RS-485

RS-485 (TIA-485-A) Standard Overview

Introduction

RS-485 (TIA-485-A / EIA-485) is a physical layer standard for balanced multipoint serial communication introduced in 1983 by the Telecommunications Industry Association (TIA).

It defines only electrical characteristics of drivers and receivers, making it protocol-independent. Higher-level protocols such as Modbus, BACnet, Profibus, and proprietary systems define framing and addressing.

RS-485 is widely used in industrial automation, building management systems, embedded networks, and instrumentation systems due to its robustness, long distance capability, and noise immunity.

Core Concept

RS-485 is based on differential signaling over a twisted pair and a shared bus architecture with tri-state drivers.

Signal is defined by voltage difference:

$V_{diff}=V_{A}-V_{B}$

Electrical Characteristics

Logic Levels

Logic 1 (MARK): $V_{diff}<-200\ {\text{mV}}$

Logic 0 (SPACE): $V_{diff}>+200\ {\text{mV}}$

Undefined: −200 mV to +200 mV

Receiver Sensitivity

±200 mV minimum differential detection

Driver Output

≥ 1.5 V across 54 Ω load

Common-mode range

−7 V to +12 V

Bus Architecture

Supported topologies:

• Linear bus (recommended)
• Multi-drop bus
• Point-to-point

Not recommended:

• Star topology (reflections)
• Ring topology

RS-485 must be implemented as a terminated transmission line.

Transmission Line Behavior

At higher speeds, RS-485 behaves as a transmission line.

Propagation delay:

$t_{prop}\approx 5\ {\text{ns/m}}$

Effects:

• reflections
• ringing
• overshoot
• signal distortion

Cable Length vs Speed

Real-world constraints depend on cable quality and capacitance:

• 10 Mbps → ~10–30 m
• 1 Mbps → ~100–300 m
• 100 kbps → up to ~1200 m

Rule of thumb:

${\text{bit rate}}\cdot {\text{distance}}\lesssim 10^{8}$

Termination

Termination must match cable impedance:

$R_{termination}=Z_{0}\approx 120\ \Omega$

Rules:

• termination at both ends only
• no intermediate termination
• required to reduce reflections

Biasing (Failsafe)

Biasing ensures a defined idle state when no driver is active.

Target condition:

$V_{diff}>200\ {\text{mV (idle)}}$

Modern transceivers often include internal failsafe circuitry, making external biasing optional in many designs.

A/B Line Polarity

RS-485 standard defines only differential signaling; it does not assign logic meaning to A and B lines.

Important:

• A/B labeling may differ between manufacturers
• polarity must be verified in practice
• oscilloscope measurement is recommended

Grounding and Common Mode

RS-485 supports differential signaling but requires a valid common-mode range:

Allowed:

• −7 V to +12 V

Considerations:

• long cable runs may introduce ground potential differences
• optional reference ground (SC/GND) may be used
• isolation recommended in industrial environments

Protection

Recommended protection methods:

• TVS diodes (ESD protection)
• common-mode chokes (EMI suppression)
• optional series resistors (10–50 Ω)

Relevant standards:

• IEC 61000-4-2 (ESD)
• IEC 61000-4-4 (EFT)
• IEC 61000-4-5 (surge)

Duplex Modes

Half-duplex

2-wire system, most common, one transmitter active at a time

Full-duplex

4-wire system, separate TX and RX pairs

Collision Handling

RS-485 does not define arbitration.

Handled by higher protocols:

• master-slave (Modbus RTU)
• token passing
• time-slot scheduling

Bus contention leads to data corruption.

Network Topology

Correct topology:

[Master]—120Ω—Device—Device—Device—120Ω

Rules:

• linear bus only
• short stubs (< 20–30 cm recommended)
• termination only at ends

Common Mistakes

• missing termination
• star topology wiring
• long stubs
• missing grounding strategy
• swapped A/B polarity
• no biasing in legacy systems

Troubleshooting

Steps:

1. measure differential voltage (A-B)
2. verify idle state stability
3. check termination resistance (~60 Ω total)
4. inspect reflections using oscilloscope
5. isolate nodes one by one

Applications

• industrial automation (Modbus, Profibus)
• PLC systems
• SCADA networks
• building automation (HVAC, lighting)
• CNC and robotics
• energy meters
• security systems
• DMX512 lighting control

Comparison with Other Standards

Feature	RS-232	RS-422	RS-485
Signaling	Single-ended	Differential	Differential
Nodes	1	10	32–256
Distance	short	long	long
Noise immunity	low	high	very high
Topology	point-to-point	point-to-point	multipoint

Advantages

• high noise immunity
• long distance support
• multi-node capability
• low cost implementation
• industrial robustness

Limitations

• no built-in protocol
• requires careful wiring
• no arbitration mechanism
• sensitive to topology errors

Conclusion

RS-485 remains one of the most widely used physical layer standards in industrial communication systems.

Its reliability depends heavily on correct implementation of:

• termination
• topology
• grounding
• biasing

Proper engineering design is required to achieve stable and high-performance communication.