RS485 Long-Distance Communication Issues

1 Overview

This document introduces the background and scope of RS485 long-distance communication (≥200m) issues encountered with ED-CM4SEN, ED-IPC2100, and ED-IPC2600 devices.

1.1 Background

n the design of the RS485 circuits for ED-CM4SEN, ED-IPC2100, and ED-IPC2600 products, our company added a ​120Ω termination resistor at the end of the RS485 circuit to ensure reliable signal transmission during high-baud-rate, long-distance communication. Users can enable this resistor for improved signal integrity in such scenarios.

However, some users reported communication failures with connected sensors when enabling the 120Ω termination resistor under high baud rates (e.g., 38400) and long distances (200m). Signal waveform testing revealed anomalies in the RS485 signal.

Our engineers conducted detailed tests on the RS485 communication of ED-CM4SEN, ED-IPC2100, and ED-IPC2600 devices:

  • ​With the 120Ω termination resistor enabled, RS485 communication failed at baud rates ≤115200 over long distances. The analysis of the above phenomena concludes that the cause is a product design defect.

1.2 Applicability

Affected products include:

  • ED-CM4SEN V1_3
  • ED-IPC21x0 V1_2

2 RS485 Usage Recommendations and Product Improvements

This section provides recommendations for using RS485 on ED-CM4SEN, ED-IPC2100, and ED-IPC2600 devices, along with our corrective actions.

2.1 RS485 Usage Recommendations

In scenarios ​without the 120Ω termination resistor enabled, we tested RS485 communication over ​1200m at various baud rates (19200, 38400, and 57600), and all communications functioned normally. Therefore, we recommend:

  • Do not enable the 120Ω termination resistor for long-distance RS485 communication.
  • If specific scenarios require enabling the 120Ω termination resistor, contact our engineering team for support.

2.2 EDATEC Measures

To resolve the RS485 long-distance communication issue, we have implemented a hardware revision. Corresponding product hardware (PCBA) will be upgraded to ​ED-CM4SEN V1_4 and ​ED-IPC21x0 V1_3.