The 4-20 mA current loop remains one of the most dominant types of analog output in the industry today.

I have been working with wiring industrial transmitters for some time now, and one thing I found out is that most people cannot wire them properly because they fail to distinguish between different types of 4-20 mA current loops.

What are the types of 4-20 mA Current Loop

There are 4 types of mA output signals
– Loop (2-Wire)

– Source (3-Wire)

– Sink (3-Wire)

– Isolated (4-Wire)

Each form uses a different reference path for the creation of mA signals, which is dependent on the controller or receiving device (i.e., PLC) to which each field device is connected.

Loop (2-Wire)

This is one of the most common 4-20 mA forms; you just need two wires for power and communication between the field device and the controller.

The controller provides the power to the loop, and the 4-20 mA signal flows from the field device to the controller through the common.

The main advantage of the 2-wire loop 4-20 mA signal is that it is easier to wire, and it will require two wires; hence, it will lower the installation cost.

The disadvantage of the 2-wire 4-20 mA loop is that it has two wires, so if the signal wire is broken, there will be no power on the field device (they use the same cable for power and signal).

There are 4 types of 4-20 mA current loops, where the two-wire loop version is by far the most common.

Although the wiring can be a little bit different, the working principle is the same; understanding how each one is wired can be fundamental to wiring them.

3-wire 4-20 mA loop (Source)

The 3-wire 4-20 mA loop uses three wires to connect the field device with the controller; here the signal has its own wire, so you have one wire for the +, one wire for the -, and one wire for the signal.

The two wires (the + and the -) are used to power the field device, while the signal wire is used to carry the field device signal to the controller.

The most important thing to note here is the current move from the field device to the controller.

The main advantage of the 3-wire 4-20 mA loop source is that the signal and the power wires are separated, so in case the power wire is disconnected, the field device can still be on.

The main disadvantage of this type of 4-20 mA signal is that it uses 3 wires, so more cable is used for wiring; hence, the cost of installation goes up.

3-wire 4-20 mA loop (Source)

The 3-wire 4-20 mA loop uses three wires to connect the field device with the controller; here the signal has its own wire, so you have one wire for the +, one wire for the -, and one wire for the signal.

The two wires (the + and the -) are used to power the field device, while the signal wire is used to carry the field device signal to the controller.

The most important thing to note here is the current move from the field device to the controller.

The main advantage of the 3-wire 4-20 mA loop source is that the signal and the power wires are separated, so in case the power wire is disconnected, the field device can still be on.

The main disadvantage of this type of 4-20 mA signal is that it uses 3 wires, so more cable is used for wiring; hence, the cost of installation goes up.

3-wire 4-20 mA loop (Sink)

This is almost the same as the three wires source type. The 3-wire 4-20 mA loop uses three wires to connect the field device with the controller; here the signal has its own wire, so you have one wire for the +, one wire for the -, and one wire for the signal.

The two wires (the + and the -) are used to power the field device, while the signal wire is used to carry the field device signal to the controller.

The main difference between the 3-wire sink and 3-wire source is that in the 3-wire sink configuration, the current signal moves from the controller to the field device.

The main advantage of the 3-wire 4-20 mA loop sink is that the signal and the power wires are separated, so in case the power wire is disconnected, the field device can still be on.

The main disadvantage of this type of 4-20 mA signal is that it uses 3 wires, so more cable is used for wiring; hence, the cost of installation goes up.

Isolated (4-Wire)

The four wires 4-20 mA current loop is my least favorite; it works almost like the 2-wire loop, but the main difference is that in 4 wires you need two power sources; in this case, the field device will need its power supply.

The current signal will be flowing from the field device to the controller, and the loop is powered by the controller in a 2-wire form.

The main advantage of the 4-wire 4-20 mA loop is that the field device and the controller use different power sources, so if the controller power source goes offline, the field device will keep working.

The main disadvantage is that you will need two power sources; the power sources are not cheap, and this will increase the cost of installation.

How do you know which type of 4-20 mA loop you need to wire?

All field devices come with user guides, and in each user guide, you should be able to see the wiring diagram.

