What is Modbus Communication? and Where you have to choose modbus for vfd control?

MODBUS IS ONE OF THE STANDARD PROTOCOL  USED IN THE FIRST TIME BY SCHNEIDER ELECTRIC .

IT IS USED  WITH THE IED RELAYS LIKE SEPAM ,X-GIPAM, DPR1000,F650,...ETC FOR DIFFERENT TYPE OF THE IED RELAYS FOR DIFFERENT COMPANY S .

BY THIS PROTOCOL WE CAN TRANSMIT ALL THE POINTS OF THE RELAY (LIKE MEASUREMENTS,STATUS,CONTROL,ALARMS,.....ETC  FROM THE IED RELAY TO THE PLC OR RTU .BY AN RS485 INTERFACE

Modbus: is a serial communications protocol for communication with industrial electronic devices such as PLC, PAC or DCS by means of a twisted pair of wires. It allows communications among approx. 240 devices simultaneously, connected to same network.

Modbus communication uses Master-Slaves technique. Only the Master generate/ initiate queries and the other devices (slaves) respond by supplying the requested data or by taking action as demanded. Master devices includes touch screens (HMI), PCs (SCADA systems) while slaves devices includes PLC, PAC, DCS etc.

Modbus for VFDs control: Normally PLC analog output cards are used to control VFDs. A modbus can be considered a good alternative to communicate the PLC with VFDs because using modbus we can:

1. Get rid of PLC analog outputs (for speed control).

2. Get rid of PLC analog inputs (current readings from VFDs).

3. Get rid of digital inputs & outputs (enable signal, running aux signal).

4. Being able to read a lot of VFDs parameters (like faults etc).

But some limitations are there for using modbus to control VFDs which are:

1. If modbus communication fails, all VFDs will be out of control.

2. One VFD could affect the communication with the rest of VFDs.

3. Troubleshooting it from remote area is a challange.

4. Modbus cannot be used for higher speed commands (frequency change or start/stop). 

Modbus is an open protocol, meaning that it's free for manufacturers to build into their equipment without having to pay royalties. It has become a standard communications protocol in industry, and is now the most commonly available means of connecting industrial electronic devices. It is used widely by many manufacturers throughout many industries. Modbus is typically used to transmit signals from instrumentation and control devices back to a main controller or data gathering system, for example a system that measures temperature and humidity and communicates the results to a computer. Modbus is often used to connect a supervisory computer with a remote terminal unit (RTU) in supervisory control and data acquisition (SCADA) systems. Versions of the Modbus protocol exist for serial lines (Modbus RTU and Modbus ASCII) and for Ethernet (Modbus TCP).

MODBUS is an industrial protocol used master-slave principles i.e. one device is a master which can initiate transactions while slaves response to the queries. Master might send broadcast messages (addressed to all receivers) or to individual device. MODBUS has two types of transmission ASCII and RTU. Advantages of using ASCII protocol is that allow long breaks between sending characters (up to 1 sec.) without causing a timeout. ACSII mode use hexadecimal coding (0-9,A-F). The frame start with character ':' follow by two character of address, function, n characters of data, two characters of LRC (Longitudinal Redundancy Check) and end of frame CR LF. While the message in RTU mode begin with time break 3.5*duration of one character. The frame in RTU mode looks follow: preamble (T1-T2-T3-T4), address (8 bits), Function (8 bits), Data ( n * 8 bits), CRC (16 bits), end of frame (T1-T2-T3-T4).  The basic functions in Modbus are: read digital input/output, read group of registers, set on/off selected digital input/output, read slave status, read message counter.

in easy way it's one type of serial communication which use in industrial area (8 bit ) for PLC and DSC and instrumentation and HMI it's have configuration : address

stop bit

even odd

more .   

modbus is a serial bus protocol used in plc, developed by midicon.in simple terms it is a method to transmiting info over serial lines between electron devices