Defining
Modbus
 Master Slave Messages

Set up an individual list of messages (commands) for each slave device.
A
Modbus®
master supports a maximum of 400 messages from slave devices. A
Modbus
 master
Modbus
 read commands cause data to be read from the slave device to the
Modbus
 master, while
Modbus
 write commands cause data to be copied from the
Modbus
 master to the slave device. Define a sequence of broadcast write commands, which a
Modbus
 master can send to multiple
Modbus
 RTU slaves without requiring an acknowledgment. The AADvance controller can control and monitor individual
Modbus
 master objects and their slave links.
Task
Procedure
Add messages for a
Modbus
 master slave
  1. In the
    Communication View
    , expand the required
    Modbus
     master, and then double-click the required
    Modbus
     master slave.
  2. The
    Communication
    property page is displayed with the
    General
     and
    Messages
     tabs.
  3. In the property page, click the
    Messages
     tab.
  4. To add a message, click
    Add Message
    , and then specify the message properties.
  5. To change message order, select the message to move, and then click
    Move Up
     or
    Move Down
    .
Remove messages from a
Modbus
 master slave
  1. In the
    Communication View
    , expand the required
    Modbus®
    master, and then double-click the required
    Modbus
     master slave.
    The
    Communication
    property page is displayed with the
    General
     and
    Messages
     tabs.
  2. In the property page, click the
    Messages
     tab.
  3. To delete messages, select the required messages in the grid, and then click
    Remove Selected
    .
Properties for adding messages are:
Command
Read Coils
Reads the coil status in a remote slave device. Each message has a maximum
Count
 of 2000 coils.
Read Discrete Inputs
Reads the discrete input (coil) status in a remote slave device. Each message has a maximum
Count
 of 2000 inputs.
Read Holding Registers
Reads the holding registers in a remote slave device. Each message has a maximum
Count
 of 125 registers.
Read Input Registers
Reads the input registers in a remote slave device. Each message has a maximum
Count
 of 125 registers.
Write Coils
If the
Master Space
 is
Coils
, forces each coil in a sequence of coils on and off in a remote slave device.
If the
Master Space
 is
Discrete Inputs
, writes to each discrete input in a remote slave device.
Each message has a maximum
Count
 of 2000 coils or inputs.
Write Holding Registers
Writes to the holding or input registers. Each message has a maximum
Count
 of 125 registers.
Master Space
Depending on the
Command
, possible values are
Coils
,
Discrete Inputs
,
Input Registers
, or
Holding Registers
.
  • For
    Read Coils
    or
    Read Discrete Input
    s, the value is
    Coils
    .
  • For
    Write Coils
    , the value is either
    Coils
    or
    Discrete Inputs
    .
  • For
    Read Input Registers
     or
    Read Holding Registers
    , the value is
    Holding Registers
    .
  • For
    Write Holding Registers
    , the value is either
    Input Registers
     or
    Holding Registers
    .
Slave Address
Possible values are 1 to 65536; the default value is 1.
Master Address
Possible values are 1 to 65536; the default value is 1.
Count
When the
Command
 is
Read Coils
,
Read Discrete Inputs
, or
Write Coils,
 the value ranges from 1 to 2000; the default value is 1.
When the
Command
 is
Read Input Registers
,
Read Holding Registers
,
or Write Holding Registers
, the value ranges from 1 to 125; the default value is 1.
TIP: Use up to 31 messages. Using more than 31 messages does not send any messages to the controller after downloading code to the controller.
Control
The address of a control register. When using a control variable, the
Modbus
 address of the coil register. Possible values range from 1 to 65536, or 0 to disable; the default value is 0. When the address of a control variable is unspecified, the link is automatically enabled once the controller starts. The application may write to the control variable to switch the state of the master slave.
True = The slave is not polled
False = The slave is polled for data
Comment
Plain text
All
Modbus
 master objects defined for a project (
Modbus
 RTU or
Modbus
 TCP) function independent of each other. Each master polls its associated slave devices, to send the messages scheduled for the slaves.
For
Modbus
 RTU, a master polls its slave devices one at a time, executing one message for each slave in turn. Different slaves attached to the same master usually need a different numbers of messages. To accommodate this, the master proceeds to work through the messages listed for the slaves, one message for each pass, returning to the first message for a slave after it has sent the last one. This means that some messages (for slaves with fewer scheduled messages) are sent more than once during a polling cycle. The polling cycle completes when the
Modbus
 master has made an equal number of polls to each slave and has sent every message; the cycle then starts again.
Example
This example displays the message schedule for a master with three slaves that require a different number of messages:
cmv_dmmsm
The total number of messages that the master must send in a complete polling cycle is equal to the largest number of messages scheduled for any slave, multiplied by the total number of slaves. In the previous example, the total number of messages is 4 x 3 = 12. Nine messages are specified for the three slaves. The message scheduling mechanism sends three of these messages twice, making a total of 12 messages sent. This table lists the sequence of messages for the serial communications link in the previous example:
Sequence Number
Slave
Message
Remarks
1
1
1
2
2
1
3
3
1
End of first pass through all the slaves
4
1
2
5
2
2
End of pass through slave 2
6
3
2
7
1
3
8
2
1
Repeat message
9
3
3
End of pass through slave 3
10
1
4
End of pass through slave 1
11
2
2
Repeat message
12
3
1
Repeat message; end of poll
Provide Feedback
Have questions or feedback about this documentation? Please submit your feedback here.
Normal