Siemens SIPART Series Operating Instructions Manual page 285

PROFIBUS PA
PA is an abbreviation of process automation. PROFIBUS PA is used in process engineering.
This fieldbus is used to control the measuring devices using a process control system. This
version of PROFIBUS is suitable for hazardous areas of zones 0 and 1. Only a weak current
flows through an intrinsically safe circuit in the bus cables, and hence sparks are not
generated even in case of a fault.
PA enhances PROFIBUS DP with an intrinsically safe transmission technique compliant with
the international standard IEC 61158-2.
Protocols
Protocols contain information about data formats, time sequences and error handling when
exchanging data between computers.
A protocol is a convention about establishing, monitoring and terminating a connection.
Different protocols are required for a data connection. Protocols can be assigned to every
layer of the reference model. Transport protocols are used for the lower four layers of the
reference mode and higher protocols are used for control, data provision and application.
Safety function
Defined function executed by a safety-instrumented system with the objective of attaining or
maintaining a safe status of the system by taking a defined hazardous incident into account.
Example: limit pressure monitoring
Safety-instrumented system
A safety-instrumented system (SIS) executes safety functions that are required to attain or
maintain the safe status in a system. It consists of sensors, logic unit/control system and final
controlling elements.
Example: a safety-instrumented system consists of a pressure transmitter, a limit signal
indicator and a servo valve.
Sensor
Converter that converts mechanical or other non-electric variables into electric signals.
SIL
The international standard IEC 61508 defines four discrete safety integrity levels (SIL) from
SIL 1 to SIL 4. Every level indicates a probability range for the failure of the safety function.
The higher the SIL of the safety-instrumented system, the higher the probability that the
required safety function works. The achievable SIL is determined by the following safety-
instrumented characteristics:
● Average probability of dangerous failures of a safety function on demand (PFDAVG)
● Hardware fault tolerance (HFT)
● Safe failure fraction (SFF)
SIPART PS2 with and without HART
Operating Instructions, 10/2013, A5E00074631-11
Glossary
283