Modbus Read/Write (MBUS) Instruction

Mnemonic (Keyboard Shortcut) = MBUS

Icon / Button =

Purpose

Perform Read or Write requests from the CPU to Modbus TCP Ethernet and Modbus RTU Serial Slave devices. This instruction is only available with P2-622 and newer (next-generation) CPUs. This instruction differs from the MRX/MWX instructions in that nearly all parameters can be modified on-the-fly, allowing a single instruction to address a large number of devices or registers, simply by changing a target IP address, slave node, or Modbus address before executing the instruction. It requires a Modbus device be configured in the Hardware Configuration window.

Instruction Parameters

Parameter	Parameter Type	Requirements	Description
Enable	Ladder Input	Must Have	Edge-driven. If Automatic Polling is selected, the instruction becomes Level-driven. When Enable is ON, the instruction will operate every scan. When Enable is OFF, the instruction is not solved, and its outputs are not updated.
Use Structure	Checkbox	Optional	Enables the use of a Structure for instruction tags.
Modbus Device	Drop-down Selection	Must select a pre-configured Modbus device	Pre-configured Modbus Devices will appear in this list. To add a new device, navigate to the Hardware Configuration window and select the Modbus tab, then drag Generic Modbus Device to the device area. A dialog will open and allow configuration. See Modbus Device Configuration for more details.
Instruction Type	Drop-down Selection	Must Select	Select Modbus Read to request data from a device. Select Modbus Write to write data to a device.
Polling Options
Automatic Polling	Checkbox	Optional	The CPU will send out a request at the specified Polling Frequency + Polling Offset Value (msec).
Polling Frequency	Numerical Tag	Must have when Automatic Poling is selected	How frequently the CPU will send a request to the specified device.(msec).
Polling Offset	Numerical Tag	Must have when Automatic Poling is selected	An offset, in msec, for the request to be delayed from the configured Polling Frequency. Multiple instances of this instruction, with the same frequency, can be configured however, the offsets must be different; the result being a ‘fanning out’ of these requests. This can alleviate communication load density.
Skip Execution if Buffer is Greater Than	Checkbox, Numerical Tag	Optional	Used with Automatic Polling, allows the user to back-off an instruction if the buffer is getting too full. Set by a percentage. More details in the Communications topic.
Instruction Status
In Progress	Boolean Tag	Optional	Status Bit that specifies a Read Request is in progress inside of the CPU (sent from the instruction but not yet sent out the Ethernet Port). The Status Bit reflects the Status of the individual Read Request.
Complete	Boolean Tag	Optional	Status Bit that specifies a Read Request has been sent and a reply has been received, but it may not necessarily have been a good reply. The Status Bit reflects the Status of the individual Read Request.
Success	Boolean Tag	Optional	Status Bit that specifies a Read Request has been sent and a good reply has been received. The Status Bit reflects the Status of the individual Read Request.
Error	Boolean Tag	Optional/p>	Status Bit that specifies a Read Request has been sent and a reply with an Error has been received from the Slave device within the specified Timeout period. Check the Exception Response String to see if it's an Error returned from the Slave device or check the Error Logs to see if it is an Internal Error. The Status Bit reflects the Status of the individual Read Request.
Timeout	Boolean Tag	Optional	Status Bit that specifies a Read Request has been sent but no reply was received within the specified Timeout value time. Timeout value is set up in the External Ethernet Port and RS232/RS485 Serial Ports of the Hardware Configuration. The Status Bit reflects the Status of the individual Read Request.
Exception Response String	String	Optional	String that holds the exception response Error. View the Communication Error Codes section of the help file for detailed information on Error Codes and possible corrective actions.
Addressing
Word Swap	Checkbox	Optional	For 32-bit tags, the Lower and Upper words are Swapped when data is Read.
Map 16-bit data to 32-bit	Checkbox	Optional	Allows user to Map 16-Bit Modbus Addresses into the low Word of 32-Bit Tags in the CPU.
Slave Modbus Starting Address	Constant	Must Have	Modbus Starting Address of the Slave.
Modbus Decimal Addressing	Selectable Option	Must Select Decimal or Zero Based Addressing	Select if Slave Modbus Starting Address field requires the standard Modicon style Addressing format range from 1 to 65536.
Zero Based Modbus Addressing	Selectable Option	Must Select Decimal or Zero Based Addressing	Select if Slave Modbus Starting Address field requires the standard Modicon style Addressing format range from 0 to 65535.
Modbus Function Code	Drop-down Selection	Must Have	Selects functions the instruction will Read (if Modbus Read mode is selected): 1:Read Coils 2:Read Input Bits 3:Read Holding Registers 4:Read Input Registers
Modbus Function Code	Drop-down Selection	Must Have	Selects functions the instruction will Write (if Modbus Write mode is selected): 5:Write Coils 6:Write Single Register 15:Force Multiple Coils 16:Write Multiple Registers
Mapping
Tag Mapping	Selectable	Must Have	Selects the type of data the Modbus request will be written to or read from on the PLC: Non-Array: Allows data to be written to or read from non-sequential tags in PLC memory. Data on the Modbus slave device must be sequential. Array: Allows data to be written to or read from an array in PLC memory. String: Allows data to be written to or read from a string tag in PLC memory. Only even numbers of characters can be sent or received.
Non-Array Mapping	Automatically selected		The Non-Array Mapping option is displayed when Non-Array is chosen as the Tag Mapping option.
Number of Tags	Constant	Must have when Non-Array Mapping is selected	Enter the number of tags to send or receive in the Modbus transaction.
Tag	Numerical Tag / Boolean Tag	Array Mapping is selected	Specifies the Tags that will Receive the response, or Send the request. Note: for Modbus function codes 3 & 4, if Non-Array mapping is selected, the max. number of tags is limited to 62, however, if the "Map 16-bit data to 32-bit" box is checked, the max. number of tags is increased 125. For Modbus function code 16, if Non-Array mapping is selected, the max. number of tags is limited to 60, however, if the Map 32-bit data to 16-bit box is checked, the max. number of tags is increased 120.
Array Mapping	Automatically selected		The Array Mapping option is displayed when Array is chosen as the Tag Mapping option.
Array Name	Numerical Array Tag	Must have when Array Mapping is selected	Specifies the Array that will Receive a response, or be the data source for a transmission
Starting Index	Integer Tag / Constant		Starting position of the selected Array
Number of Elements	Integer Tag / Constant		Number of Array elements that will be filled by incoming data, or sent to a Modbus slave device For Reads: • Max. 2000 for Boolean arrays • Max. 125 for 8-bit Integer arrays • Max. 125 for 16-bit Integer arrays • Max. 62 for 32-bit Integer arrays For Writes: • Max. 1968 for Boolean arrays • Max. 120 for 8-bit Integer arrays • Max. 120 for 16-bit Integer arrays • Max. 60 for 32-bit Integer arrays
String Mapping	Automatically selected		The String Mapping option is displayed when String is chosen as the Tag Mapping option.
String Name	String Tag / Constant	Must have when String Mapping is selected	Specifies the String tag that will Receive a response, or is the source for a transmission
Number of Characters	Integer Tag / Constant Even Number Only	Must have when String Mapping is selected	This value, whether a tag or a constant, must be an even number. Maximum value: 128
Byte Swap	Checkbox	Optional	Every Two Characters will be Swapped when data is Read or Sent

