Hello all, I am looking for recommendations on the best type of cable to use for RS-485 2-wire + GND connections, including Data+, Data-, and GND. I have been considering using Belden 9841 or 9842 with AWG24, but I have concerns about using screw terminals for intermediate connections. It is important to maintain the cable's characteristic impedance of 120 ohm for RS-485. While my runs will not be very long, around 30m or less, it is crucial not to ignore the "GND" connection, especially if both systems do not share the same 0V reference. Additionally, it is advised not to use the shield for the GND connection. Thank you.
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In a previous project, I utilized Belden 8770, a 3-conductor shielded twisted pair (STP) cable with a 70 Ohm impedance. To optimize signal transmission, I employed two 120 Ohm resistors in parallel as terminating resistors, achieving a close match for the configuration. This setup successfully operated at a baud rate of 38.4K for a point-to-point connection. For more detailed technical information, refer to the datasheet available at https://www.farnell.com/datasheets/2331478.pdf.
Thank you, gentlemen, for your much-appreciated assistance.
The Belden 8760 is my go-to choice for connecting my 50 RS-485 instruments on site. Any issues I've encountered were due to my own errors. Find this high-quality audio cable on RS Online for a reliable and efficient connection.
I appreciate your advice on the RS-485 connection. In my research, I found that while the "GND" connection is labeled as optional in many sources and manuals, it is recommended to connect it when the 0V reference may vary between ends. Most RS-485 ports already have the GND terminal available. I am looking for a 1.5 pair cable, ideally LSZH-rated to address potential safety concerns. I believe the Belden 9842LW cable may meet my requirements. There is a trade-off to consider with the small AWG24 size as it relates to screw-terminals. However, the cable specifications mention a wide range of suitable sizes from AWG26 (or even AWG30, 0.2mm2) to AWG 12 (4mm2), providing flexibility in installation.
Your considerations are on point and I concur that maintaining the impedance and ensuring the correct GND connections are critical. Belden 9841 or 9842 are indeed solid choices and come highly recommended. Alternatively, you might want to consider an equivalent, such as West Penn 253240. This offers AWG24 and conforms to the same impedance standards. In terms of the screw terminals concern, I've found that using a high-quality terminal block with good clamping force provides reliable results. Ultimately, having a proper wire preparation and termination procedure might mitigate any issues that you're worried about. And yes, using shield as GND could lead to ground loops, creating noisy signals. Therefore, it is always better to refrain from doing this unless absolutely necessary.
The Belden 9841 cable should work quite well for your purposes; it's specifically designed for low-capacitance data transmission for RS-485 and offers great quality for both Data+ and Data- connections. Granted, AWG24 is a bit on the thin side, but given your relatively short run length, it should be fine. As for the GND connection, I completely agree, it’s often overlooked but very important. Might I suggest using a separate dedicated wire for your GND connection, just to avoid any potential ground loop issues? And a heads up with the screw terminals, make sure to avoid over-tightening as it may cause deformation, affecting your characteristic impedance. Ensure you do a solid termination job there. Cheers!
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Answer: - The recommended cable types for RS-485 2-wire + GND connections include Belden 9841 or 9842 with AWG24 to maintain the characteristic impedance of 120 ohms.
Answer: - The cable runs for RS-485 connections can be around 30 meters or less.
Answer: - Maintaining the GND connection is crucial, especially if both systems do not share the same 0V reference, to prevent potential issues with signal integrity.
Answer: - It is advised not to use the shield for the GND connection in RS-485 setups to avoid potential grounding issues.
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