PROFIBUS PA network sizing
A PROFIBUS PA segment is one power supply shared by every instrument on it: the coupler current, the voltage left at the farthest terminal and the total cable length all come out of the same budget. This page shows how the segment is checked — current with headroom, voltage drop section by section, length by cable type, spur limit by number of spurs, FISCO in hazardous areas, cyclic bytes and cycle time — with the limits of IEC 61158-2.
When to use
When distributing field instruments among DP/PA coupler ports, when deciding where to place the junction boxes along the trunk, when checking whether a segment with valve positioners still leaves 9 V at the farthest device, and when checking a FISCO supply against the zone and gas group of the area.
What gets sized in a PA segment
A PROFIBUS PA segment starts at a coupler or link port, runs as a trunk through junction boxes and reaches each instrument through a spur — the magnetic flow meters, the differential pressure transmitters, the radar level transmitters and the temperature transmitters mounted on thermowells. The same pair carries data and power, so every new instrument consumes current, adds voltage drop and adds cable to the total length. Sizing means checking that this single budget closes on every axis at once: current, voltage, length, spurs, attenuation, data and time.
Current and voltage: the same budget
The segment current is the sum of the quiescent currents plus a fixed 20 mA headroom and the FDE current, and it must stay below the coupler output. The voltage is then followed section by section from the coupler: each trunk run loses I × R × L with the current of what is downstream of it, and each spur loses the current of its own instrument. The farthest instrument must keep at least 9.0 V — or the minimum the datasheet asks for, which is usually higher for positioners and analysers. Above the 32 V MBP ceiling the segment is reported as not assessed rather than approved.
Length, spurs and topology
Trunk plus spurs count against the maximum length of the cable type; when cables are mixed, the fractions are added. Take the real routed lengths from the cable tray layout rather than straight-line distances. The spur limit falls as the number of spurs grows, and with intrinsic safety the column is stricter. The bus has two ends: the terminator integrated into the coupler and the one at the junction box that closes the trunk. A trunk that branches has more ends than terminators, and the tool flags it.
Hazardous area and FISCO
The protection level follows the zone: zone 0 asks for ia, zone 1 for ib and zone 2 admits ic. With that and the gas group, the tool reads the Io ceiling from the FISCO tables and checks Uo, Po and the 1000 m length limit. A segment declared in a classified zone without intrinsic safety fails, whatever the supply.
What this tool does not cover
It checks the FISCO supply, not the field devices: the Ii, Pi, Ci and Li of each instrument and the terminator values still have to be confirmed against the certificates. Entity-concept calculations, FNICO and high-power trunks with barriers are outside the model. The DP side of the coupler — the master’s address space, bus parameters and DP cycle — is checked in the PROFIBUS DP network sizing tool, and the space for couplers, power supplies and switches in the control room in 19” rack sizing. For new instrument networks that carry Ethernet all the way to the device, the single-pair successor of the PA segment is checked in Ethernet-APL network sizing.
Formulas and fundamentals
I_seg = Σ I_b + I_headroom + I_FDE ≤ I_coupler Σ I_b is the quiescent current of the instruments — 15 mA for a bus-powered 2-wire device and 10 mA with external supply when the datasheet value is not entered. The headroom is an absolute 20 mA (expansion or change of manufacturer) and the fault disconnection current (FDE) is 0 mA unless the vendor states otherwise. Margin below 10 % of the coupler current is flagged.
ΔV = I_downstream × R_loop × L ; V_node = V_parent − ΔV Applied from the coupler outwards along the tree (trunk → junction box → spur). Each instrument has its own minimum terminal voltage, 9.0 V by default; 10.5 V is the recommended headroom.
V_conservative = V_coupler − Σ I_b × R_loop × L_total The textbook formula, which assigns the whole current to the whole length. It always overestimates the drop and is shown alongside as the check to use while the topology is not yet defined.
