Ensure that your Modbus RTU network is properly terminated for optimal performance and reliability.
The master node patiently awaits a response from each of the connected nodes based on a designated timer. During this waiting period, it attempts to send requests to other nodes. I'm uncertain about the concept of "silence time," as it's not something I've encountered before. Do you believe that challenges like these could arise when the master node is from a different vendor than the slave nodes? We have examined the network both with and without terminating resistors, and found no significant differences.
An RS485 network is designed to accommodate up to 32 devices without the need for a repeater. However, once you reach 26 connected devices, you are nearing this threshold. If the termination of your network isn't properly configured, or if the impedance of the connections is not correct, your RS485 network may struggle to support the maximum number of devices. Additionally, it's crucial to note that the total cable length should not exceed 4,000 feet; otherwise, you might find it necessary to implement a repeater for optimal performance.
1. Does the system include a dedicated signal ground wire in addition to the A/B driver lines? I’m not referring to a shield but rather a third wire specifically designated for the signal ground.
2. In a multidrop daisy chain configuration featuring numerous slave devices, the polling speed of the master device is limited to the response time of the slowest slave. Typically, the master unit has an adjustable timeout setting, which determines the wait time for a response before initiating a poll of the next slave. Have you considered extending the timeout duration to allow the slower slave sufficient time to transmit its response before the master polls again?
3. Steve Bailey offers valuable insights based on his experience. If I were managing your network, I would suggest installing an isolating repeater at the midpoint of the line to assess whether this modification enhances performance.
1: Unfortunately, there is no third wire signal ground available. We conducted tests including this configuration, but we observed no reduction in packet loss. This raises an interesting question: if Modbus utilizes RS-485 and often incorporates A/B and a signal ground, why does Profibus never implement a third wire signal ground despite also being based on RS-485?
2: You’re absolutely correct. We have tested adjusting this parameter up to 100ms, but it made no noticeable difference.
3: The nearest node experiencing issues is only around 60 feet away, so I believe a repeater is unnecessary in this case. Thank you for all your valuable feedback!
This is a common issue faced by many users. There's a 98.4% likelihood that your difficulty stems from the absence of bias resistors in your setup. In a point-to-point, four-wire configuration, both the master and slave transmitters actively drive the bus to a known state. However, in a multidrop configuration, there's no active signal driving the bus between requests and replies, which causes the RS422/RS485 drivers to enter tri-state mode. This results in the bus being in an undefined state, making it highly susceptible to interference. Consequently, your system may generate erroneous ones and zeros, misleading the receivers into looking for non-existent messages while overlooking actual ones.
Integrating bias resistors can help by establishing a known state for the A/B lines. One resistor pulls one wire up to +5V, while the other pulls the second wire down to 0V, working alongside the standard termination resistor positioned at the center of the line.
It's important to note that high-quality RS485 converters typically come with these bias resistors pre-installed for multidrop configurations, alleviating the need for external installation. Furthermore, your organization should have included them with your Modbus setup, as omitting them in a multidrop system represents a significant hardware design flaw. Ensuring proper installation of these components is crucial for optimal performance and reliability in your serial communication systems.
Pete.S. remarked: "This is a common issue. With a confidence level of 98.4%, it appears that the root cause of your problem is the absence of bias resistors. In a point-to-point, 4-wire configuration, both the transmitter on the master and slave units are actively driving the bus to a defined state. However, in a multidrop setup, there is no active drive on the bus during periods between requests and responses. This results in the RS422/RS485 drivers entering tri-state mode, leaving the bus in an undefined state. Consequently, the system becomes highly susceptible to electromagnetic interference, causing erratic bits—consequently fooling receivers into searching for nonexistent messages while overlooking actual ones.
To resolve this, bias resistors can be employed to establish a defined state on the A and B lines. Specifically, one resistor will pull one wire to +5V, while the other pulls it down to 0V, along with a standard termination resistor placed between the two. For optimal performance in multidrop configurations, quality RS485 converters typically come with these bias resistors pre-installed, eliminating the need for external installations. In fact, your company should have included them in your Modbus hardware design, as their absence represents a significant oversight in hardware configuration.
I appreciate your insights, and I plan to integrate this solution into our network setup. We often utilize Phoenix Contact converters for 485/485 or 232/485 communications, and I'll verify whether those converters include the necessary internal balancing resistors. Can you recommend any specific devices dedicated to achieving this balancing functionality?"
This is a boxed model; however, by using a couple of resistors along with a 5V DC power supply, you can achieve similar functionality. For more details, visit this [Phoenix Contact link](https://www.phoenixcontact.com/online/portal/fi?uri=pxc-oc-itemdetail:pid=2313944&library=fifi&tab=1). Additionally, it’s crucial that you test the configuration with 120-ohm resistors at both ends of the bus. I’ve observed that if there isn’t proper bias applied when required, none of the queries will function correctly.
