5V Low Power RS-485 / RS-422 Differential Bus Transceiver# Technical Documentation: LMS1487CMX RS-485/RS-422 Differential Bus Transceiver
Manufacturer : National Semiconductor (NSC)
Document Revision : 1.0
Date : October 26, 2023
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## 1. Application Scenarios
### 1.1 Typical Use Cases
The LMS1487CMX is a monolithic, low-power differential line driver and receiver pair designed for bidirectional data communication on multipoint bus transmission lines. It is fully compliant with the TIA/EIA-485 (RS-485) and TIA/EIA-422 (RS-422) standards.
* Point-to-Point and Multipoint Data Transmission: The device's driver features three-state outputs, allowing multiple drivers to be connected to a common bus. The receiver inputs feature high input impedance and a failsafe design, ensuring a logic-high output when the inputs are open, shorted, or terminated but not driven.
* Half-Duplex Communication Networks: In a single twisted-pair configuration, the LMS14877CMX facilitates bidirectional data flow. The driver's enable (`DE`) and receiver's enable (`/RE`) pins allow the device to switch between transmit and receive modes, making it ideal for master/slave architectures like Modbus RTU.
* Noise-Immune Long-Distance Communication: Utilizing differential signaling, the device provides excellent common-mode noise rejection (typically ±7V), enabling reliable data transmission at rates up to 10 Mbps over distances exceeding 1200 meters at lower speeds.
### 1.2 Industry Applications
* Industrial Automation & Process Control: Found in Programmable Logic Controllers (PLCs), Distributed Control Systems (DCS), and sensor/actuator networks (e.g., PROFIBUS, Modbus physical layer).
* Building Automation: Used in HVAC control systems, lighting control networks, and security system data backbones.
* Telecommunications Infrastructure: Employed in base station controllers, network routers, and switching equipment for internal subsystem communication.
* Point-of-Sale & Retail Systems: Connects terminals, cash drawers, and receipt printers in a robust, multi-drop network.
* Automotive Test & Measurement: Used in diagnostic equipment and data acquisition systems within manufacturing and service environments.
### 1.3 Practical Advantages and Limitations
Advantages:
* Robustness: Meets or exceeds RS-485 standards for driver output voltage (±1.5V min differential into 54Ω) and receiver input sensitivity (±200mV).
* Low Power Consumption: Typically draws 300 µA of supply current when quiescent (both driver and receiver disabled), suitable for power-sensitive applications.
* Thermal Shutdown Protection: The driver includes internal thermal shutdown circuitry to protect against excessive power dissipation.
* ESD Protection: Device pins offer a level of Electrostatic Discharge protection, enhancing handling and field reliability.
* Wide Common-Mode Range: Receiver input voltage range of -7V to +12V allows significant ground potential differences between nodes.
Limitations:
* Half-Duplex Only: The LMS1487CMX is designed for half-duplex operation. Full-duplex systems require a separate transceiver pair (e.g., a driver and a receiver on independent channels).
* Limited Data Rate: While suitable for most industrial protocols, its maximum 10 Mbps data rate is insufficient for very high-speed serial applications.
* External Components Required: Robust networks require external termination resistors (typically 120Ω across the differential pair at both ends) and may need bias resistors to establish a failsafe state.
* No Integrated Isolation: Does not include galvanic isolation. Applications in environments with large ground shifts require external isolation barriers (e.g., digital isolators or isolated power supplies).
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