Skip to main content

MODBUS ASCII Communication Protocol Explained

 n this post, we will study the concept of the Modbus ASCII protocol.


Modbus is a communication protocol that is widely used in PLCs. It is a type of serial communication.


Nowadays, apart from PLC’s, it is widely used in various ranges of instruments like PID controllers, sensors, various types of electronic instruments, VFD’s, etc.


Modbus works in master-slave mode. That means, suppose there is a PLC that acts as a master and three VFD’s which act as slaves.


If you understand it in simpler terms, it is a turn-by-turn protocol. The PLC requests information from the first slave; then the second slave and at last, the third slave.


It requests for data in a set period of time; and if the response is not received (timeout time), then it proceeds for the next slave and so on.


After the last slave, it goes back again to the first slave. Also, apart from reading, it also writes data to slaves one by one.


MODBUS ASCII Communication Protocol

Modbus works on three hardware layers for serial communication –


RS-232

RS-422

RS-485

In Modbus serial protocol, there are two types of transmission –


RTU

ASCII

In this post, we will have a look at the Modbus ASCII protocol.


Before proceeding ahead with the data structure in Modbus ASCII protocol, let us have a look at the basic data frame structure of Modbus protocol.


The frame starts with the address field, followed by the function code field, then the data field, and at last, CRC (Cyclic Redundancy Check) field.


Address field determines the slave address; function code determines what type of action to perform on data; data field is the data to be sent or received, and CRC or error checking field is the result of redundancy checking calculation to check if the frame being communicated has any error or not.


ASCII

ASCII stands for American Standard Code for Information Exchange. It uses ASCII characters to begin and end messages. ASCII characters are hexadecimal characters.


The format of the message starts with a colon ‘:’ and ends with a carriage return and line feed. Due to this, the space between the messages becomes variable.


As compared to Modbus RTU, ASCII uses the less effective LRC (longitudinal redundancy check) error checking method. If we look at its frame format, the first byte consists of start bit; then 2 bytes for station address; then 2 bytes for function code; then ‘n’ bytes for data; then 2 bytes for LRC and at last, 2 bytes for stop bit (carriage return and line feed).


Disadvantages of Modbus ASCII Communication

Modbus ASCII is used very rarely and is less efficient as compared to Modbus RTU. The main reason for this is the error checking method, which is much stronger in RTU.


So it is used in very less devices. It is a depleting type of protocol that is available in Modbus serial communication. But, some of the major applications where it is used are in serial modems and weight balance check machines.  


In this way, we had a basic overview of the Modbus ASCII protocol.

Labels:

Comments

Post a Comment

Popular posts from this blog

Ferrules and Cross Ferruling

 Ferrules are identification labels provided for every wire terminations in an instrument, equipment, or electrical/instrumentation control panels. These tube-shaped sleeves can be inserted easily on each individual wire in a multi-core cable. In earlier days fixed digits/letters are used as ferrules, but now Instrumentation engineers/technicians prints out desired ferrules by using a ferrule printing machine. Typical Ferrule The numbers/ letters on the ferrules will be given as per the approved electrical hook up or loop diagrams. This helps technicians to easily identify a particular loop/wiring from a series of terminal blocks and to troubleshoot the desired terminal connection. Separate numbers on the ferrules distinguish the positive and negative polarities of wires, thus ensure the polarity protection of the instrument. Cross Ferruling  As a wire is connected on its both ends, it is quite useful to use a cross reference method for wire identification. Unlike normal ferru...
1 comment
Read more

What is a Torbar? – Averaging Pitot Tubes

 The Torbar is employed for flow measurement of liquids, gases, or steam in circular, square, or rectangular section ducts for large flow rates. The Torbar is an insertion type multi-port self-averaging primary sensor for flow measurement. Torbar TORBAR is a set of Pitot tubes mounted on a bar across the pipeline with no moving parts. An averaging Pitot tube is a technology, while TORBAR is a manufacturing brand name. There are several brands available in the market with VERABAR, ANNUBAR, etc. Averaging Pitot Tube Principle Purpose Averaging Pitot tube can be employed when the average velocity of the flow profile, rather than the velocity in a specific point in the cross-section is desired. Averaging Pitot Tubes Principle It measures the differential pressure between the static pressure tap and the tap of full pressure of a stream. Thus such magnitude of differential pressure is directly proportional to the square of the flow rate. Working The TORBAR is designed in such a way that ...
7 comments
Read more

Dissolved Oxygen Analyzer Working Principle

 What is dissolved oxygen ? Dissolved oxygen refers to oxygen dissolved in water. Its concentration is expressed as the amount of oxygen per unit volume and the unit is mg/L. Biologically, oxygen is an essential element for respiration of underwater life and also acts as a chemical oxidizer. The solubility of oxygen in water is affected by water temperature, salinity, barometric pressure, etc. and decreases as water temperature rises. Measurement of dissolved oxygen by the membrane electrode method The membrane electrode method measures a diffusion current or reduction current generated by the concentration of dissolved oxygen or partial pressure of oxygen to obtain the concentration of dissolved oxygen. This method is not affected by the pH value of water being measured, oxidation and reduction substances, color, turbidity, etc. and the measurement method offers good reproducibility. When a sensor is inserted into water, an air layer forms on the membrane (Teflon membrane). The ox...
1 comment
Read more