Two common interfaces of Modbus protocol

Question-solving class: Why do we recommend RS485 instead of 4-20mA in modern water quality monitoring?

In the design and integration of a water quality online monitoring system, choosing which output signal is the first step. Out of habit, many engineers and design institutes give priority to the classic 4-20mA analog output. It is simple and direct. One instrument corresponds to one parameter, which seems to be clear at a glance. However, as monitoring parameters increase and requirements for data accuracy and equipment functions increase, the limitations of 4-20mA become increasingly prominent. Today, we want to discuss with you why, in most cases, RS485 Modbus digital communication is the better and more future-proof choice.

In the design and integration of a water quality online monitoring system, choosing which output signal is the first step. Out of habit, many engineers and design institutes give priority to the classic 4-20mA analog output. It is simple and direct. One instrument corresponds to one parameter, which seems to be clear at a glance. However, as monitoring parameters increase and requirements for data accuracy and equipment functions increase, the limitations of 4-20mA become increasingly prominent. Today we want to discuss with you why RS485 Modbus digital communication is the better and more future-proof choice in most cases.

1. From “Country Road” to “Information Highway”: Revolution in Cabling Engineering

Let’s start with the most intuitive pain point – wiring.

• 4-20mA mode: This is a "dedicated line" mode. Each parameter that needs to be monitored (such as turbidity, pH, residual chlorine) requires a separate set of two-core or three-core cables to be pulled from the on-site sensor back to the collector in the control room. Scenario: If you have a sensor with 5 parameters, you need to pull 5 sets of wires. The number of cables is large, the cost is high, and the construction is complicated. Later troubleshooting line faults is like "finding a needle in a haystack".

4-20mA mode: This is a "dedicated line" mode. Each parameter that needs to be monitored (such as turbidity, pH, residual chlorine) requires a separate set of two-core or three-core cables to be pulled from the on-site sensor back to the collector in the control room.

• Scenario: If you have a sensor with 5 parameters, you need to pull 5 sets of wires. The number of cables is large, the cost is high, and the construction is complicated. Later troubleshooting line faults is like "finding a needle in a haystack".
• RS485 Modbus mode: This is a "bus line" mode. All sensors that support RS485 can be connected to the same set of twisted pairs hand in hand, and finally returned to the collection port in the computer room. Scenario: Whether you have 5 or 10 parameters, you only need 1 set of wires. This greatly saves cable costs, bridge space and construction time, making the system clear and easy to maintain.

RS485 Modbus mode: This is a "bus line" mode. All sensors that support RS485 can be connected to the same set of twisted pairs hand in hand, and finally returned to the collection port in the computer room.

• Scenario: Whether you have 5 or 10 parameters, you only need 1 set of wires. This greatly saves cable costs, bridge space and construction time, making the system clear and easy to maintain.

Conclusion: In multi-parameter monitoring scenarios, RS485 has overwhelming cost and efficiency advantages in wiring.

2. The dispute over accuracy: the truth about “range” and “reading”

4-20mA transmits a "relative value", and its accuracy depends heavily on the preset "range".

• 4-20mA Dilemma: In order to ensure that the meter does not exceed the range under extreme circumstances, a large margin is usually left during design. For example, a turbidity meter's actual measurement range is 0-10 NTU, but the range may be set to 0-100 NTU. Here comes the question: When the actual turbidity is 0.5 NTU, the output current value is approximately 4 + (0.5 / 100) * 16 = 4.08 mA. This tiny current change (0.08mA) is easily affected by line loss, electromagnetic interference, and the collector's own errors, resulting in large fluctuations in the final reading and inaccuracy.

4-20mA Dilemma: In order to ensure that the meter does not exceed the range under extreme circumstances, a large margin is usually left during design. For example, a turbidity meter's actual measurement range is 0-10 NTU, but the range may be set to 0-100 NTU.

