware of some important things, though.

 

1. Remove The Isolation With A Profibus Stripping Tool

The first thing to do is to remove the isolation of your Profibus cable. You can use a special Profibus stripping tool or if you are used to work with cables, you can use a knife or a wire cutter. The Profibus stripping tool is worth the money, because it will give you a sharp and perfect stripping of the Profibus cable needed for the connectors. It can be difficult to strip the cables correctly with a knife, so that both the shield and the two wires inside are visible.

 

PROFIBUS cable stripping tool

 

2. Connecting The Profibus Cable And Connectors

The next thing to do is to lay the Profibus cable in the Profibus connector so that the shield and the wires are connected correct. At this point we need to know what pins we need to use. As described before the connectors has 9 pins, but we will only be using some of them. Anyway, we can start by taking a look at how Profibus uses the different pins in the 9-pin D-sub connector:

Pin # Function
Description
1 Shield Shield and functional earth
2 NC Not in use
3 RxD/TxD-P Data recieve and transmit (positive)
4 CNTR-P Control signal to repeater (positive)
5 DGND Reference potential for +5 volt and data
6 VP +5 volt for terminating resistors (active termination)
7 NC Not in use
8 RxD/TxD-N Data recieve and transmit (negative)
9 CNTR-N Control signal to repeater (negative)

 

At first, this might look a bit confusing. It seems like the Profibus uses 7 out of 9 pins in the D-sub connectors. But how is that possible when the Profibus cable only has two wires and a shield?

The answer is that we do not always use all the pins. Not even all 7 pins. For most purposes we will only be using pin 3, 5, 6 and 8. So these are the four pins we will begin with.

 

2 Pins For Data Transmisstion

Pin 3 and pin 8 are used for the data transmission. This is where you will connect the green and the red wire. The green wire is usually the positive or channel A. So, when you are connecting the Profibus cable you should be connecting the green wire to pin 3. The red wire is considered the negative or channel B. Pin 8 is for the green wire.

You can connect the two wires the opposite way or use your own wire colors. As long as you use the same colors in each end of the cables and are consistent. Most Profibus connectors though has a green indicator for where channel A goes and a red indicator for where channel B goes. Using different colors or switching the green and red can cause a lot of confusion.

 

2 Pins For Active Termination

Then we have pin 5 and pin 6. They are used for termination. More on termination of the cables later.

Now, let’s have a look at the standard Profibus connectors. The ones made specially for Profibus. This is how they usually look like:

 

Profibus D-sub connectors for Profibus cable

 

In And Out Of Profibus Connectors

As you can see it is not only clear where channel A and B should be connected. Most of them have two connections for IN and two connections for OUT.

The OUT is where your network will start. When you are building your network topology you should always begin the network from the OUT or output of the first connector. The OUT is also where the network continues. If you have a station connected to the master in a network and you want the network connected to a new slave you should use OUT.

The IN or the input of the connector is where the “network line” ends. So except from the master, all cables should be connected to IN. When extending the “network line” you should use OUT. Here is how it should look:

 

PROFIBUS connectors and cabling IN and OUT

 

Connecting the Profibus cable to the connector should be done with extra caution. Remember, that it is very important not only to connect the wires correct, but also to connect the shield correct and even to strip the wire correct. Take a look at this great instruction video if you’re in doubt about the connector:

 

 

The next thing to consider is the termination of the cables.

 

Termination Of Profibus Cables

To minimize signal reflections the Profibus cable has to be terminated in the end. Signal reflection occurs when in a signal transmitted by a transmission media, such as copper cable or an optical fiber, some of the signal energy may be reflected back, instead of being passed all the way along the cable to the second end. Just like grounding this is such an issue that wrong termination of the Profibus cables will prevent the bus from working.

A Profibus termination is done by inserting a 220 Ω resistor in each end of the Profibus line. So, you insert a 220 Ω resistor in the first and one in the last station. The reason for using a 220 Ω resistor is that the two 220 Ω resistors are connected parallel to each other.  The parallel connection of the two resistors make a total resistance of 110 Ω. The loop resistance of the standard Profibus DP cable is 110 Ω/km. Be aware that a standard Profibus PA cable has a loop resistance of 44 Ω/km.

