How to Quickly Resolve RS485 Network Communication Issues

The advantages of the RS-485 bus are well-established, and isolation RS-485 modules are typically employed to ensure communication stability. However, during actual RS-485 network deployment, issues such as communication failures, error occurrences, or damaged RS-485 transceivers may arise to varying degrees. This article provides an in-depth analysis of the underlying causes of these networking issues.

When a communication error occurs or communication fails, first determine whether the application falls under the application scenarios listed in Table

Table 1 RS-485 Bus Application Scenarios

Table 1 outlines four application scenarios corresponding to issues involving terminal resistance, pull-up/pull-down resistors, control pins, and logic input level. A detailed analysis follows below.

1、Terminal resistor issue

If a terminal resistor is connected to the RS-485 bus and the RS-485 transceiver uses a threshold level of ±200 mV, the abnormal phenomena described in Table 2 may occur.

Table 2 Abnormal phenomena caused by terminal resistance

Figure 1 RS232/485 bidirectional converter

The cause of anomalies due to terminal resistance:

The RS-485 transceiver has a reception threshold level of ±200 mV. Specifically, when the differential voltage between AB exceeds +200 mV, the output is high; when it falls below-200 mV, the output is low; when the voltage falls within ±200 mV, the output state becomes indeterminate—it may output either a high level (indicating normal communication) or a low level (indicating communication anomaly). Consequently, when the bus is idle and the AB differential voltage lies within this threshold range, anomalies may occur with a certain probability.

Table 2 Phenomenon 1 Analysis: The board communicates normally, but after networking, the presence of a termination resistor on the RS-485 bus causes the differential voltage on the bus to remain within the threshold level during idle states, resulting in communication anomalies. When this occurs, first verify whether a termination resistor is present on the bus.

Table 2 Phenomenon 2 Analysis: During single-board testing, when either the board or its communicating device is connected with a terminal resistor, communication anomalies may occur with a certain probability if the differential voltage between AB falls within the threshold level.

Table 2: Analysis of Phenomenon 3: This phenomenon is also caused by terminal resistance. The AB pins of the RS-232/485 bidirectional converter (as shown in Figure 1) typically incorporate small-value pull-up/pull-down resistors (e.g., 560Ω). When using the RS-232/485 bidirectional converter for communication, the voltage during the RS-485 bus idle state is determined by voltage division between the pull-up/pull-down resistor and the terminal resistance. In this case, the AB differential voltage during idle exceeds 200 mV, causing the RS-485 transceiver to output a stable high level without triggering communication errors. Figure 2 illustrates communication between two RSM485PCHTs with a 60Ω resistor connected between AB and the RS-232/485 bidirectional converter's test waveform; the idle voltage measures 520 mV, preventing errors. Figure 3 shows the test waveform with a 60Ω resistor between AB during communication between two RSM485PCHTs, revealing an AB differential voltage of 40 mV during idle—within the threshold range—but still potentially causing communication errors.

Figure 2: Add two 120Ω resistors between AB and install an RS-232/485 bidirectional converter.

Figure 3: Only two 120Ω resistors are added between AB.

There are primarily three solution methods, as detailed in Table 3:

 Table 3 – Solutions to Terminal Resistance Issues