Connection technology
The field of application of a fieldbus system is essentially determined by the choice of transmission medium and the physical bus interface. In addition to the requirements for transmission security, the expense and work involved in acquiring and installing the bus cable is of crucial significance. The PROFIBUS standard therefore allows for a variety of implementations of the transmission technology while retaining a uniform bus protocol.
Cable-based transmission:
This version, which accords with the American EIA RS-485 standard, was specified as a basic version for applications in production engineering, building management and drive technology. A twisted copper cable with one pair of conductors is used. Depending on the intended application area (EMC aspects should be considered) the shielding may be omitted.
Two types of conductor are available, with differing maximum conductor lengths (see the RS485 table).
RS-485 transmission according to the PROFIBUS standard | |
|---|---|
Network topology | Linear bus, active bus terminator at both ends, drop lines are possible |
Medium | Shielded twisted cable, shielding may be omitted, depending upon the environmental conditions (EMC) |
Number of stations | 32 stations in each segment without repeater. Can be extended to 125 stations with repeater |
Max. bus length without repeater | 100 m at 12 Mbit/s |
Max. bus length with repeater | Repeaters can increase the bus length up to 10 km. The number of possible repeaters is at least 3 and can be up to 10 depending on the manufacturer. |
Data transfer rate (adjustable in steps) | 9.6 kbit/s; 19.2 kbit/s; 93.75 kbit/s; 187.5 kbit/s; 500 kbit/s; 1500 kbit/s; 12 Mbit/s |
Connector | 9-pin D-sub connector for IP20 |
Cable-related faults
Note the special requirements on the data cable for baud rates greater than 1.5 Mbaud. The correct cable is a basic requirement for correct operation of the bus system. If a simple 1.5 Mbaud cable is used, reflections and excessive attenuation can lead to some surprising phenomena. For example, a connected PROFIBUS station does not establish a connection, but can do so again after disconnecting the neighboring station. Or there may be transmission errors when a specific bit pattern is transmitted. The result of this can be that when the equipment is not operating, PROFIBUS works without faults, but that there are apparently random bus errors after start-up. Reducing the baud rate (< 93.75 kbaud) corrects this faulty behavior.
If reducing the baud rate does not correct the error, then in many cases this can indicate a wiring fault. The two data lines may be crossed over at one or more connectors, or the termination resistors may not be active, or they may be active at the wrong locations.
 | Pre-assembled cable from BECKHOFF The pre-assembled cables from BECKHOFF simplify installation considerably. Wiring faults are avoided, and commissioning is more rapidly completed. The BECKHOFF range includes fieldbus cables, power supply cables, sensor cables and accessories such as termination resistors and T-pieces. However, field-assembled plugs and cables are also available. |
The following diagram shows the cabling between two stations and how the D-sub connections are assigned:
| Termination resistors In systems with more than two stations all devices are wired in parallel. The bus cable must always be terminated with resistors at the cable ends in order to avoid reflections and thus transmission problems. |
Distances
The bus line is specified in EN 50170. This yields the following lengths for a bus segment.
Baud rate in kbits/sec | 9.6 | 19.2 | 93.75 | 187.5 | 500 | 1500 | 12000 |
|---|---|---|---|---|---|---|---|
Cable length in m | 1200 | 1200 | 1200 | 1000 | 400 | 200 | 100 |
Drop lines up to 1500 kbaud <6.6 m; drop lines should not be used at 12 Mbaud.
Bus segment
A bus segment consists of at most 32 devices. 125 devices are permitted in a PROFIBUS network. Repeaters are required to refresh the signal in order to achieve this number. Each repeater is counted as one device.
IP-Link is the subsidiary bus system of the Fieldbus Boxes whose topology is a ring structure. The coupler modules (IP230x-Bxxx or IP230x-Cxxx) contain an IP-Link master to which up to 120 extension modules (IExxxx) can be connected. The distance between two modules may not exceed 5 m. When planning and installing the modules, note that because of the ring structure the IP-Link master must be connected again to the last module.
Installation guidelines
When installing the modules and laying the cable, observe the technical guidelines of the PROFIBUS user organization (PROFIBUS-Nutzerorganisation e.V.) for PROFIBUS-DP/FMS (see: https://www.profibus.com).
Check the PROFIBUS cable
A PROFIBUS cable (or a cable segment when using repeaters) can be checked with a few simple resistance measurements. The cable should meanwhile be disconnected from all stations:
- Resistance between A and B at the beginning of the line: approx. 110 Ohm
- Resistance between A and B at the end of the line: approx. 110 Ohm
- Resistance between A at the beginning and A at the end of the line: approx. 0 Ohm
- Resistance between B at the beginning and B at the end of the line: approx. 0 Ohm
- Resistance between shield at the beginning and shield at the end of the line: approx. 0 Ohm
If these measurements are successful, the cable is okay. If, in spite of this, bus malfunctions still occur, this is usually a result of EMC interference. Observe the installation instructions of the PROFIBUS User Organization (www.profibus.com).