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Using Modbus I/O Servers (DSC Module or Real-Time Module)

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Using Modbus I/O Servers (DSC Module or Real-Time Module)

Modbus is an application-level messaging protocol that provides client-server communication between devices connected on different types of buses or networks. You can create a Modbus or Modbus Slave I/O server to read data from or write data to Modbus devices. For example, you can create a Modbus Slave I/O server on an NI real-time controller and use this controller as a Modbus slave device. You also can create a Modbus I/O server on a host computer and use this server as a Modbus master device to communicate with a Modbus slave device.

(DSC Module) Refer to the Modbus Fundamentals VI in the labview\examples\lvdsc\Distributed System Connectivity\Modbus Fundamentals\Modbus Fundamentals.lvproj for an example of connecting to and interacting with a Modbus device.

 Open examples  Find related examples

Use the View I/O Items dialog box to view the data items to which the I/O server can read or write. You also can monitor these data items by using the NI Distributed System Manager. To launch the System Manager from LabVIEW, select Tools»Distributed System Manager.

Modbus I/O servers use six-digit addresses. You can convert a five-digit address to a six-digit address by adding a zero between the first and the second digits of the five-digit address. For example, you can convert address 45001 to 405001.

Modbus and Modbus Slave I/O server data items use the following denotations:

  • A—Denotes an array.
  • D—Denotes a 32-bit unsigned integer.
  • F—Denotes a 32-bit floating-point number.
  • L—Denotes the length of an array. The Maximum Data Points Per Command values that you specify in the Advanced Attribute Settings dialog box determine the maximum length of the array. If the array elements are 32-bit integers or floating-point numbers, the maximum length of the array is half the value you specify for the corresponding Maximum Data Points Per Command value.
  • S—Denotes a 16-bit signed integer.
  • SD—Denotes a 32-bit signed integer.

The following table lists the data items that a Modbus or Modbus Slave I/O server supports. The Example column of this table explains the relationship between data items and their physical I/O point addresses on a Modbus device. This column uses the following format: Data Item = {I/O point address}. Note that one 32-bit data item occupies two I/O points.