Σ (L_x / L_max,x) ≤ 1 Trunk plus every spur, per cable type. L_max is 1900 m for type A, 1200 m for B, 400 m for C and 200 m for D; intrinsic safety (FISCO) caps the segment at 1000 m, and so does zone 0.
A = (L_spur × 0.15 nF/m + C_d) × 0.035 dB/nF ≤ 14 dB Capacitance of the spur cable plus that of the device, converted into attenuation. Above 12 dB the spur is flagged, above 14 dB it fails.
Tc = 10 ms × N + 10 ms + 1.3 ms × ceil(bytes / 5) N instruments, 10 ms of class-2 acyclic services and 1.3 ms per set of 5 cyclic bytes (in plus out). Each cyclic variable costs 5 bytes.
Standards & methods
- IEC 61158-2 — MBP physical layer (cables A to D, Table 5 spur lengths)
- PROFIBUS Design Guideline, PI Order No. 8.012 v1.27 — FISCO segment length
- IEC 60079-11, Annex E — FISCO supply limits (Tables E.1 and E.2)
- IEC 60079-14 — equipment selection by EPL (zone → ia / ib / ic)
- IEC 60079-25 — intrinsically safe systems
Typical reference values
| Quantity | Typical range | Note |
|---|---|---|
| Cable A / B / C / D — loop resistance | 44 / 112 / 264 / 40 Ω/km | 0.8 / 0.32 / 0.13 / 1.25 mm² |
| Cable A / B / C / D — maximum segment length | 1900 / 1200 / 400 / 200 m | trunk + all spurs; 1000 m with FISCO or in zone 0 |
| Spur limit, non-IS | 120 m (≤ 12 spurs) · 90 m (13–14) · 60 m (15–18) · 30 m (19–24) | 25 or more spurs leaves practically no spur (1 m) |
| Spur limit, intrinsic safety | 60 m (≤ 18 spurs) · 30 m (19–24) | — |
| Instrument quiescent current (when not declared) | 15 mA bus-powered · 10 mA externally powered | — |
| Terminal voltage | 9.0 V floor · 10.5 V recommended · 32 V MBP ceiling | — |
| FISCO supply, zones 0 and 1 (ia/ib) | Uo ≤ 17.5 V · Po ≤ 5.32 W · Io ≤ 75 mA (IIC) / 213 mA (IIB) at 17.5 V | IEC 60079-11 Table E.1; 183 / 380 mA at 14 V |
| FISCO supply, zone 2 (ic) | Uo ≤ 17.5 V · Io ≤ 112 mA (IIC) / 319 mA (IIB) at 17.5 V | Table E.2, power not restrictive |
| Cyclic data | 5 bytes per variable · 244 bytes in and 244 out | — |
Worked example
Two junction boxes, 12 instruments, non-hazardous area
Inputs
- Coupler output
- 19 / 400 V / mA
- Cable
- type A 44 Ω/km
- Trunk CP-01 → JB-01 → JB-02
- 350 + 300 m
- JB-01 spurs (FT-01, FT-02, PT-01, PT-02, LT-01, TT-01)
- 25, 25, 15, 15, 30, 20 m
- JB-02 spurs (TT-02, PT-03, FV-01, FV-02, LT-02, TT-03)
- 20, 20, 40, 40, 35, 60 m
- Consumption
- 15 (positioners FV-01/02: 13) mA
- Cyclic variables
- 1 each (FT: 2 in; FV: 1 in + 1 out) —
Results
- Segment current (176 + 20 headroom)
- 196 mA of 400
- Voltage at JB-01 / JB-02
- 16.29 / 15.15 V
- Farthest terminal (TT-03)
- 15.11 V (floor 9.0)
- Closed-form check
- 11.29 V
- Total length (650 trunk + 345 spurs)
- 995 m of 1900
- Longest spur
- 60 m of 120 (12 spurs)
- Highest spur attenuation
- 0.32 dB
- Cyclic bytes in / out
- 70 / 10 bytes
- Cycle time
- 150.8 ms
The segment closes with room to spare: 204 mA of current margin, 15.11 V at the farthest instrument and half the length allowed for type A cable. Even the conservative closed formula stays above 10.5 V. What limits this segment is time, not power: 12 instruments and 16 sets of bytes give about 151 ms of cycle. If FV-01 closes a flow loop that needs to be faster, it is the cycle that justifies splitting the instruments over another coupler port.