If in the user manual you cannot figure out which type of 4-20 mA your device or controller has, please contact the manufacturer of your device, and they should be able to tell you how to wire it.

Conclusion: Types of 4-20 mA Current Loop

That is it; those are the types of 4-20 mA current loops. Depending on the type, the flow of current and the wiring can change a little.

If you have one of those and you need some help, please post your question below, and we will get back to you.

Modbus is one of the most common communication protocols in industrial automation. In this post, I will share with you what Modbus is, its types, advantages, when to avoid using it, and how to diagnose it.

What is ModBus communication protocol?

Modbus communication protocol is a serial communication protocol developed by Modicon® in 1979 for use with its programmable logic controllers (PLCs).

In simple terms, it is a method used for transmitting information over serial lines between electronic devices, one being the master (the one that initiates the communication) and the other the slave (the one that responds to a communication).

How does Modbus work?

In a few words, this is how the Modbus protocol works. The Modbus protocol exchanges data using a request/response mechanism between a master and a slave.

The master/slave principle is a type of communication protocol in which a device (the master) controls one or more devices (the slaves).

Why is Modbus so popular?

Modbus is popular among engineers and technicians because it is so easy to understand; you do not need to be a programmer to understand it.

I remember when I was providing training to new hires, I would tell them that Modbus RTU is very simple: connect A to A and B to B, and everyone was able to wire it on the first day of class.

Is Modbus dead?

No, Modbus is not dead; this is a myth. It will continue to live on, as there are millions of Modbus devices, and every day many of them are being built and implemented.

Is the Modbus protocol industry-specific?

No, the Modbus protocol is not industry-specific and can be used in different types of industries such as factory automation, building automation, process control, oil & gas, traffic & parking, agriculture & irrigation, water & wastewater, pharmaceutical and medical, material handling, etc.

When should you not use Modbus?

Don’t use Modbus if you have a lot of data to transfer. The packets are limited to around 120 bytes maximum.

Transferring 1K requires almost ten messages. It’s just not efficient for any kind of large data transfer.

What are the advantages of Modbus?

– Longer distances.

– Higher speeds.

– The possibility of multiple devices on a single multi-drop network.

Types of Modbus Communication Protocols

Several versions of the Modbus protocol exist for the serial port and Ethernet and the most common are:

– Modbus RTU

– Modbus ASCII

– Modbus TCP

– Modbus Plus

Modbus RTU (Remote Terminal Unit)

Modbus RTU is the most common implementation available for Modbus, it is used in serial communication and it makes use of a compact, binary representation of the data for protocol communication.

Modbus ASCII (American Standard Code for Information Interchange)

This is the type of Modbus that is used in serial communication and makes use of ASCII characters for protocol communication.

The ASCII format uses a longitudinal redundancy check checksum. Modbus ASCII messages are framed by a leading colon (‘:’) and trailing newline (CR/LF).

Modbus TCP/IP or Modbus TCP

This is the type of Modbus protocol that is used for communications over TCP/IP networks.

The Modbus data is wrapped around TCP/IP internet protocols and then the data is transmitted over standard internet.

Modbus Plus (Modbus+ or MB+)

Modbus Plus is a peer-to-peer protocol that runs at 1 MBS. The Modbus Plus protocol specifies the software layer as well as the hardware layer. This remains proprietary to SCHNEIDER ELECTRIC.

Modbus RTU

This is the most commonly used type of Modbus in industrial automation; let us answer a few questions about this type of Modbus.

What is a Modbus RTU?

Modbus RTU is an open serial protocol derived from the master/slave architecture (now client/server) originally developed by Modicon (now Schneider Electric). It is a widely accepted protocol due to its ease of use and reliability.

How many slaves can be connected in Modbus RTU?

Modbus RTU will support up to 247 slaves from addresses 1 to 247 – address 0 is reserved for broadcast messages.

What is the difference between Modbus RTU and Modbus TCP?

The main difference between MODBUS RTU and MODBUS TCP/IP is that MODBUS TCP/IP runs on an Ethernet physical layer, and Modbus RTU is a serial protocol.

