Selection and Design of RS-485 Chip for Communication Interface

The selection and design of an RS-485 interface is critical for robust industrial, automotive, and building automation systems.

Here is a comprehensive guide covering the selection criteria, key design considerations, and a step-by-step design process.

1. Understanding the RS-485 Standard

Before selecting a chip, understand what you need to achieve:

• Differential Signaling: Uses two wires (A and B) to transmit a signal. Noise coupled onto the cable appears equally on both lines and is rejected by the receiver. This enables long distances and noise immunity.

• Multi-Drop: Supports up to 32 unit loads on a single bus. Many modern transceivers are 1/4 or 1/8 unit load, allowing for 128 or 256 devices on the bus.

• Half-Duplex: Uses one pair of wires for both transmitting and receiving. Requires direction control.

• Full-Duplex: Uses two pairs of wires (four wires), one for transmitting and one for receiving. Allows for simultaneous communication.

2. RS-485 Transceiver Selection Criteria

When choosing a chip, evaluate the following parameters against your system requirements:

A. Electrical Characteristics

1. Supply Voltage (Vcc):

   • 5V: Traditional, still very common.

   • 3.3V: Standard for modern microcontrollers (STM32, ESP32, etc.).

   • Wide Supply (3V to 5.5V): Offers flexibility and compatibility.

2. Data Rate (Speed):

   • Standard Speed: Up to 500 kbps (e.g., SN65HVD72). Sufficient for most industrial protocols (Modbus, Profibus).

   • High Speed: Up to 20+ Mbps (e.g., MAX13487E). Needed for high-bandwidth applications. Higher speed often reduces cable length.

3. Number of Drivers/Receivers:

   • Half-Duplex (1 Driver + 1 Receiver): Most common. Requires a single control pin for direction (DE/RE#).

   • Full-Duplex (1 Driver + 1 Receiver): Less common, used in point-to-point or 4-wire systems.

   • Multi-Driver/Receiver ICs: Some chips integrate multiple transceivers to save space.

4. Unit Load (UL):

   • Defines how much load a device places on the bus. Standard is 1 UL.

   • 1/4 UL (e.g., MAX13487E) or 1/8 UL (e.g., SN65HVD78): Allows you to connect many more devices (128 or 256) on the same bus without exceeding the 32 UL standard limit.

5. ESD Protection:

   • Critical for robustness. Look for high ESD ratings on the bus pins (A, B).

   • ±15kV (HBM): Good. ±30kV (HBM): Excellent. Some chips offer IEC 61000-4-2 (Level 4) protection, which is very robust.

6. Fault Protection:

   • Bus Pin Fault Protection: Look for chips that can withstand voltages beyond the supply rail (e.g., -7V to +12V) on the A/B pins. This protects against miswiring or faults.

7. Operating Temperature:

   • Commercial (0°C to 70°C): For office equipment.

   • Industrial (-40°C to 85°C): The standard for most field equipment.

   • Extended/Automotive (-40°C to 125°C): For harsh environments.

B. Functional Features

1. Slew-Rate Limiting:

   • Non-Slew-Rate Limited: For high-speed operation.

   • Slew-Rate Limited (e.g., MAX487): Intentionally slows down the edge rates of the digital signal to reduce EMI and reflections, allowing for error-free communication on imperfect cables. Essential for lower-speed, long-distance networks.

2. Fail-Safe Biasing:

   • Receiver Fail-Safe: Guarantees the receiver output (RO) is in a known high state (logic 1) when the bus is open (no devices transmitting) or short-circuited.

   • True Fail-Safe: Some chips have internal biasing resistors to ensure this state without external components.

3. Low Power Modes:

   • Shutdown/Standby Mode: Many modern transceivers have a low-power mode where the quiescent current drops to microamps. Crucial for battery-powered devices.

3. Popular Chip Families & Manufacturers

4. Interface Design and Layout Considerations

Selecting the chip is only half the battle. Proper design is key to reliability.

A. Schematic Design

1. Termination Resistors:

   • Why? To prevent signal reflections at the end of the cable. Always use at both ends of the bus.

   • Value: Typically 120Ω, matching the characteristic impedance of the twisted-pair cable.

2. Bias Resistors:

   • Why? To ensure a known logic state on the bus when no device is transmitting (failsafe).

   • How? A pull-up resistor (e.g., 1kΩ) to Vcc on Line A and a pull-down resistor (e.g., 1kΩ) to GND on Line B. Only required on one node on the network, often the master. Many modern transceivers have integrated biasing, making external resistors optional.

3. Protection Components:

   • TVS Diodes: Place a bi-directional TVS diode (e.g., SMAJ12A) between A-GND and B-GND to clamp high-energy voltage transients (ESD, surges).

   • Series Resistors: Small series resistors (e.g., 10Ω) on the A and B lines can help limit current during transient events.

4. Power Supply Decoupling:

   • Place a 0.1µF ceramic capacitor as close as possible between the Vcc and GND pins of the transceiver. A larger bulk capacitor (e.g., 10µF) may also be needed for isolated designs.

B. PCB Layout

1. Keep it Tight: Place the transceiver, termination resistors, and decoupling capacitor very close to each other.

2. Thicken Traces: Use wide traces for the A and B lines leading to the connector.

3. Ground Plane: Use a solid ground plane underneath the transceiver circuitry for a stable return path.

4. Isolation Gap: If using an isolated transceiver, maintain a clear gap (e.g., >4mm) between all copper on the primary and secondary sides of the isolation barrier. Follow the IC manufacturer's layout guidelines exactly.

C. Cabling and Connection

1. Use Twisted-Pair Cable: This is mandatory. The twisting provides common-mode noise rejection.

2. Use a Shielded Cable: Highly recommended in electrically noisy environments. Connect the shield to earth ground at one point only (usually the master end) to avoid ground loops.

3. Daisy-Chain Topology: Avoid stubs. The main trunk should run from one device to the next. Long stubs cause signal reflections.

5. Step-by-Step Selection and Design Process

1. Define System Requirements:

   • Data Rate? Distance? Number of nodes?

   • Power supply (3.3V or 5V)?

   • Operating environment (temp, noise level)?

   • Is isolation required? (Different buildings? Large ground potential differences?)

2. Select the Transceiver:

   • Use the criteria above to narrow down to 2-3 potential chips from major manufacturers (TI, Analog Devices, Maxim, Silicon Labs).

3. Design the Schematic:

   • Include termination, biasing, and protection circuits.

   • Don't forget the direction control (DE/RE#) logic from your microcontroller.

4. Plan the PCB Layout:

   • Group related components together. Prioritize the placement of the transceiver and its decoupling capacitor.

5. Implement Software Control:

   • Ensure your firmware correctly controls the DE (Driver Enable) pin. You must be in receive mode (DE=0) when not actively transmitting. A common mistake is leaving the driver enabled, which blocks the entire network.

By carefully considering these selection criteria and design principles, you can create a highly robust and reliable RS-485 communication interface.