Profibus Cable, Connector And Termination Tips

Profibus or Process Field Bus is a field bus communications standard for the automation industry. Its industrial computer network is designed to withstand high levels of noise. With its two-wire cable, Profibus supports different network topologies such as a star or a tree, a line or a combination of these.

There are four variants of Profibus. Each one has a different purpose, as the following list shows:

• PROFIBUS DP (Decentralized Peripherals) drives sensors and actuators through a central controller. It supports data speeds up to 12 Mbit/s over twisted pair cables, with fibre optic cables also available as an option.
• PROFIBUS PA (Process Automation) monitors measuring equipment through a process control system. This variant suits explosive areas (Ex-zone 0 and 1). A weak current flows through the bus lines in an intrinsically safe circuit, preventing sparks even if a fault occurs. Its disadvantage is the slow data speed of 31,25 kbit/s.
• PROFIsafe is used in safety applications, usually alongside safety PLCs or safety relays.
• PROFIdrive is used for motion control.

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

In this article, I will focus on the Profibus DP network. It is the most used bus network and is among the fastest and most widespread industrial networks in use.

Choosing a popular network such as Profibus DP gives you a system used in many industrial applications, with plenty of people with Profibus experience and a lot of resurces. The network is widespread and taught at most schools and educational institutions. Profibus also benefits from its organisation and clearly defined network standards, which make documentation easy to find. In my experience, a quick Internet search for most Profibus issues brings up plenty of useful material. Standardisation also gives us a large choice of components and helps keep their prices down.

The Profibus Cable: How To Make Your Own

Making a Profibus cable requires very little effort. The cable has only two wires and a shield, so the connections are fairly simple to make. Even so, take care with every connection. The simple appearance does not leave room for mistakes: each wire must connect correctly. An incorrect connection will not damage any components. However, I can guarantee that the network will not work if the wires are connected incorrectly.

The Cable

Profibus networks use a shielded AWG 22 or 0.34 mm² twisted pair cable. Profibus DP cable normally has purple insulation, with green and red wires inside the twisted pair. These colours matter less than the cable specifications. A Profibus installation needs the correct cable, so the official Profibus organisation publishes the following 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.

The full specification is available on the Profibus website. You can find further Profibus cable specifications in IEC 61784 (previously EN 50170 chapter 8-2).

The Connectors

Most installations use 9-pin D-sub connectors at the cable ends to connect devices, as recommended in EN 50170. The 9-pin D-sub is the default Profibus connector on most components. Personally, the only connectors I have encountered are the 9-pin D-sub and, on rare occasions, the Profibus M12 connector. The following diagram shows the 9 pins in a 9-pin D-sub connector:

Making your own Profibus cables is straightforward. With just a few tools, you can reduce the price of the cables considerably compared with buying them pre-made. Pre-made Profibus cables are quite expensive, and making them yourself also lets you cut each cable to the exact length needed. However, there are several points you need to pay attention to when making your own cables.

1. Remove The Isolation With A Profibus Stripping Tool

Start by removing the insulation from your Profibus cable. You can use a special Profibus stripping tool or, if you are used to working with cables, a knife or wire cutter. The Profibus stripping tool is worth the money because it strips the cable cleanly to fit the connectors. With a knife, it can be difficult to expose both the shield and the two internal wires correctly.

2. Connecting The Profibus Cable And Connectors

Next, lay the Profibus cable in the connector so that the shield and wires connect correctly. We now need to identify which pins to use. The connector has 9 pins, as described earlier, but we need only some of them. The table shows how Profibus uses each pin in the 9-pin D-sub connector:

This may look confusing at first: Profibus seems to use 7 of the 9 D-sub pins, yet the cable has only two wires and a shield. How does that work?

The explanation is that we do not always use every pin on the connector. We do not even need all 7 of the listed pins. For most applications, we use only pin 3, 5, 6 and 8. These four pins are therefore our starting point.

2 Pins For Data Transmission

Pin 3 and pin 8 carry the data transmission and are where the green and red wires connect. The green wire is usually positive, or channel A. When connecting the Profibus cable, connect the green wire to pin 3. The red wire is considered negative, or channel B. Pin 8 is for the green wire.

You can reverse the wires or choose your own colours, provided you use the same colours at both cable ends and remain consistent. However, most Profibus connectors mark channel A in green and channel B in red. Different colours, or reversed green and red wires, can therefore cause considerable confusion.

2 Pins For Active Termination

Next are pin 5 and pin 6, which are used for termination. We will cover cable termination later.

Now consider the standard connectors made specifically for Profibus. The following picture shows what these connectors usually look like:

In And Out Of Profibus Connectors

On these connectors, the markings clearly show where channels A and B connect. You can also see that most connectors have two connections for IN and another two for OUT.

