BUS-485-120R

RS-485 / Profibus / Modbus Twisted Pair Bus Cable

Impedance-controlled twisted pair for RS-485, Profibus DP and Modbus RTU segments.

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Lead time15–25 working days
Minimum500 m per size
SampleFree up to 5 m

Illustrative cross section. Construction layers listed below.

A serial bus fails for reasons that have nothing to do with conductor size. RS-485 and Profibus need a controlled characteristic impedance so that the signal edge is not reflected back down the segment, a low and predictable capacitance per metre so the maximum segment length is met, and a screen with a low-resistance path so common-mode noise does not push the receiver outside its −7 V to +12 V window.

Voltcore supplies both impedance families: 120 Ω for RS-485 and Modbus RTU, and 150 Ω for Profibus DP, which is the value the Profibus physical layer specifies. The Profibus construction is the familiar purple-sheathed Type A cable with a solid 0.64 mm conductor, foil plus braid screen and a drain wire; the 120 Ω construction is supplied with stranded conductors where the segment is terminated in a panel.

Specification

RS-485 / Profibus / Modbus bus cable — Technical Specification
Item	Value
Bus standard	RS-485, Modbus RTU, Profibus DP / FMS
Characteristic impedance	120 Ω ± 10 % (RS-485, Modbus) or 150 Ω ± 10 % (Profibus DP)
Conductor	Class 5 stranded tinned copper, or solid 0.64 mm for Profibus Type A
Construction	1 twisted pair (2 cores), or 2 pairs where power and data share one cable
Cross-section range	0.22 – 1.0 mm² (24 – 18 AWG)
Insulation	Foam-skin polyethylene or solid PE, dielectric constant ≈ 2.3
Nominal capacitance	≤ 45–60 pF/m
Screen	Aluminium foil plus tinned copper braid, ≥ 85 % braid coverage, with tinned drain wire
Sheath	PVC purple (Profibus), grey or black (RS-485, Modbus)
Max. continuous conductor temperature	70 °C
Test voltage	1.5 kV rms core to core, 1 kV rms core to screen, 5 min
Minimum bending radius	10 × cable overall diameter

Layer-by-layer construction

Listed from the conductor outward. Read this alongside the specification table — the two together tell you whether the cable will survive its installation method.

1. ConductorStranded tinned copper Class 5, or solid 0.64 mm bare copper for Profibus Type A
2. InsulationFoam-skin or solid polyethylene at 0.5–0.8 mm wall, dimensioned to hold the characteristic impedance
3. Pair twistingTwo cores twisted at a short, controlled lay length to fix impedance and cancel induced noise
4. Foil screenAluminium-laminated polyester foil applied longitudinally with overlap, 100 % coverage
5. Braid screenTinned copper wire braid at ≥ 85 % coverage with a tinned drain wire for termination
6. SheathPVC purple for Profibus DP, grey or black for RS-485 and Modbus applications

Reference dimensions and weights

RS-485 bus cable — 1 pair, dimensions and weight (reference figures)
Construction	Nominal cross-section (mm²) / AWG	Conductor structure (No./mm)	Characteristic impedance (Ω)	Nominal capacitance (pF/m)	Approx. overall diameter (mm)	Approx. weight (kg/km)	Max. loop resistance (Ω/km)
1 pair (2 cores)	0.22 / 24 AWG	7/0.20	120 ± 10	45	6.2	52	178
1 pair (2 cores)	0.34 / 22 AWG	19/0.15	120 ± 10	48	6.8	68	115
1 pair (2 cores)	0.50 / 20 AWG	16/0.20	120 ± 10	52	7.4	88	78.0
1 pair (2 cores)	0.75 / 19 AWG	24/0.20	120 ± 10	55	8.1	112	52.0
1 pair (2 cores)	1.00 / 18 AWG	32/0.20	120 ± 10	58	8.8	140	39.0
1 pair (2 cores), Profibus Type A	0.34 / 22 AWG	1/0.64 solid	150 ± 10	30	8.0	78	115

Reference figures for preliminary sizing and drum planning. Overall diameter and weight vary with strand construction and sheath thickness tolerance; certified drawings are issued with each quotation.

Engineering characteristics

Characteristic impedance held to 120 Ω ± 10 % for RS-485 and Modbus, or 150 Ω ± 10 % for Profibus DP
Nominal capacitance of 30–58 pF/m, which is what allows 1 200 m at 9.6 kbit/s and 100 m at 12 Mbit/s
Foil screen for 100 % coverage plus a tinned copper braid for a low-resistance shield path
Tinned drain wire so the screen can be terminated without disturbing the braid
Short, controlled twist lay length keeps impedance uniform along the drum
Purple sheath on Profibus DP construction for immediate identification on site
Impedance and capacitance verified on every production length

Typical applications

Profibus DP segments between PLC masters and remote I/O, drives and valve islands
Modbus RTU multidrop networks across water, HVAC and energy metering schemes
RS-485 links to variable frequency drives, soft starters and power meters
Building management system trunk cabling between controllers and field devices
Long serial runs in solar, wind and substation monitoring installations

Ordering options

Profibus Type A construction: solid 0.64 mm conductor, 150 Ω, purple sheath
Two-pair version carrying 24 V DC power alongside the data pair
Armoured version for outdoor and inter-building runs
LSZH sheath for tunnels, stations and enclosed public buildings
Pre-terminated lengths with DB9 or M12 connectors and matching bus terminators

Standards manufactured to

EIA/TIA-485-A (RS-485 electrical layer)
Profibus DP physical layer specification (EN 50170)
IEC 61158 (fieldbus)
IEC 60228 (conductor)
IEC 60332-1-2 (flame propagation)
RoHS and REACH compliant compounds

Manufactured to other standards on request. Send the standard number with your RFQ and we will quote against it rather than against a near-equivalent.

Price and availability for RS-485 / Profibus / Modbus Bus Cable

Include cross-section, core count, voltage grade, route length and installation method — those five determine price more than anything else on the page.

Engineer-reviewed before pricing
Reference drawings issued with the quotation
Free sample up to free sample up to 5 m

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RS-485 / Profibus / Modbus Bus Cable — frequently asked questions

Can I use ordinary instrumentation cable for Profibus?

Not reliably. Profibus specifies a 150 Ω characteristic impedance and a capacitance of about 30 pF/m, while general instrumentation cable sits near 120 Ω and 60 pF/m. On a short segment it may work; at 12 Mbit/s the mismatch produces reflections that cut the usable length well below 100 m.

Why 120 Ω for RS-485 and 150 Ω for Profibus?

They are different specifications written at different times. The RS-485 electrical standard settled on 120 Ω as the nominal cable impedance and its terminator value; Profibus DP later defined 150 Ω with a lower capacitance to reach 12 Mbit/s. Terminate each segment with a resistor matched to the cable you installed.

How long can a Modbus RTU segment be?

RS-485 allows up to 1 200 m, but the practical limit is set by the baud rate and the capacitance of the cable. At 9 600 bit/s with cable below 60 pF/m, 1 000–1 200 m is realistic; at 115.2 kbit/s plan for roughly 100–200 m and check the voltage drop across the termination and bias resistors.

Do I need terminating resistors and where do they go?

Yes, at both physical ends of the segment, matched to the cable impedance — 120 Ω or 150 Ω. Fit them only at the two ends, never at intermediate nodes, and keep the stub to each device as short as possible so it does not act as a reflection point.