RS485 2 Wire Network Setup and Best Practices

Setting up an RS485 2 wire network can seem daunting, but it's actually quite straightforward. The key is to understand the basics of the network and follow some best practices.

To start, RS485 is a differential communication standard that uses two wires: a transmit pair and a receive pair. This allows for reliable communication over long distances.

The transmit pair consists of a positive wire and a negative wire, which are used to send data between devices. The receive pair does the opposite, receiving data from other devices on the network.

In a 2 wire setup, the transmit and receive pairs are combined into a single cable, reducing the number of wires needed. This makes it ideal for applications where space is limited.

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RS485 wiring arrangements can be confusing, but let's break it down simply. There are three common arrangements: Write only, 4-wire (full duplex), and 2-wire (half duplex).

The Write only arrangement is straightforward, but the other two can be a bit more complicated. In a 4-wire full duplex arrangement, both Transmit Data (TX) and Receive Data (RX) signals are converted to RS485 or RS422.

The 2-wire half duplex arrangement is a bit more limited, using only the TX and RX signals, just like the 4-wire setup. This is a more common setup, especially in RS485 2 wire applications.

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To prevent issues with your RS485 2 wire setup, it's essential to keep the cable length short. A maximum length of 1.2 kilometers is recommended to avoid signal degradation.

Noise can be a significant issue with RS485 2 wire, so it's crucial to use proper shielding and grounding techniques. This will help to reduce electromagnetic interference and keep your signals strong.

RS485 2 wire is a differential signal, which means it's resistant to noise and interference. However, it's still possible for noise to creep in, so it's essential to use a proper termination method to prevent reflections and ensure a clean signal.

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Preventing Voltage Reflection is crucial in serial communication. Signals sent down a cable always reflect back up the cable, and the reflection is bigger as signals' edges get faster and lines get longer.

This reflection can cause issues with signal integrity. The reflection is bigger as signals' edges get faster.

To minimize this reflection, you need to terminate the ends of the line with the characteristic impedance of the system. This is 120 ohm for RS485.

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Signal names can be a source of confusion when working with RS485 connections. In some cases, the names A and B are used, while others use + and -. This inconsistency can lead to misconnections, which can be frustrating to troubleshoot.

RS485 signal names are not consistently applied by manufacturers, so it's essential to double-check the documentation for each device. This can save you time and headaches down the line.

A common mistake is assuming that A on the computer will always connect to A on the instrument. However, this may not be the case, and you may need to connect A on the computer to B on the instrument instead.

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When wiring your RS-485 2-wire connections, keep in mind that conduit can make a huge difference in reducing interference with the communication wires.

To minimize problems, keep parallel runs of power and RS-485 wires as far apart as is practical. This will help reduce the chances of interference.

If possible, use shielded CAT5, CAT5e, or CAT6 cable. These cables have 4 twisted pairs of wires, and each RS-485 connection will use one of the twisted pairs.

Shielded cable protects the communications in electrically noisy environments. This is especially important if you're working in a marine or humid environment.

To ensure reliable connections, apply dielectric grease to the connection points. This will prevent corrosion and make sure the connections last as long as possible.

Here are some tips for reducing interference:

The SN75HVD05 schematic can be a bit tricky when working with RS485 2 wire cables.

RS485 cables typically use three wires, including a signal ground (SG), but in some cases, only two wires are available. In this scenario, the explicit connection of SG to the cable needs to be stopped.

For example, if the cable length is up to 350 meters, it's necessary to take measures to match the SG level of each RS485 circuit as an alternative.

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One possible solution is to loosely ground the SG of each RS485 circuit with a high resistance of mega ohm to match the SG of each 485 circuit while suppressing the influence of GPD.

The common-mode operation voltage range of the SN75HVD05 device is -7 V to +12 V, which aligns with the minimum requirements of the RS-485 standard.

This means that as long as the ground potential differences do not cause the voltages seen by each transceiver to exceed this range, the two-wire connections can work.

Here are some possible transceivers with wider common mode ranges that can support larger GPDs:

Note that each RS-485 receiver presents a high (but not infinite) resistance from the bus to its own ground, which can help the grounds to drift towards similar voltages if the grounds between systems are all totally independent and "floating" with respect to one another.