Your network starts at OUT, the output of the first connector. Always begin there when building the network topology. OUT is also the connection used to continue the network. If a station is already connected to the master and you want to add a new slave, use OUT.

The connector’s IN connection is where the “network line” ends. Apart from the master, all cables therefore connect to IN. Use OUT when extending the “network line”, as shown here:

Take extra care when fitting the Profibus cable to its connector. Strip the wire correctly and check the wire connections as well as the shield connection. If you are unsure about the connector, watch this instruction video:

After making the connections, the next part of the installation to consider is how to terminate the cables.

Termination Of Profibus Cables

To reduce signal reflections the Profibus cable must be terminated at its ends. Signal reflection occurs when some energy in a signal travelling through a medium, such as copper cable or optical fibre, reflects back instead of reaching the far end. As with grounding, incorrect Profibus cable termination prevents the bus from working.

To terminate Profibus, insert a 220 Ω resistor at each end of the line: a 220 Ω resistor in the first station and another in the last. The 220 Ω value is used because the two 220 Ω resistors connect in parallel, giving a total resistance of 110 Ω. Standard Profibus DP cable has a loop resistance of 110 Ω/km. In comparison, a standard Profibus PA cable has a loop resistance of 44 Ω/km.

Most Profibus connectors come with a termination option. With these connectors, there is no need to fit your own resistor manually. Instead, you activate or deactivate termination using a switch, usually on top of the connector. This helps with two parts of the work. During installation, you can finish all the connections first, then flip the switches wherever the Profibus cable needs termination. During troubleshooting, you can check whether the termination is correct without disconnecting anything from the network.

When installing a bus network such as Profibus, termination is where the most common installation errors occur.

Active Termination In Bus Networks

Profibus uses an extended form of termination called active termination. This is needed for installations with very long communication cables. With active termination, you can increase the line voltage on the bus.

Active termination uses the 220 Ω resistor and adds a 5V connection between VP(6) and DGND(5). This creates a defined ground signal when none of the stations are active. There are three ways to make an active termination:

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

We call the termination active because it uses the +5 volt connection. Usually, you create the active termination simply by flipping the termination switch on the connector. The following Profibus connector diagram shows the connections involved and makes this arrangement clearer:

Switching on the termination microswitch connects pins 5 and 6 to create an active termination. For more about active or electrical termination, read the article on voltage regulation on Wikipedia. Active termination is a voltage regulator that keeps the voltage across the terminating resistor constant.

Profibus And Grounding

Connect the shield correctly to avoid noise. Profibus will not work if its shields are connected incorrectly. All stations also need a ground connection. Machinery sold in the European Union needs a grounding wire of at least 16 mm². Grounding ensures that all stations in the installation work (IEC 60204-1). It also prevents potential differences between stations. Using different power supplies at each station can itself create a potential difference. If a shield joins two stations at different potentials, current flows through it and creates noise. The shield must carry no current, so we must make sure the ground connections are sufficient.

Profibus Addressing

Assign an address to every station before powering on the Profibus network. Addresses range from 0 to 125. Depending on the station’s specifications, you can set its address in several ways:

• 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)

Structure of the Profibus addresses (unwritten rules of addressing)

1. Low addresses are reserved for Master Class 1 & 2 (addresses: 0,1,2) and are often the default addresses.
2. Avoid addresses 124 & 125 because some slaves use them as their default addresses.
3. Address 126 is reserved for service use.
4. A new address takes effect only after the system is rebooted (OFF/ON). Addressing through Master Class 2 is an exception to this rule.

What Is The Transmission Speed Or Baudrate In Profibus?

The baudrate, or transmission speed, applies to the whole network. Every Profibus station must use the same speed to communicate. Set the default baudrate in Master Class 1, between 9.6kbit/s and 12Mbit/s. A faster setting is not always better because transmission speed limits cable length. Some stations may have a maximum baudrate of 9.6kbit/s, which limits the entire Profibus network to 9.6kbit/s.

Depending on the station, you can set the baudrate using the following methods:

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

Increasing the transmission speed also reduces the length you can use for the cables or segments in the network. A segment is the group of stations or devices located between two Profibus repeaters. Profibus repeaters strengthen the signal so you can extend the network. A segment can contain up to 32 stations (31 and a repeater). After 32 stations, another repeater is needed to continue the bus line. A single Profibus network supports a maximum of 126 stations.

When you use a repeater, you need termination in two places. The Profibus cable going into the repeater (IN) needs to be terminated. The beginning of the continuing cable going out of the repeater (OUT) also needs termination.

More About Profibus

For more information, visit the Profibus official website, where you can download material about Profibus. Leave a comment below with any questions about Profibus.