PA (Process Automation) Basics


What are the components of a PA network?

PROFIBUS PA ( Process Automation) was designed to work along with PROFIBUS DP (Decentralized Periphery) to make a consolidated network using DP for high-speed communications and PA for for process environment. A network consists of a DP master (controller), a DP segment, a DP/PA coupler and the process network with slave devices.

Fast DP controllers connected to slow PA instruments. How does that work?

The device that brings DP and PA together is called a  coupler. While the physical layers are very different for each network, the content of the communication telegrams are  the same. The DP/PA coupler translates the signals between the different physical layers and provides data coordination at the interface between the parts of the network.

DP PA network

DP and PA device setup

The setup of  devices for input and output is based on GSD files (General Station Description) that describe all available features for data communications by the devices. The GSDs are used by the configuration tool to specify the input and output configurations of the device. The  tool used depends on the vendor of the PLC / DCS being used for the controller.

PA Engineering tools

There are “additional” features inside PA devices that are accessed by specialized engineering tools These features are things like temperature units, pressure units, alarm levels, and asset tags. One type of engineering tool uses Electronic Device Description Language files (EDDLs) to describe the available features of the device.  PDM from Siemens uses EDDLs . The other type of engineering tool uses Device Type Manager files (DTMs) to access PA features. PactWare and FieldCare use DTMs.

High-speed Network for Process Automation

Originally, PROFIBUS DP was to be the main network trunk with PA  branches (sub-networks).  PROFINET was designed to be the  successor technology to DP so PROFINET also became the replacement backbone instead of  DP at the trunk level of the network . PROFINET and PA profile 4.0 bring additional possibilities for PA operations with improvements to the original DP in connection-technology and speed. The following table shows PROFIBUS PA, PROFIBUS DP and PROFINET features to allow a head to head comparison of all three.

PROFIBUS PA DP and PROFINET comparison

 PROFIBUS PAPROFIBUS DPPROFINET
Communication technologyMBP-IS RS-485 Ethernet
Bandwidth31.25 kbps9.6 kbps to 12 Mbps100 Mbps
Typical distances up to 1900 m total (1000 m for IS)100 m at 12 Mbpsup to 100 m between connections
Network topologyline, star, treeline (daisy-chain)line, star, tree, ring for redundancy
combination
Fiber optical available?noyesyes
Data packet sizeup to 244 bytesup to 244 bytesup to 1440 bytes
Number of stations on network64 devicesup to 126 devicesunlimited
Intrinsically Safeoptionaloptional, limited to RS-485 IS devices and infrastructure.optional / planned