Thank you for your inquiry! I have the item available at the following link: [Phoenix Contact Product](https://www.phoenixcontact.com/onli...k=P-08-13-02-02-01&tab=1&selectedCategory=ALL). Based on the circuit schematic, it appears that the balancing component is integrated internally.
Orense noted that the master device waits for a response from each node based on a predefined timer. In the interim, it attempts to request data from additional nodes. I am uncertain about the concept of silence time, as it is unfamiliar to me. Do you believe that challenges like this can arise when the master and slave devices come from different manufacturers?
In principle, mixing devices from various vendors should not cause issues, provided they adhere to the established standards. For instance, not adhering to the silence period can lead to communication failures. Imagine sending a request, but your software takes too long to compute the checksum. When the checksum is finally transmitted, there's a delay of two character times. The receiving device might discard the packet portion that lacks the checksum because it detects a silence longer than 1.5 characters, resulting in an incomplete transmission.
Now, consider another scenario: Your software sends a packet to one device but fails to wait for 3.5 character times before dispatching data to a second device. The first device may merge both packets into one, which could render it invalid.
In summary, ensure that your software sends all data of a packet promptly and allows sufficient time before transmitting the subsequent packet. This attention to timing will help maintain effective communication and prevent packet loss.
Orense mentioned that the master device waits for a response from each node based on a predetermined timer. During this wait, it actively seeks requests from other nodes. However, this raises a concern for me. Does this imply that while the master is waiting for a response from one node, it is simultaneously reaching out to others? If that is the case, it could potentially lead to data collisions. Jerry
JerryH commented: "This doesn't seem accurate to me. Are you suggesting that while awaiting a response from one node, you are simultaneously attempting to connect with other nodes? If that’s the case, it could potentially lead to data collisions." Jerry went on to clarify: "It's quite common practice to implement a timeout mechanism and switch to another node if the current one is unresponsive. If a node doesn’t reply within a few seconds, it's likely that it won’t."
OkiePC commented: It's quite typical to implement a timeout timer that allows transitioning to the next node in the network. If a node fails to respond within a few seconds, it likely won't. The original poster mentioned that the polling rate has been increased to 100ms. It's crucial that the polling rate exceeds the timeout for the slave node. If a new query is sent before the previous slave node has responded to the earlier request or before the timeout elapses, it can lead to collisions on the bus, resulting in communication issues.
Archie remarked that Modbus RTU relies on silence intervals to identify the conclusion of data packets and also plays a role in clearing receive buffers. Certain devices may follow this protocol stringently, potentially disregarding a packet if the buffer is cleared too soon due to silence. Does your system comply with the 3.5 and 1.5 character silence rules?
I have reviewed the specifications for the slave devices and confirmed that they adhere to a 3.5 character silence period. According to the slave's manual: "The end of the frame is recognized when no transmission occurs during the duration required for the transmission of three and a half characters (3.5 times the character delay time) as detailed in the Modbus Protocol Reference Guide."
Does this indicate that the master device must also be configured to observe the same 3.5 character silence period? Thank you!
Orense inquired: "Does this imply that the master must also implement a silence period of 3.5 characters?"
To clarify, the answer is yes. When operating on an RS485 communication network, all slave devices receive identical data transmissions. For instance, if the master device sends a data packet directed at node 1 and then quickly follows it with another packet aimed at node 2 without a silence gap of 3.5 characters, every node will interpret this as a single, invalid concatenated packet. This highlights the importance of adhering to the required silence intervals for proper communication integrity in RS485 networks.
In my opinion, Modbus RTU is missing a "bytes to follow" field in its packet header, a feature that is present in Modbus TCP. Incorporating this element would eliminate the need for the unnecessary silence of 3.5.
In my opinion, Modbus RTU is missing a "bytes to follow" field in its packet header—a feature that Modbus TCP includes. This absence makes the 3.5 seconds of silence unnecessary. When operating as a master, it is responsible for controlling the data flow. After sending a request, the master can determine the expected number of bytes to be returned, which typically involves 9 bytes for RTU responses or error messages.
Slaves can estimate how many bytes will follow once a few initial bytes have been received, but they must still perform error checking before responding. It's also possible to identify the end of a message before the designated silence period concludes.
Interestingly, I've observed that a response time of 100 milliseconds is permitted for slaves before the master issues another message, aside from after a broadcast. For low-power slaves—especially those using GPIO pins for RS-485 control rather than dedicated integrated circuits—this 100ms waiting period may not be ideal.
To diagnose problematic slaves, logging all requests and responses can be beneficial. A straightforward approach is to extend the response wait time to 2 seconds prior to sending the next request. If issues resolve during this extended timeframe, it indicates that the master should consider increasing its wait time from the current 100ms.
