Hello, I am faced with the challenge of connecting a RS232 Modbus RTU slave, which only has Tx, Rx, and GND signals, to a Modbus RTU master. I have found that the RS232 to RS485 converters available require the complete set of RS232 signals to work properly. However, after numerous attempts and hours of experimentation, I accidentally stumbled upon a wiring setup that successfully connected the devices. To confirm this unexpected success, I tested the setup with two different serial boards on a Windows PC (a PCIe board and a USB board) and achieved the same positive results. The configuration looked like this: Windows PC -> Modbus RTUMBPOLL (RS232C) -> PCIe or USB to RS485 Converter -> Slave The wiring setup I used was as follows: - Pin 1 (A+) connected to GND - Pin 2 (B-) connected to Rx - Pin 5 (GND) connected to Tx This configuration worked effectively, although I am unsure of the specific reasons why. Before recommending this solution to the customer, I would like to understand the underlying principles that make it successful. I appreciate any advice or insights you can provide on this matter. Thank you.
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It has been over ten years since I last connected a Modbus RTU slave to a Master using an RS-232 port. I believe that a USB/RS-232 converter with an FTDI chipset should work with just Tx, Rx, and GND connections, but I am unable to confirm as I have only worked with RS-485 in recent years. Nevertheless, I am pleased to hear that you were able to find a solution.
Hello Dan, I hope you are well. I would like to share some interesting information with you and potentially others. This particular refinery has a large number of Rosemount FF transmitters, specifically for pressure and temperature readings. They are using an older Azbil DCS that only supports RS232C, so they have added an RS232 to RS485 media converter as the devices are located far from the control system. The converter is linked to a Rosemount 3420 unit, which acts as a gateway by converting FF data to Modbus RTU for the Azbil system. However, the Rosemount 3420 has been discontinued, and the client is looking to replace it with FG-200 without needing to make changes to the Azbil DCS. This transition will require a converter that is externally powered, and I plan to provide a more robust industrial-grade solution. While I source the necessary equipment for the client, I will use my current setup to configure the FG-200 and complete the necessary mapping. Feel free to take a look at the setup in my lab, which may be a bit chaotic at the moment. Wishing you a wonderful weekend ahead.
Great job on your experimenting! My understanding is that most RS232 to RS485 converters use the RTS signal of the RS232 port to switch between send/receive modes on the RS485 bus. It seems like your Modbus RTU master may not be utilizing this feature, hence the standard converters didn't work. In your setup, you're handling the A and B data lines of RS485 directly with the Tx and Rx lines of the RS232. Handling the direction control manually can lead to abnormal behavior, but it certainly will work in applications where full duplex communication isn't expected. Proceed with caution after assessing the reliability of this setup.
It seems you've actually stumbled upon a common solution used in the field when dealing with Modbus RTU communication over RS232 and RS485 interfaces. Typically, RS232 uses the Tx, Rx, and GND lines for communication and doesn't always interface smoothly with RS485 which uses A+ (or D+), B- (or D-), and GND. Your wiring setup tricked the converter into functioning correctly. In your configuration, the Pin 1 (A+) to GND connection serves as a "handshake" signal to satisfy the RS232 to RS485 converter. Pin 2 (B-) connected to Rx enables communication from the master to the slave device, while Pin 5(GND) connected to Tx allows data from the slave back to the master. The fact that it worked on both PCIe and USB boards with separate converters lends credibility to your solution. It could be beneficial to run more extensive functional tests to rule out potential glitches but I believe you've stumbled upon a viable solution for your scenario.
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Answer: - Typically, RS232 to RS485 converters require signals such as Tx, Rx, GND, RTS, and CTS for proper functionality.
Answer: - Yes, it is possible to convert RS232 devices with only Tx, Rx, and GND signals to RS485 with a specific wiring setup, as described in the thread content.
Answer: - The successful wiring setup included connecting A+ (Pin 1) to GND, B- (Pin 2) to Rx, and GND (Pin 5) to Tx.
Answer: - The specific reasons behind the success of the unconventional wiring setup may involve signal polarity, voltage levels, or communication protocol compatibility. Further analysis may be needed to fully understand the underlying principles.
Answer: - To replicate the successful setup, one can use a Windows PC, a Modbus RTUMBPOLL (RS232C), a PCIe or USB to RS485 converter, and follow the wiring configuration mentioned in the thread content
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