Quick Guide to RS-485
What is RS-485
RS-485 is an industrial specification that defines the electrical interface and physical layer for point-to-point communication of electrical devices. The RS-485 standard allows for long cabling distances in electrically noisy environments and can support multiple devices on the same bus.
RS-485 Cable Selection and Other Parameters
Polar Monitoring Gateways rely on RS-485 communication through RS-485 half-duplex, differential signaling at a 9600 baud rate. There are different baud rates that are used with RS-485 communications, it ranges from 9600bps, up to 38400bps, and it can go higher. The baud rate is determined by distance, noise, and the requirements of the application.
When connecting RS-485 wires to the gateway, it is important to make sure that the polarity that you are connecting the wires in, is correct. Unfortunately, there is no set standard, and the polarity may vary from manufacturer to manufacturer. Thus it is important to view the user manual or use our wiki guides to make sure that communication does not fail.
All Polar Gateways have their standard as A+ and B-.
While short distances (<30m) in electrically quiet environments may not require specific cable considerations, long distances, noisy environments, and other factors will require careful cable selection. This is why it is important to use the Twisted Belden RS-485 cable.
Frequently Asked Questions
Q: Can I run my RS-485 cable along my Mains wire?
A: No. This is not recommended at all. This will be a guaranteed way to cause interference.
Q: Can I connect multiple devices on the same RS-485 cable?
A: Yes. The RS-485 standard can support multiple devices on the same bus provided each device has been set with a unique address.
Q: Is it necessary for me to have a common ground if I have multiple devices that have 0V?
A:e s. There should always be a common ground when there are multiple devices connected as the 0V across the devices could be slightly different, if the devices try to equalize their 0V across the A or B wire, it would cause a lot of interference and errors in the readings.
Q: Are there some devices that can cause more interference than others?
A: Yes. Some devices are more prone to interfering than others. For example, the PCS inverter range from Atess has quite large transformers, therefore their interference is quite significant. That is why we do not recommend that you run the RS-485 cable under or through these kinds of devices.
Q: Is grounding and shielding the same thing?
A: No. Shielding is the foil that surrounds the wires, and it is used to try to reduce the noise susceptibility, Ground is another wire within the cable, and it refers to connecting a circuit or electrical device to the Earth or a reference point with a low resistance. The shield is also only connected on one end of the ground wire in order to prevent a grounding loop which will cause noise on the communication wires.
You will see that the shielding is often connected to the ground wire.
Q: What are termination resistors?
A: Termination resistors in RS-485 communication play a crucial role in ensuring reliable and efficient data transmission. They serve various functions that improve noise immunity, and overall enhancement of signal quality by addressing distortions. Termination resistors are connected at both ends of the RS-485 bus to match the transmission line’s impedance, meeting standards and ensuring interoperability. In long-distance communication, they are particularly essential, maximizing signal quality and minimizing errors.