Note  When reading or writing valid and invalid data items simultaneously, Modbus and Modbus Slave I/O servers identify all data items as invalid.
Data Item Data Type Modbus Modbus Slave Description Example
Read Write Read Write
000001–065535 Boolean value Yes Yes Yes Yes Accesses single-bit coils. 000001 = {000001}
100001–165535 Boolean value Yes No Yes Yes Accesses single-bit discrete inputs. 100002 = {100002}
300001.1–365535.16 Boolean value Yes No Yes Yes Accesses individual bits of input registers and interprets them as logical TRUE or FALSE values. The least significant bit is 1. The most significant bit is 16. 300001.1 = {the first bit of 300001}
300001–365535 16-bit unsigned integer Yes No Yes Yes Accesses 16-bit input registers as unsigned integers ranging from 0 to 65,535. 300001 = {300001}
400001.1–465535.16 Boolean value Yes No Yes Yes Accesses individual bits of holding registers and interprets them as logical TRUE or FALSE values. The least significant bit is 1. The most significant bit is 16. 400002.16 = {the 16th bit of 400002}
400001–465535 16-bit unsigned integer Yes Yes Yes Yes Accesses 16-bit holding registers as unsigned integers ranging from 0 to 65,535. 400002 = {400002}
A000001L1–A065535L1 Array of Boolean values Yes Yes Yes Yes Accesses arrays of single-bit coils. A000001L2 = {000001, 000002}
A100001L1–A165535L1 Array of Boolean values Yes No Yes Yes Accesses arrays of single-bit discrete inputs. A100005L3 = {100005–100007}
A300001L1–A365535L1 Array of 16-bit unsigned integers Yes No Yes Yes Accesses arrays of 16-bit input registers as arrays of unsigned integers. A300001L2 = {300001, 300002}
A400001L1–A465535L1 Array of 16-bit unsigned integers Yes Yes Yes Yes Accesses arrays of 16-bit holding registers as arrays of unsigned integers. A400005L3 = {400005–400007}
AD300001L1–AD365534L1 Array of 32-bit unsigned integers Yes No Yes Yes Accesses arrays of 32-bit unsigned integers. Each 32-bit unsigned integer in the array is composed of two adjacent 16-bit input registers. AD300001L1 ={300001, 300002}
AD400001L1–AD465534L1 Array of 32-bit unsigned integers Yes Yes Yes Yes Accesses arrays of 32-bit unsigned integers. Each 32-bit unsigned integer in the array is composed of two adjacent 16-bit holding registers. AD400002L3 = {400002–400007}
AF300001L1–AF365534L1 Array of 32-bit floating-point numbers Yes No Yes Yes Accesses arrays of 32-bit floating-point numbers. Each 32-bit floating-point number in the array is composed of two adjacent 16-bit input registers. AF300001L2 = {300001–300004}
AF400001L1–AF465534L1 Array of 32-bit floating-point numbers Yes Yes Yes Yes Accesses arrays of 32-bit floating-point numbers. Each 32-bit floating-point number in the array is composed of two adjacent 16-bit holding registers. AF400002L3 = {400002–400007}
AS300001L1–AS365535L1 Array of 16-bit signed integers Yes No Yes Yes Accesses arrays of 16-bit input registers as arrays of signed integers. AS300001L1 = {300001}
AS400001L1–AS465535L1 Array of 16-bit signed integers Yes Yes Yes Yes Accesses arrays of 16-bit holding registers as arrays of signed integers. AS400002L3 = {400002–400004}
ASD300001L1–ASD365534L1 Array of 32-bit signed integers Yes No Yes Yes Accesses arrays of 32-bit signed integers. Each 32-bit signed integer in the array is composed of two adjacent 16-bit input registers. ASD300001L1 = {300001, 300002}
ASD400001L1–ASD465534L1 Array of 32-bit signed integers Yes Yes Yes Yes Accesses arrays of 32-bit signed integers. Each 32-bit signed integer in the array is composed of two adjacent 16-bit holding registers. ASD400002L3 = {400002–400007}
CommFail Boolean value Yes No Yes No Represents a signal the Modbus I/O server generates. The signal is TRUE in the following cases:
  • Errors occur when the Shared Variable Engine fails to communicate with a Modbus device.
  • Errors occur on the serial ports or Ethernet interface of a Modbus Slave I/O server.
 N/A
D300001–D365534 32-bit unsigned integer Yes No Yes Yes Accesses two adjacent 16-bit input registers as one 32-bit unsigned integer ranging from 0 to 4,294,967,295. D300001 = {300001, 300002}
D400001–D465534 32-bit unsigned integer Yes Yes Yes Yes Accesses two adjacent 16-bit holding registers as one 32-bit unsigned integer ranging from 0 to 4,294,967,295. D400002 = {400002, 400003}
ErrorStatus 32-bit signed integer N/A N/A Yes No Represents the error status of the Modbus Slave I/O server. If CommFail is FALSE, this data item is 0. If CommFail is TRUE, this data item returns an error code. N/A
F300001–F365534 32-bit floating-point number Yes No Yes Yes Accesses two adjacent 16-bit input registers as one 32-bit floating-point number. F300001 = {300001, 300002}
F400001–F465534 32-bit floating-point number Yes Yes Yes Yes Accesses two adjacent 16-bit holding registers as one 32-bit floating-point number. F400002 = {400002, 400003}
OffHook Boolean value Yes Yes N/A N/A Specifies that a Modbus object retain exclusive use of a communication port when the value of OffHook is TRUE. If the value is FALSE, the Modbus object does not retain exclusive use of the communication port. Modbus Slave I/O servers do not support this data item. N/A
S300001–S365535 16-bit signed integer Yes No Yes Yes Accesses 16-bit input registers as signed integers ranging from –32,768 to 32,767. S300001 = {300001}
S400001–S465535 16-bit signed integer Yes Yes Yes Yes Accesses 16-bit holding registers as signed integers ranging from –32,768 to 32,767. S400002 = {400002}
SD300001–SD365534 32-bit signed integer Yes No Yes Yes Accesses two adjacent 16-bit input registers as one 32-bit signed integer ranging from –2,147,483,648 to 2,147,483,647. SD300001 = {300001, 300002}
SD400001–SD465534 32-bit signed integer Yes Yes Yes Yes Accesses two adjacent 16-bit holding registers as one 32-bit signed integer ranging from –2,147,483,648 to 2,147,483,647. SD400002 = {400002, 400003}
UpdateNow Boolean value No Yes N/A N/A Specifies that the Modbus I/O server refresh the Modbus device once if the value of UpdateNow changes from FALSE to TRUE. Modbus Slave I/O servers do not support this data item. N/A
UpdateRate 64-bit floating-point number Yes Yes N/A N/A Specifies how often the Modbus I/O server refreshes a Modbus device, in seconds. You can specify a non-integer value for this data item. If the value of this data item is zero, the Modbus I/O server does not refresh the device. Modbus Slave I/O servers do not support this data item. N/A
Updating Boolean value Yes No Yes No Represents a signal the Modbus or Modbus Slave I/O server generates. The signal is TRUE while the Modbus I/O server polls a Modbus device or the Modbus Slave I/O server is being updated. N/A

 Understanding Modbus Communication in LabVIEW

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