• Here comes the question: When the actual turbidity is 0.5 NTU, the output current value is approximately 4 + (0.5 / 100) * 16 = 4.08 mA. This tiny current change (0.08mA) is easily affected by line loss, electromagnetic interference, and the collector's own errors, resulting in large fluctuations in the final reading and inaccuracy.
• Advantages of RS485: It directly transmits the final digital value calculated by the sensor. No matter your actual value is 0.3499 NTU or 35.82 NTU, what is sent back through RS485 is the precise "0.3499" or "35.82". Advantages: There is no concept of measuring range, no intermediate conversion, and the "source data" is obtained directly. The accuracy is completely guaranteed by the sensor itself and is not affected by the transmission process.

Advantages of RS485: It directly transmits the final digital value calculated by the sensor. No matter your actual value is 0.3499 NTU or 35.82 NTU, what is sent back through RS485 is the precise "0.3499" or "35.82".

• Advantages: There is no concept of measuring range, no intermediate conversion, and the "source data" is obtained directly. The accuracy is completely guaranteed by the sensor itself and is not affected by the transmission process.

Conclusion: For projects that operate at low values ​​for a long time, RS485 can provide far more stable and accurate data than 4-20mA.

3. Function expansion: Leap from “reading” to “dialogue”

This is the dimensional gap between digital communications and analog signals.

• Capability boundary of 4-20mA: The only thing it can do is pass a current measurement value. You can't get any more information through this line, nor can you make any settings on the field device.

Capability boundary of 4-20mA: The only thing it can do is pass a current measurement value. You can't get any more information through this line, nor can you make any settings on the field device.

• The infinite possibilities of RS485: Due to the two-way digital communication, you can not only read data, but also write commands to realize rich device management functions: Read multiple data: One command can obtain multiple parameters such as turbidity, temperature, TDS, etc. at the same time. Read equipment status: Obtain the working status, fault code, calibration record, service life, etc. of the sensor to achieve predictive maintenance. Remote setting and calibration: You can remotely modify the device address, range, and even trigger the calibration procedure without going to the site. Firmware upgrade: Provides the possibility for subsequent function iterations of the product.

The infinite possibilities of RS485: Due to the two-way digital communication, you can not only read data, but also write commands to achieve rich device management functions:

• Read multiple data: One instruction can obtain multiple parameters such as turbidity, temperature, TDS, etc. at the same time.

Read multiple data: One instruction can obtain multiple parameters such as turbidity, temperature, TDS, etc. at the same time.

• Read equipment status: Obtain the working status, fault code, calibration record, service life, etc. of the sensor to achieve predictive maintenance.

Read equipment status: Obtain the working status, fault code, calibration record, service life, etc. of the sensor to achieve predictive maintenance.

• Remote setting and calibration: You can remotely modify the device address, range, and even trigger the calibration procedure without going to the site.

Remote setting and calibration: You can remotely modify the device address, range, and even trigger the calibration procedure without going to the site.

• Firmware upgrade: Provides the possibility for subsequent function iterations of the product.

Firmware upgrade: Provides the possibility for subsequent function iterations of the product.

Conclusion: If you only use the 4-20mA interface, you are equivalent to using less than half of the functions of our sensor, which is a huge waste of resources.

4. Comprehensive comparison table: clear choice at a glance

Conclusion and suggestions

We understand and respect traditional design habits. 4-20mA still has value in simple situations with a single parameter, short distance, and no interaction. But facing today's multi-parameter, high-precision, and intelligent water quality monitoring needs, the RS485 Modbus protocol is undoubtedly a more economical, more accurate, and more powerful solution.

YuanGan Technology sincerely recommends that you give priority to using the RS485 interface to connect to our water quality sensors when designing new projects or renovating old systems. This can not only save a lot of cost and post-maintenance energy for the project, but also fully unleash the full potential of smart sensors, providing a more solid data foundation for your smart water and environmental monitoring platforms.

—— YuanGan Technology will work with you towards a new era of accurate and intelligent digital monitoring!