Examples of PROFIBUS PA architectures

The following graphical representations illustrate possible PROFIBUS PA configuration variants with DP/PA coupler and PA Link routers on the:

• PROFIBUS DP master, simple design (AS Single Station)
• PROFIBUS DP master, redundant design (AS Redundancy Station)

If the DP/PA coupler is operated independently as a PA router, then the connection is then directly on the coupler instead of via the interface module.

The number of PROFIBUS PA devices is limited according to the specifications in the "Technical specifications" section.

PROFIBUS PA on an AS Single Station as PROFIBUS DP master

PROFIBUS PA on an AS Redundancy Station as PROFIBUS DP master

Line architecture with single coupler

In the line architecture with individual couplers, each line segment is connected to one DP/PA coupler each.

If the PA router is an independent DP/PA coupler, then a PROFIBUS PA line (line segment) can be connected. A maximum of 5 line segments can be operated via single couplers (max. 3 for mixed configurations with ring or coupler redundancy) on a PA link as PA router, equipped with up to 5 DP/PA couplers.

The PA router can be connected to a single or redundant PROFIBUS DP, depending on the version (see figures).

The FDC 157-0 is the first choice as the DP/PA coupler. When using this coupler, the PA?devices can be integrated into the line segment via AFD active field distributors, e.g. AFD4, AFD4 RAILMOUNT, AFD4 FM and AFD8 (approval for Ex zone 2/22) and AFDiSD (approval for Ex zone 1/21). The PA devices are connected to these field distributors via short-circuit-proof spur lines.

Alternatively, it is possible to operate up to 8 AFD field distributors, up to 5 AFDiSD field distributors or any combination of up to 5 AFDiSD and AFD field distributors in a line segment. With mixed AFDiSD/AFD operation, however, extended fieldbus diagnostics of the AFDiSD in the PROFIBUS PA is not possible. The last field distributor at the end of the line leading away from the DP/PA coupler automatically activates its bus terminating resistor.

Intrinsically-safe PA devices in hazardous areas in accordance with Ex zone 1/21 or 0/20 are preferably integrated into a bus segment by means of AFDiSD active field distributors. For PA devices in Ex zone 1/21, connection via a line segment on the DP/PA coupler Ex [i] (in the PA Link or independently) is a possible alternative. The devices are integrated separately into the line segment using SpliTConnect taps (via spur line or directly via SpliTConnect M12 outlet). A SplitConnect terminator is required for the bus termination of the segment.

By grouping individual devices in different line segments, Flexible Modular Redundancy is possible at device level.

Line architecture with redundant coupler

The PA Link operable as a PA router on a single or redundant PROFIBUS DP can only be equipped with one redundant DP/PA coupler pair (up to 3 single couplers can also be optionally configured). The redundant DP/PA coupler pair can be used either for a line architecture with Active Field Splitter (AFS) or for a ring architecture.

With a line architecture, the AFS is connected to the redundant DP/PA coupler pair (2 x FDC 157-0) in the PA router. It connects the line segment connected to it to the active of the two redundant DP/PA couplers. A DP/PA coupler can be replaced without interrupting the ongoing operation.

The PA devices are integrated in the line segment as for a line architecture with single couplers via active AFD or AFDiSD field distributors. The limits with respect to the number of field distributors are also identical (up to 8 AFD, up to 5 AFDiSD or up to 5 AFDiSD and AFD combined; for mixed AFDiSD and AFD operation, extended fieldbus diagnostics for the AFDiSD is not possible).

Ring architecture with coupler and media redundancy

With the redundant DP/PA coupler pair (2 x FDC 157-0) of a PA router, a ring segment with automatic bus termination can also be implemented instead of a line segment with AFS. Apart from the ring segment, only line segments with individual couplers can be configured on this PA router. The PA router can be connected to a single or a redundant PROFIBUS DP.

Integration of the PA field devices into the ring segment is carried out via active AFD or AFDiSD field distributors whose number is limited as with the line architectures (up to 8 AFD, up to 5 AFDiSD or up to 5 AFDiSD and AFD combined; for mixed AFDiSD and AFD operation, extended fieldbus diagnostics for the AFDiSD is not possible). These field distributors have galvanically isolated, short-circuit-proof spur line connections for connecting the PA devices.

At the device level, flexible modular redundancy is possible by grouping individual devices on different field distributors.

Special advantages of the ring architecture:

• High availability
• Transparent redundancy management of the intelligent DP/PA couplers FDC 157?0 for the host system
• Active bus terminators for automatic bus termination in the FDC 157?0 DP/PA couplers and the AFD and AFDiSD active field distributors enable:
  • Automatic, smooth isolation of faulty subsegments in the event of a short-circuit or open-circuit
  • Modification of the ring configuration or instrumentation during operation, including the addition or removal of ring segments
• Safety-related and fault-tolerant applications with low device and cabling requirements

Cable lengths of bus segments and spur lines

The PROFIBUS PA is based on electrical transmission components. A shielded two-wire cable is used for digital data transmission and for the power supply of the field devices.

With line, tree and ring topologies, bus segments up to approx. 1.9 km can be configured. If AFD active field distributors are used, both the length of the spur lines for connecting devices and the quality of the cable used must also be considered when calculating the total length of the bus segment. Spur lines on the AFDiSD are not relevant to the total length of the bus segment.

For bus segments with active field distributors, the spur lines can have the following maximum lengths:

• Up to 120 m in accordance with IEC 61158?2
• Up to 120 m in accordance with IEC 60079-27 (FISCO)

With AFD active field distributors, these maximum values may be reduced depending on the number of spur lines of the bus segment (for details, see the "Technical specifications" section). With AFDiSD active field distributors, this reduction is canceled by the integrated repeater function.

The SIMATIC Fieldbus Calculator provides help in calculating and designing fieldbus segments:
https://support.industry.siemens.com/cs/ww/en/view/53842953

Intrinsically-safe PA devices in hazardous areas are preferably integrated into a bus segment by means of AFDiSD active field distributors. For PA devices in Ex zone 1/21, connection via a line segment on the PA router with DP/PA coupler Ex [i] is a possible alternative. In such a configuration the max. possible length per spur line is reduced to 30 m and per bus segment to 1 km.

Bus segments are terminated either automatically (for architectures with AFD or AFDiSD active field distributors) or with the passive terminating element for PROFIBUS PA (SpliTConnect terminator).