Common mistakes
- Applying the closed formula to a finished design. Charging the full current over the full length condemns segments that close comfortably: in the example below it gives 11.29 V where the per-section calculation gives 15.11 V.
- Reading the spur limit by number of instruments. Table 5 of IEC 61158-2 counts spurs: with 13 spurs the limit drops from 120 m to 90 m, and with 25 the spur practically disappears.
- Leaving the coupler at its non-Ex default inside a hazardous area. A 19 V / 400 mA output delivers 7.6 W, above the 5.32 W FISCO ceiling of zones 0 and 1, and its 400 mA exceed the Io allowed at any tabulated voltage.
- Branching the trunk. A PA segment is a bus with two ends and two terminators; two junction boxes hanging from the same point create a third end that no terminator closes.
- Sizing only power and forgetting time. Each instrument adds about 10 ms to the cycle; a segment with 12 devices takes around 150 ms, too slow for a fast flow loop.
Frequently asked questions
How many instruments fit on a PROFIBUS PA segment?
There is no fixed number — it is a current and voltage budget. With 15 mA instruments, a 400 mA coupler and 20 mA of headroom, current allows up to 25; the voltage at the farthest terminal, the spur limit (which falls as the number of spurs grows) and the cycle time usually limit it earlier. In hazardous areas, the FISCO current ceiling cuts the number much further.
Why does the voltage per section differ from the textbook formula?
The closed formula multiplies the whole segment current by the whole cable length, as if every instrument were at the end of the line. Per section, each run carries only the current of what is downstream of it. The first is a safe pre-design check; the second is the real voltage once the topology is drawn.
What is the maximum spur length in PROFIBUS PA?
It depends on how many spurs the segment has (IEC 61158-2, Table 5): 120 m up to 12 spurs, 90 m with 13–14, 60 m with 15–18 and 30 m with 19–24, outside intrinsic safety. With intrinsic safety the limit is 60 m up to 18 spurs and 30 m up to 24.
What does the tool check under FISCO?
In a classified zone without intrinsic safety the segment fails. With intrinsic safety it checks Uo ≤ 17.5 V and the Io allowed by Table E.1 (zones 0 and 1, which also limit Po to 5.32 W) or E.2 (zone 2) of IEC 60079-11 Annex E, for the declared gas group, and caps the segment at 1000 m. Between tabulated voltages it takes the next higher row, which is the conservative reading.
Does the 244-byte limit fail the segment?
No — it is shown as a warning. The 244 bytes in and 244 out are a ceiling of the DP slave that the coupler or link represents, and the tool does not know which DP master the segment sits behind. Above the ceiling, check the link configuration.
Glossary
- MBP
- Manchester Bus Powered — IEC 61158-2 physical layer of PROFIBUS PA, 31.25 kbit/s with data and power on the same pair.
- Segment
- Everything fed by one coupler port — trunk, junction boxes and spurs — with one terminator at each end.
- Spur
- Branch from a junction box (or from the coupler) to a single instrument; it has no terminator.
- FDE
- Fault Disconnection Electronics — current the device draws in a fault before isolating itself from the bus.
- FISCO
- Fieldbus Intrinsically Safe Concept — intrinsic safety by supply limits (Uo, Io, Po) and certified devices, without entity calculation per loop.
- DP/PA coupler
- Gateway between PROFIBUS DP (RS-485) and the PA segment, which it also powers.