Defining a
Modbus
 Master Slave

Define
Modbus®
master slaves for existing
Modbus
 masters. A controller and a
Modbus
 master can have a maximum of 64 master slaves. When adding a master slave, this creates a link between the
Modbus
 master and slave device. Each master slave device represents a server. When configuring a
Modbus
 master, define general and messages properties.
IMPORTANT: If the
Modbus
 master is configured for
Modbus
 TCP, only create one master slave link. If reconfiguring
Modbus
 RTU master with multiple slave links to
Modbus
 TCP, keep only one master slave and delete the other links.
Every
Modbus
 master slave link contains a pair of holding registers: a control register and a slave register. The control register enables the application to control the link and port, while the status register retrieves the status information. Each register is found at a unique address and is defined using the
Control Address
 and
Status Address
. Registers are optional. To use a register, declare a variable for the register in the application variables and the address must also be specified in the holding registers.
General properties are:
Property
Description
Name
Name of the
Modbus
 master slave. Possible values are any combination of alphanumeric characters and punctuation including spaces.
Slave ID
Unique identifier for the
Modbus
 master slave.
When the protocol of the master is RTU, possible values range from 1 to 247; the default value is 1.
When the protocol of the master is TCP, possible values range from 1 to 255; the default value is 255.
Control Address
The address of a control register. When using a control variable, the
Modbus
 address of the holding register. If a control variable address is unspecified, the link is automatically enabled once the controller starts. The application may write the control variable to switch the state of the master slave.
0 = inactive, the slave link is disabled and the slave is not polled or pinged.
1 = standby, the slave is forced to operate in standby mode.
Modbus
 continues to ping the slave to ensure communications are possible.
2 = active, the slave is forced to operate in active mode and is polled for data.
Possible values are 1 to 65536, or 0 to disable; the default value is 0.
Status Address
The address of a status register (a holding register). The status register is a UINT returned by the
Modbus
 driver to let the application monitor and act on faults. If a status register address is not specified, the master slave link does not report the status information to the application.
0 = healthy, the
Modbus
 slave is operating normally; the link is active and no errors are reported.
1 = initializing, the slave link is initializing. This value is also reported if the slave device has failed to respond to a ping on a standby link, if the
Modbus
 master is in error, or if the master is set to inactive.
2 = error, the last message to the slave resulted in an exception response.
Possible values are 1 to 65536, or 0 to disable; the default value is 0.
Slave Wait Interval (ms)
The minimum amount of time to wait from the start of one message to the beginning of the next message. Possible values range from 0 to 65535 milliseconds; the default value is 100 ms.
Retries
The number of times the master tries to retransmit a message when receiving no response from the slave. This property is used in conjunction with the Modbus master
Message Wait Interval
 value.
Possible values range from 0 to 10; the default value is 3.
Slave IP
For TCP connections, the network address used by master slave.
Possible values range from 0.0.0.0 to 255.255.255.255.255; the default value is 0.0.0.0.
Slave Port
For TCP connections, the port number on which the slave device is offering services.
Possible values range from 0 to 65535; the default value is 502.
Modbus
 slaves having the same
Modbus
 master must use unique port numbers.
Ping Mode
Tests the communications between the master and slave and determines the action the master takes if no response is received from the slave after the specified number of Retries.
The AADvance controller supports a
Function Code 08
 ping mode for
Modbus
 RTU slaves.
Function Code 08
 sends a diagnostic message to the operational slave. If supported by the slave, select
Function Code 08
 since it is a short message. Alternatively, the
Read Holding Register
mode reads data from the holding register. Also set a
Do Not Ping
 setting for both protocols.
For
Modbus
 RTU, the possible values are
Do Not Ping
,
Function Code 08
, and
Read Holding Register
.
For
Modbus
 TCP, the possible values are
Do Not Ping
 or
Read Holding Register
.
Read Holding Register
 is the only method available to ping a Modbus TCP slave.
When the
Ping Mode
 is
Do Not Ping
 or
Function Code 08
, the
Ping Address
 property is disabled.
It is not recommended to use the
Do Not Ping
 mode since the master does not look for the slave after disconnection. When using
Do Not Ping
, communications only restart when there is a change to the control register of the master or slave.
Ping Interval
The maximum time between each initiation of a
Read Holding Register
 or
Function Code 08
 diagnostic message and specified individually for each master slave.
Possible values range from 0 to 65535 milliseconds; the default value is 5000 ms.
Ping Address
For use with the
Read Holding Register
ping mode and specified individually for each master slave; it is the
Modbus
 address of the slave holding register.
Possible values range from 1 to 65536, or 0 to disable; the default value is 0.
Statistics Mode
Indicates the statistics reporting mode for the master slave. Possible values are:
  • Disabled
    , the default reporting mode.
  • Last Rate
    , reports the length of the most recent scan time.
  • Maximum Rate
    , reports the longest scan time.
  • Average Rate
    , reports the average scan time.
    Statistics are reported in hundredths of a second.
    For the
    Last Rate
    ,
    Maximum Rate
    , and
    Average Rate
     reporting modes, specify the
    Data Address
     and
    Reset Address
     properties.
Data Address
The holding register of the data variable. The data variable is reported in hundredths of a second.
Possible values are 1 to 65536, or 0 to disable; the default value is 0.
Reset Address
The holding register of the reset variable. Any non-zero value in the reset variable resets the data variable to zero.
Possible values are 1 to 65536, or 0 to disable; the default value is 0.
Comment
Plain text
To define a
Modbus
 master slave
  1. In the
    Communication View
    , expand the required controller, and then expand the
    MODBUS
    element.
    The
    MODBUS Slave
     and
    MODBUS Master
     elements are displayed.
  2. Expand the
    MODBUS Master
     element, right-click the existing
    Modbus
     master, and then click
    Add New Master Slave
    .
  3. Double-click the master slave instance, and in the
    Communication
    property page, define the required properties and messages:
    1. On the
      General
       tab, define the required general properties.
    2. On the
      Messages
       tab, specify the messages for the slave.
Provide Feedback
Have questions or feedback about this documentation? Please submit your feedback here.
Normal