What type of cable is recommended for RS-485 communication networks?

Created by Patrícia Siqueira, Modified on Tue, 9 Jan at 11:08 AM by Patrícia Siqueira

For RS-485 communication, it is recommended to use twisted pair and shielded cables, considering their application in environments with high levels of interference. The use of this type of cable prevents the action of electromagnetic interference (EMI) on communication systems, as EMI commonly causes distortions that can alter and cause failures in data acquisition, leading to errors and false alarms in various equipment.

In specific aspects of RS-485 communication, a wide range of different cable types with equivalent characteristics can be used.

The "twisted pair" characteristic of the cable consists of two insulated metallic conductors twisted together with a specific pitch along the longitudinal axis of the cable.

ℹ️ This arrangement improves immunity to electromagnetic disturbances because it forms a series of successive coils, each one facing in the opposite direction to the next. In this way, any magnetic field in the environment passes through each pair of coils in opposite directions, and its effect is therefore greatly reduced. Theoretically, the effect on each coil is exactly the opposite of the next one, and as a consequence, it cancels out.

The "shielded" characteristic of the cable refers to the metallic covering that surrounds the conductors. This can be formed by a braided mesh of fine conductive wires or a foil consisting of a metal film wrapped around the conductors, which may include a drain (a bare wire or cable that facilitates grounding the foil). Both types are functionally equivalent, each with its peculiarities, and both types can be combined to achieve better characteristics.

ℹ️ This arrangement allows for electrostatic shielding, ensuring that little or no electric and magnetic field exists inside this conductor. This ensures that the digital signal being transmitted does not suffer from "noise" to the point of causing substantial degradation of its integrity.

Recommended Characteristics:

Flexible copper conductor;
18 AWG (approximately 1.2 mm²);
Maximum length of 1200 m;
Impedance of up to 100Ω;
Twist pitch of 50 to 75 mm;
300 Volts;
Operating temperature from -40 °C to +75 °C;
PVC insulation;
Shielding:
- Braided mesh (70% to 95% coverage);
- Tape (laminated aluminum with polymeric film);
- Combined shielding (recommended).

⚠ The choice of shielding model will depend on the intensity of local electrical noise.