How to Quickly Resolve RS485 Network Communication Issues (Part 2)
The previous section has analyzed several RS-485 networking issues; below are the corresponding solutions for these problems.
II. Waveform Testing Method
Given the widespread use of RS-485 buses, the issues encountered extend beyond those mentioned above. After addressing these issues, the location of communication anomalies can be identified by testing the bus waveform, allowing for the determination of the underlying cause.
1.Check the transmission function of the RS-485 transceiver
During communication anomalies, measuring the waveform correlation between the RS-485 bus AB differential voltage and the TXD/RXD pins of the module enables identification of the faulty location. Using the testing method illustrated in Figure 4, the waveform shown in Figure 5 was obtained: when TXD is high, A-B is high; when TXD is low, A-B is low, and the module output level remains normal, confirming proper transmission functionality.
Figure 4: Test whether the sending function is working properly
Figure 5: Normal waveform of the transmission function test
2.Check the receiving function of the RS-485 transceiver
Using the testing method shown in Figure 6, the waveform depicted in Figure 7 was obtained. When A-B is at a high level, RXD is also at a high level; when A-B is at a low level, RXD is at a low level. Moreover, the output level of module RXD is normal, indicating that the module's reception function is functioning properly.
Figure 6: Test whether the receiving function is normal
Figure 7: Normal waveform of reception function test
3.Check the logical relationship between the RS-485 transceiver control pins and TXD/RXD pins
For the RSM485PCHT, the CON pin must be set to either a low or high level at least 25 μs before sending or receiving signals, and the transmit/receive state should only switch after data transmission/reception is completed.
Figure 8 TXD and CON Test
Figure 9: Data transmission CON waveform
Figure 10: Reception data CON waveform
III. Transceiver damage
1.Damage caused by the AB module pins exceeding the common-mode voltage range
The RS-485 transceiver's AB pins typically require a common-mode voltage range of-7V to +12V; exceeding this range can damage the chip. In industrial environments, ground currents often carry transient high currents. If the transceiver's RGND pin is improperly connected, the common-mode voltage at the AB pins may exceed their tolerance threshold, leading to module failure. The following analysis uses the RSM485PCHT as an example.
Figure 11 Schematic diagram of RGND multi-point grounding (incorrect connection)
When U1 sends a high level, using RSM485PCHT as an example:
Since both modules are directly connected to the chassis or ground, when a significant current flows through the chassis or ground, a V-earth voltage difference is generated between the RGND pins of U1 and U2. When U1 transmits data to U2, the voltage at U2's A pin is:
Since the VA1 voltage is 5V during transmission, exceeding 7V on VEARTH may cause damage. Therefore, in practical applications, the RGND between nodes can be connected via the shield of a shielded twisted pair cable, with the shield grounded through a RC single-point connection as shown in Figure 12.
Figure 12 RGND Recommended Connection
2.High-level static electricity and surges cause module damage
In application environments with high levels of static electricity and voltage surges, relying solely on an RS-485 transceiver chip or module may cause damage to the device. In such cases, a peripheral protection circuit must be added to safeguard the transceiver. However, this protection circuit requires reliable grounding to dissipate static and surge energy effectively. Taking a common-mode surge test as an example (Figure 13), if the protection circuit is not grounded, the surge energy (shown in red) is released through the isolation module, and higher surge levels can easily damage the module. When the protection circuit is grounded (Figure 14), the surge energy is first dissipated to ground via the GDT, then released through the TVS and capacitors, with only a minimal amount remaining before being discharged through the module, thereby providing effective protection.
Figure 13: The protection circuit is not connected to ground
Figure 14: Protection circuit grounded