Is Modbus RTU serial?

Yes, Modbus RTU is an open, serial (RS-232/422/485) protocol derived from the Master/Slave architecture.

What is Modbus RTU speed?

The majority of Modbus RTU devices only support speeds up to 38400 bits per second.

Modbus TCP IP

What is Modbus TCP/IP?

Modbus TCP/IP ( is simply the Modbus RTU protocol with a TCP interface that runs on Ethernet.

The Modbus messaging structure is the application protocol that defines the rules for organizing and interpreting the data independent of the data transmission medium.

What is the difference between Ethernet and Modbus TCP/IP?

The main difference between Ethernet and Modbus TCP/IP is that Modbus TCP/IP combines a physical network (Ethernet), with a networking standard (TCP/IP), and a standard method of representing data (Modbus as the application protocol).

Essentially, the Modbus TCP/IP message is simply a Modbus communication encapsulated in an Ethernet TCP/IP wrapper.

How to troubleshoot Modbus communication failure?

Troubleshooting Modbus failure can be the most difficult troubleshooting because it means that no activity is being recognized between the slave and master.

Basic Checks for No-response from slave error:

Check that communication settings parameters are correct

This is the most common error I found in many Modbus communication, you need to set the same baud rate in the master and the slave, also double check if the protocol selected is Modbus ( most field devices can communicate via different communication protocols.

Check that the slave’s address

If you have more than one field device, you need to assign them different addresses, most field devices come with a default address of 1, if you do not change it, you will have a duplicated address problem and this will cause a communication error.

Also, check on the controller side, the number of addresses on the datalogger should be equal to the number of field devices connected.

Check Modbus wiring

Just to be sure check your wiring, make sure there are no loose cables or open circuits, and also make sure that the cable distance is less than 2000 ft ( 660 meters).

Avoid using T-Taps, if you have more than one field device, you must daisy chain them.

Check for reversed polarity on RS485 lines

Wiring Modbus devices is simple; they have two terminals, A and B, just wire A to another A and B to another B. But sometimes manufacturers will use different terminology (some use TX and RX). If uncertain, just try swapping them.

Conclusion

That is it, in this post, we defined what Modbus is and how it works, and we answered a few common questions about the Modbus communication protocol.

If you have questions please feel free to let us know and will answer them as soon as we can.

Modbus is one of the most used industrial automation protocols. In this post, I will explain what a Modbus heartbeat is and how it is used. 

What is Modbus Heartbeat?

A heartbeat is either a bit or a holding register that changes state for external devices to tell that the industrial controller (mostly the PLC) is running. 

What is the main function of the Modbus Heartbeat?

The main function of the heartbeat is to facilitate the detection of communication problems in programming environments where the transport layer communication error information is unavailable.

This increments approximately every 5 to 10 seconds. It is the responsibility of the system integrator to notify plant personnel if a Modbus master (the PLC or DCS system) fails to communicate with the transmitter. This register can facilitate this notification.

How is Modbus Heartbeat implemented?

The implementation of a heartbeat depends upon the master and slave in question. For most master applications, a heartbeat can be any simple message that is sent out to each slave to ensure that even if they have nothing new to report, at least they are online and in communication.

The timer register in the master will be expecting the information every 5 seconds (some manufacturers have this time of 10 seconds).

The external device that is communicating with this device would compare over time to see if the register is changing. If it does not, then the external device would declare either a master failure or a comm fail.

In most of the applications, there is no need to implement the heartbeat. This is because most of the Modbus slave units do not respond to Modbus commands unless they are operating. In such cases, just use a normal Modbus Read function.

What is heartbeat in PLC?

A PLC heartbeat signal is a signal that is periodically sent by a programmable logic controller (PLC) to indicate that it is still functioning properly. The signal is used by the PLC’s watchdog timer to confirm that the PLC is running correctly.

Conclusion

The Modbus heartbeat is like a watchdog that will make sure that there is communication between the master and slave in a Modbus communication.

This bit or register is not widely used in industrial communication, as most slave devices will not respond to master queries if they are not in communication.