Most Profibus connectors has a termination option. This means that you don’t have to connect your own resistor manually. You can activate or deactivate termination in the connector by flipping the switch, usually placed at the top of the connector. This is useful in two ways. First of all it makes termination very easy, since you can make all your connections and then flip the switches where you need to terminate the Profibus cable. The second reason this is smart is that troubleshooting becomes easier. Without disconnecting anything you can check if the termination is done correct.

The most common errors when installing a bus network like Profibus is termination.

 

Active Termination In Bus Networks

Profibus uses an extended way of terminating called active termination. This is needed when you have very long communication cables. Active termination can be used to increase the line voltage of the bus.

The only difference between normal termination and active termination is that besides the 220 Ω resistor you also have to connect 5V to the termination (between VP(6) and DGND(5)). This will create a defined ground signal when non of the stations are active. You can make active terminations in three ways:

  • In the connector
  • In the station
  • With a seperate terminator

 

The reason we call this an active termination is that we use the +5 volt in the termination. Usually active termination happens when the termination switch on the connector is flipped. To make that more clear, let us take a look at a scheme for a Profibus connector:

 

Profibus cable active termination in connector

When the microswitch for termination is switched on the connection to pin 5 and 6 will be made. Now you have an active termination. If you want to read more about active termination or electrical termination you should read the article about voltage regulation on Wikipedia. Because active termination is a voltage regulator that makes sure that we have a constant voltage over the terminating resistor.

 

Profibus And Grounding

The connection of the shield is crucial to avoid noise. In fact your Profibus will not even work if you connect the shields incorrect. In addition to the shield, it is also important to connect all stations to ground. In machinery sold in the European Union the grounding wire has to be at least a 16 mm² wire. The simple reason for grounding is to make sure all the stations in out installations works (IEC 60204-1). Another reason is to avoid potential differences between stations in our installation. Normally we wouls use different power supplies in each station, and this alone can create a potential difference. If the shield is connected to two stations with a difference in their potential, a current will flow in the shield and that will make noise. No current should run in our shield and that is why we be absolutely sure that our ground connections are sufficient.

 

Profibus Adressing

Before powering on the Profibus network you have to assign an address to each of the stations. The addresses go from 0 til 125. You can set the address of a station in several ways depending on the specifications of the station:

  • Via a DIP-switch or digit wheel
  • Via. the display
  • In the software by using master class 2.
  • 1 or 3 (micro switch)
  • 1 or 2 (microswitch)

 

Stucture of the Profibus addresses (unwritten rules of addressing)

  1. Low addresses are reserved for Master Class 1 & 2 (addesses: 0,1,2) often default addresses.
  2. Do not use the addresses 124 & 125, since some slaves uses these as default addresses.
  3. The address 126 is reserved for service.
  4. New addresses are only in use after a reboot of the system (OFF/ON). This does not apply to addressing via. Master Class 2)!

 

What Is The Transmission Speed Or Baudrate In Profibus?

The baudrate or the transmission speed is the speed of the whole network. All the stations in a Profibus network has to run at the same transmission speed to be able to communicate. The default baudrate for your Profibus network is set in Master Class 1 and can be between 9.6kbit/s and 12Mbit/s. You may initially think that the faster the speed, the better. But this is not always true, since the speed of your Profibus will influence how long your cables can be. Some stations may even run a a maximum baudrate of 9.6kbit/s. This means that the used baudrate in your Profibus network can only be at maximum 9.6kbit/s.

The baudrate can be set in the following ways:

  • DIP-switch on the station
  • In a display
  • Autoselect

 

Faster transmission speed also shortens the lenght of your cables or segments. A segment is defined as the stations or devices between two Profibus repeaters. Profibus repeaters works by strengthening the signal, so you can expand the network. Up to 32 (31 and a repeater) stations can be in one segment. After 32 stations you would need another repeater to continue the bus line. One Profibus network can have a maximum of 126 stations.

When using repeater you have to terminate two times. You have to terminate the Profibus cable going into (IN) the repeater. But you also have to terminate at the beginning of the continuing Profibus cable going out of the repeater (OUT).

 

More About Profibus

For further information on Profibus you can visit the Profibus official website, where you can download a lot of great material about Profibus. Feel free to comment below if you have any questions about Profibus.

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