PROFIBUS-FMS and PROFIBUS-DP are device level protocols. PROFIBUS-PA is a controller level protocol. PROFINET is a controller level and control center level protocol. PROFIBUS International (PI) differentiates the protocol usage as PROFIBUS-DP is mainly for factory automation and PROFIBUS-PA is primarily for process automation.
All of the PROFIBUS protocols are used primarily in manufacturing, but they are also found in water, oil, gas and other vertical markets. The PROFIBUS heritage as a German protocol is evident in its widespread use in Europe. It has expanded to be a worldwide protocol, in a large part due to Siemens support.
The Process Field Bus (PROFIBUS) specification was first issued in 1989 by a consortium of primarily German companies and organizations. This consortium is now known as PROFIBUS International (PI). The original specification was PROFIBUS Field bus Message Specification (PROFIBUS-FMS). This specification still exists but is largely replaced in practical use by the simpler and faster PROFIBUS Decentralized Peripherals (PROFIBUS-DP) that was issued in 1993.
PROFIBUS Process Automation (PROFIBUS-PA) sits on top of PROFIBUS-DP-V1 and adds the Control Level capabilities in the form of Application Profiles. PROFIBUS-PA was developed for controller to controller communication in a decentralized control system or for DCS to controller communications in a centralized solution.
PROFIBUS-PA is also viewed as a specification for interoperability between controller or DCS vendors.
The desire to leverage the low cost and widespread availability of Ethernet led to the development of PROFINET in 2003.
The PI organization claim an installed base of over 20 million devices and over 2,500 different products that support PROFIBUS or PROFINET.
PROFIBUS DP is a master/slave protocol with the controller as the master and the instruments as the slaves.
The PROFIBUS specification primarily covers the application and data link layers, although there are physical layer specifications as well.
The PROFIBUS data link layer is referred to as the Field bus Data Link (FDL) and as the security layer, although no true security measures are included in this specification. The data link layer is a token passing system to provide predictable and deterministic communication performance, and it supports the PROFIBUS master/slave approach.
The Application Layer
- DP-VO provides the basic PROFIBUS functionality with predictable, cyclic data exchange and diagnostics. Masters read input from slaves cyclically, and masters write output to slaves cyclically. Up to 126 PROFIBUS-DP-VO devices can be connected to a fieldbus.
- DP-V1 adds features for non-cyclic communication to the DP-V0 specification, such as parameter assignment, visualization and alarm handling. DP-V1 features are helpful for process automation and are used in PROFIBUS-PA. Non-cyclic communication has a lower priority than cyclic-communication and is sent in gaps.
- DP-V2 provides isochronous mode and data exchange broadcast (slave-to-slave communication)
PROFINET is essentially PROFIBUS over industrial Ethernet. It has two versions:
- Component Based Automation (PROFINET CBA)
- Input / Output (PROFINET IO)
Like most industrial Ethernet protocols, Ethernet’s collision detection is replaced with a token based collision avoidance protocol that guarantees communication time slots and provides predictable performance needed for cyclic communication.
Where PROFINET differs from most other industrial Ethernet protocols it has significantly different communication methods for critical and non-critical communication. A realtime channel is available for time critical communications, typically cyclic communications for factory automation with response times less than 10 ms. Non-time-critical communications, typically process automation communications with response times closer to 100 ms, are sent using standard TCP and UDP over IP.
There is also a very high performance mode called Isochronous Real Time (IRT) communication used in motion control and with response time of less than 1 ms. The PROFINET standard uses the term Real Time (RT) for time critical communications. RT communications is identified in layer 2 by the Ethertype 0x8892. If RT communications needs to be sent between two networks it is encapsulated in UDP for routing.
All PROFINET field devices must support UDP over IP data communications. TCP over IP communication support is optional. PROFINET ports are:
- TCP/UDP 34962 PROIFINET RT Unicast
- TCP/UDP 34963 PROFINET RT Multicast
- TCP/UDP 34964 PROFINET Context Manager
Special effort has been placed on integrating serial, legacy fieldbuses with PROFINET through defined proxies. The initial fieldbus support was PROFIBUS (DP and PA), AS-i, INTERBUS and DeviceNet.
In March of 2005, a PROFINET Security Guideline was released. This document is a general guideline document on control system security and leverages the ISA 99 Technical Reports. Section 7 lists the PROFINET security requirements in broad terms.
The PROFINET TCS3/WG18 is responsible for security in the protocol, and the chair of that working group has indicated there are no plans to add security to the protocol.
Siemens makes the ERTEC family of ASIC’s with the PROFINET stack, and since continues to be a very active supporter of PROFIBUS and PROFINET. The ERTEC chips are likely the most widely deployed implementation.
Hilscher offers the PROFINET stack on a FPGA, and Softing has implementations that use DSP chips and processor board solutions with code libraries.
It is unlikely that a vendor would develop their own PROFIBUS or PROFINET protocol stack due to the complexity and multiple lower cost solutions.