Has anyone used a Mitsubishi QPLC to HMI system? If you check the 8 pin connector which goes from the HMI Rs-232 port to the PLC, you can see a considerably macro plastic projection inside the connector. What’s the use of it? Well, there’s nothing particular per se. The plastic projection makes it impossible to use the connector with any other brand of PLC. So, if you need to use a Mitsubishi 8-pin connector with a Delta PLC, plug and play usability is just out of the window. What we concentrate today isn’t related to the hardware part. That’s there just to give you an example. Different companies have different communication protocols for their hardware. Especially industrial automation products like PLC, HMI etc. And just like the hardware limitation which we observed with the Mitsubishi cable, most of the communication protocols are limited to their home brands.

In 1979, Schneider electric (which was then called Modicon) released a serial communication protocol called Modbus, where they introduced the system of addressing to the registers of the PLC directly, where each data register and each coil has a unique address in the PLC memory. In this system where each bit is sent as a voltage, zeros are sent as positive and ones are sent as negative voltages. In Modbus, a concept of Master and Slave is applicable, where the device requesting the information is called the Modbus Master and the devices supplying information are called Modbus Slaves. In a standard Modbus network, there is one Master and up to 247 Slaves, each with a unique Slave Address from 1 to 247. The Master can also write information to the Slaves. After developing this protocol, Schneider did something wonderful. They made the protocol open source or practically, free. This meant that everyone was welcome to test this type of communication out and configure their devices accordingly. This made a revolutionary advancement in the field of communication protocols, as Modbus became the ‘Windows’ of protocol technology. Every major and minor companies began introducing this protocol as a standard along with their existing system, making Modbus a universal standard for communication protocols. Nowadays, you simply cannot see a control system without the Modbus protocol.

There are 3 major types of Modbus in use today and they are:

i. Modbus ASCII

ii. Modbus RTU

iii. Modbus TCP/IP


What to note is that If the Master is of Modbus ASCII mode, all the slaves must be in Modbus ASCII and not in RTU. No two different types of Modbus Protocols are combatable with each other.

Learning Modbus protocol isn’t as tough as you think it would be as we are addressing to the internal memory of the controller directly. Since for all controllers the type of data accessing is same, learning Modbus would open up the gateway to easiness in using all the controllers alike.

And as always,

Thanks for reading.