Another potential challenge arises when slaves release the communication line and enter tri-state mode after sending their message. If the master is significantly faster, it may receive the response, process it, and immediately send the next command. However, if the slave has not had enough time to release the bus, it could cause a conflict, as it continues to try and drive the communication line while the master attempts to transmit its next message.
These observations highlight the intricacies of the Modbus RTU protocol and the importance of effective timing and error handling in communication systems.
Mark commented: "I found it intriguing that a 100ms response time was permitted for a slave device before the next message is transmitted (except following a broadcast), especially without additional context regarding the slaves, their speed, or whether any slaves are powered down. For low-performance slave devices, particularly those using I/O pins for RS-485 control instead of a dedicated integrated circuit, a 100ms response window is generally not advisable."
He continued: "A straightforward test you could conduct is to extend the allowed response time to 2 seconds before sending the next request. If you run this test for a duration longer than the typical failure time and the issue is resolved, it suggests that the master device should consider increasing the response time beyond 100ms." I completely agree with this assessment.
Orense noted that the nearest problematic node is roughly 60 feet away, so there's no immediate requirement for a repeater. The primary concern is not the need for repeater functionality, but rather the necessity for isolating from potential ground loops, which can fluctuate over time, as well as for biasing purposes. Many isolators come equipped with built-in biasing features.
For detailed guidance on performing a voltmeter check for additional biasing, refer to this application note: [DC Bias Tech Note](https://www.icpdas-usa.com/documentation/DC%20Bias%20Tech%20Note.pdf). ICP-DAS offers a module featuring screw terminals that can provide either termination or biasing. It's important to locate the V+ and GND connections on the RS-485 chip and connect them to the module, a requirement applicable regardless of the devices involved.
For more information, you can access the module's specification sheet here: [Module Spec Sheet](https://files.icpdas-europe.com/products/K126689/web/icpdas/datasheet-tm-sg4_cr.pdf). It's worth noting that biasing is implemented at a single point within a 485 multi-drop network, and the DIP switch settings for bias wiring can be found in the provided resources.
When it comes to troubleshooting RS485 communication buses, it's essential to have the right tools at your disposal. Here are the key items you'll need:
1. **RS485 Transceiver**: This device is crucial for connecting your PC to the RS485 network.
2. **Communication Monitoring Software**: Utilize software designed to record and analyze data transmissions.
3. **Digital Storage Oscilloscope (DSO) or Analog Oscilloscope**: These instruments are vital for visualizing the signal integrity on the bus.
Having the capability to "sniff" the communication on the RS485 bus is fundamental for diagnosing issues. This allows you to monitor what data has been transmitted and the timing of those transmissions.
Additionally, analyzing the waveform signals is critical to identifying any potential electrical problems, such as bias resistor issues, which can be easily detected with an oscilloscope.
Lastly, it is crucial that the Modbus master is implemented correctly to ensure proper error handling and retransmission in the event of communication failures. This is why the Modbus protocol incorporates checksums and acknowledgment messages to maintain data integrity.
By equipping yourself with these essential tools and knowledge, you'll be well-prepared to effectively troubleshoot RS485 communication systems and ensure optimal performance.
Pete S. stated: "This is a well-known issue. With a certainty of 98.4%, the root of your problem likely lies in the absence of bias resistors. In a point-to-point, four-wire configuration, both the master and slave transmitters drive the bus to a defined state. However, in a multidrop setup, there's no active signal between requests and replies. The RS422/RS485 drivers operate in tri-state mode, resulting in the bus being in an undefined state. This lack of stability makes the system extremely vulnerable to interference, leading to numerous false signals that can confuse receivers into searching for non-existent messages while overlooking actual ones.
Introducing bias resistors helps establish a known state on the A/B lines. One resistor pulls one wire up to +5V, while the other pulls the second wire down to 0V, with a standard termination resistor placed in between.
High-quality RS485 converters often come with built-in bias resistors designed for multidrop applications, eliminating the need for external components. Ideally, your company should have incorporated these resistors in your Modbus setup. The omission of bias resistors in a multidrop configuration represents a significant hardware design oversight.
Thank you for sharing this information. We integrated a 485/485 galvanic isolator that had internal bias resistors, and after installation, our communication improved notably. We did adjust the polling time accordingly, but since then, we've experienced no issues.
Does anyone have recommendations for websites where I can find reliable information on polling times, message delays, and related topics?"
Orense inquired: Are there any websites that provide reliable information on polling times and message delays? Click to expand... I've never encountered a manufacturer that shares specifications regarding response times for polling. The response time of a slave device is simply fixed. One challenge in creating these specifications is defining what a 'Modbus task' truly entails. For instance, an FC16 write operation for 50 registers may take significantly longer to complete than an FC03 read operation for just 2 registers. Additionally, different devices may face varying loads, which can impact their execution time and response rates. When specifications are published, expectations are set; without a spec, there are no expectations. It is what it is.