Configuration Settings

The configuration settings for a slave for a
Modbus®
master include communications and status reporting. When configuring a slave for a
Modbus
 master, define these properties:
Property
Description
Communications
Station ID
,
Modbus
 communications identity for the slave. This value must be unique for all slaves connected to a single
Modbus
 master. For Ethernet-based slaves, possible communications are only single point-to-point. In this case, usually use an ID of 1.
Retries
, number of attempts the
Modbus
 master makes to communicate with a slave before declaring a communications failure.
Min Packet Gap
, time the
Modbus
 master waits between messages sent to the slave, in milliseconds. For slaves that must not be accessed by the
Modbus
 master at full communications speed.
Pinging
Method used by the
Modbus
 master to test communications with an individual slave. The
Modbus
 master pings a non-communicating slave at regular intervals. Once the non-communicating slave responds to a ping, then polling of the slave restarts on the next poll cycle. When a slave returns to the polling sequence, the first message that is sent to the slave is the first message defined in the INI file for that slave.
Repeat Rate
, time delay the
Modbus
 master waits between each attempt to communicate with the slave.
Command
, method the
Modbus
 master uses for testing communications. Possible methods are:
  • Use
    Modbus
     08 Command
    , (Query Data), subcommand 0. Compatible slaves echo this command exactly, similar to an Ethernet ping.
  • Use Slave Register
    , reads data from a slave register to test communications to the slave, when the slave does not support
    Modbus
     08 Command. When using a register for testing communications, discards the data read from the register.
Slave Control
Modbus
 address of an application variable used to control the slave device. If the application does not control the slave device, leave unchecked. These are the valid values for a slave control holding register. Invalid values result in the inactive state.
Use different
Modbus
 addresses for all control and status holding registers, and control variable coils.
State
Value
Description
Inactive
0
Do not poll or ping the slave
Standby
1
Ping the slave to ensure communications are possible
Active
2
Poll the slave for data
Status Reporting
Modbus
 address of an application variable which is used to report the status of the communications link to the application. If status reporting is not required, leave unchecked. These are the valid values for slave status holding register.
Use different
Modbus
 addresses for all control and status holding registers, and control variable coils.
State
Value
Description
Healthy
0
1. Slave is responding normally
2. Slave is set to inactive
Unavailable
1
1. Slave failed to respond to a ping on a link declared as standby
2. Master is in error or is initializing
3. Master is set to inactive
Communications Fail
2
Slave is being polled for data and not responding
Slave Error
3
Illegal values are being requested from slave
To define communications and status reporting for a slave of a
Modbus
 master
  1. Click the
    Configuration
    tab.
  2. In the
    Communications
     area, specify these properties:
    • Station ID
      ,
      Modbus
       communications identity for the slave
    • Retries
      , number of attempts the
      Modbus
       mastermakes to communicate with a slave
    • Min Packet Gap
      , time the
      Modbus
       master waits between messages sent to the slave
  3. In the
    Pinging
     area, define the pinging method the
    Modbus
     master uses to test communications with slaves:
    1. Indicate the
      Repeat Rate
       time delay the
      Modbus
       master waits between communication attempts with slaves.
    2. Specify the
      Command
      method the
      Modbus
       master uses for testing communications:
      • Use
        Modbus
         08 Command
      • Use Slave Register
  4. (optional) In the
    Applications Interface
     area for
    Slave Control
    , select the
    Application Controls
     check box, and then indicate the
    Modbus
     address for the communications control variable to enable the application to control communications for the slave.
  5. (optional) In the
    Applications Interface
     area for
    Status Reporting
    , select the
    Report Status
     check box, and then indicate the
    Modbus
     address for the status reporting variable to report the status of the communications link to the application.
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