Modbus RS485
Modbus RS485 Interface
Operating Guide
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Modbus RS485
Modbus RS485
MODBUS is an open application layer messaging protocol, which is deployed in areas of
manufacturing automation, process automation and building automation. It provides client/server
communication between devices connected over a RS485 connection.
Modbus RS485 networks consist of two different devices, a Master and a Slave.
Master Device - Master devices determine the data traffic on the network. They can send data without
an external request.
Slave Device - Slave devices are peripheral devices. They do not have their own access rights to the
data traffic on the network and only send their data due to an external request from a master. The
MXD70 Series operates as a slave device on the network.
Modbus Telegram Structure - The data is transferred between the master and slave by means of a
telegram. A request telegram from the master contains the following four telegram fields:
ï‚· Slave address - The slave address can be in an address range from 1 to 247. The master talks to
all the slaves simultaneously by means of the slave address 0 (broadcast message).
ï‚· Function code - The function code determines which read, write and test operations should be
executed by means of the MODBUS protocol.
ï‚· Data - Depending on the function code, the following values are transmitted in this data field:
Register start address (from which the data is transmitted), Number of registers, Write/read data, Data
length, etc.
ï‚· Checksum - The telegram check sum forms the end of the telegram.
The master can send another telegram to the slave as soon as it has received an answer to the previous
telegram or once the time-out period set at the master has expired. This time-out period can be
specified or modified by the user and depends on the slave response time.
If an error occurs during data transfer or if the slave cannot execute the command from the master, the
slave returns an error telegram (exception response) to the master.
The slave response telegram consists of telegram fields which contain the requested data or which
confirm that the action requested by the master has been executed. It also contains a check sum.
MXD70 Series Modbus communications is indicated in the top of the screen by the following symbol:
Supported Modbus Function Codes
Function Code Type Function
2
Read Discrete Inputs Reads one or more discrete inputs of the MXD70 Series
1 to a maximum of 2000 consecutive registers can be
read with a telegram.
The discrete inputs in the response message are packed
as one discrete input per bit of the data field.
Status is indicated as 1= ON and 0= OFF.
! Note. If the returned output quantity is not a multiple of
eight, the remaining bits in the final data byte will be
padded with zeros.
Application: For reading the status of the instrument and
its error messages.
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