Modbus RTU is an open (RS232 or RS 485) protocol derived from the Master/Slave architecture. This is a widely recognized protocol because of its ease of use and reliability. This broad acceptance is due in large part to MODBUS RTU’s simple use.

MODBUS RTU messages really are a simple 16-bit CRC (Cyclic- Checksum). The simplicity of these messages is always to ensure dependability. As a result of this ease, the basic 16-bit MODBUS RTU register construction may be used to pack in ASCII text, tables, floating-point, queues, along with other unrelated data.

MODBUS is considered an application layer messaging protocol, providing Master/Slave communicating between devices linked through buses or networks. At level 7, MODBUS is positioned on the OSI model. MODBUS protocol is supposed to be a request/response protocol and offers services specified by function codes. The function codes of MODBUS are elements of MODBUS’ request/answer PDUs (Protocol Data Unit)

A MODBUS transaction must be initiated by the customer, so that you can build the MODBUS program data unit. It is the function which informs the host about which form of activity to perform. The MODBUS program protocol establishs a petition initiated by way of a Master’s format. Only codes within the number of 1 through 255 are considered valid, with 128-255 being earmarked for exception answers. It’s the function code area which advises the server of what sort of action to perform when a note is sent by the Master to the Slave.

To define multiple activities, some functions may have sub- function codes added to them. For example, the Master is able to read the ON/OFF states of a group of inputs or discreet outputs.  When the Master gets the Slave response, the Slave uss the function code field to signal either a mistake- an exception answer or free response. The Slave repeats to the petition of the primary function code in the event of a normal response.

Data Item Properties
MODBUS RTU converter packets are merely intended to send information; they do not have the potential to send parameters like point name, resolution, components, etc. In the event the power to send such parameters is needed, you ought to inquire alternative protocols that are modern, EtherNet/IP, or a BACnet.

MODBUS RTU versus Other Protocols

Despite the restrictions of MODBUS RTU, there are nonetheless several good reasons as to the reason why it is still a challenger among industrial automation protocols. For one, MODBUS RTU is a dominating force in the market place and is a lot easier to execute than newer protocols. MODBUS RTU also requires considerably less memory. To implement MODBUS RTU, it is possible to fit the necessary size of 2Kb on a small 8- bit CPU or PIC processor, whereas with BACnet and EtherNet /ip address, you could possibly require 30-100Kb of memory.

Standard MODBUS RTU node addresses are 1-254, with 0 write just and being reserved for broadcast messages. Yet the 0 address is rarely used because of the fact there is no evidence the message was properly received in the slave node. This doesn’t have much influence if your physical layer is RS-232 as just one node can be implemented anyway. RS-485 while some some drivers will allow you to extend the sum of money limits the amount of nodes to 32.


The simplest difference between MODBUS RTU and MODBUS TCP communication protocols (also called MODBUS IP, MODBUS EtherNet, and MODBUS TCP/ip address) is that MODBUS TCP runs on an Ethernet physical layer and Modbus RTU is a serial level protoCol. Modbus TCP also uses a-6 byte header allowing routing.