NOTE: This instruction is only compatible with P2-622 and newer (next-generation) CPUs. On CPUs with multiple ethernet ports and Port 2 (Remote) configured as User Defined, when a selected Modbus device is configured as an Ethernet device, communications are routed through the appropriate port automatically, based on the target IP address and the CPU’s gateway settings, if applicable.

Modbus devices configured as Serial will communicate using the port selected and node value configured in the Modbus device properties. The node value can be changed throughout the program, and this instruction will communicate using the node value at time of execution.

NOTE: During a runtime transfer any Modbus communications in operation will indicate a timeout bit. These will clear once the project transfer is complete.

Instruction Configuration

When the Modbus Read/Write Instruction is selected the window shown below opens.

Parameter Configuration Table

Modbus Read/Write
Instruction

(MBUS)

Notes

* This icon indicates using 1D Array data types as indicated in Data Type column.

Modbus Device

Select a preconfigured Modbus device to target. This is set up in the Modbus tab of the Hardware Configuration window.

Instruction Type

Select Modbus Read or Modbus Write to match the intended instruction behavior.

Automatic Polling

Enable this checkbox to request a Read or Write action at a regular frequency

Polling Frequency

Enter the desired polling frequency, in msecs

Polling Offset

Enter the desired polling offset in msecs

Skip Execution

Select and enter % Value.

Word Swap

Select if Word Swap is desired.

Map 16-bit data to 32-bit

Select if 16-bit to 32-bit conversion is desired.

Slave Starting Address

Enter a Starting Address for Slave Modbus.

Modbus Decimal Addressing

Select desired Addressing

Zero Based Modbus Addressing

Modbus Function Code

Select desired Function Code The available options will change based on the Instruction Type.

Tag Mapping

Select the desired type of data to read or write

Number of Tags

Enter desired Tag(s) to be Mapped.

Tag Table

Enter a list of tags to populate from received data, or tags to send in one transaction

Array Name*

Enter Tagname of Array to be Mapped.

Starting Index

Enter the Starting Point of Mapped Array.

Number of Elements

Enter the number of array elements to receive into, or send from.

String Name

Enter the desired string Tagname to be Mapped for this Project.

Number of Characters

Enter the number of Characters to be read or written

Byte Swap

Select if Byte Swap is desired.

Use Structure

Select if Structures are used.

In Progress

Select a Boolean Tag for Progress

Complete

Select a Boolean Tag for Complete.

Success

Select a Boolean Tag for Success

Error

Select a Boolean Tag for Error

Timeout

Select a Boolean Tag for Timeout

Exception Response String

Select a String Tag for Response Errors.

Modbus Ethernet Throughput (messages per second)

To determine the total number of Modbus messages that can be sent per second (Ethernet throughput), use the following calculation:

Example

Number of Modbus Masters/Slaves = 8

'Peak Scan Interval' System Tag = 10msec

8 x 10 msec. (100 scans) = 800 messages per second.

NOTE: The number and size of the tags used in a specific Modbus message will increase the scan time, thus affecting the overall throughput.

Timing Charts

NOTE: Another side effect of the queuing mechanism is the instruction status bits may not be seen in the ladder. When an instruction has completed (Success, Exception Response, or Timeout), the CPU firmware will set the appropriate status bits. At the point that the CPU firmware pulls another message off the queue, it will reset the status bits of that instruction. The timing can work out in such a way that the status bits get set but the CPU firmware pulls off the instruction that contains those same status bits and resets them to start another iteration before the ladder code examines those bits.
Two things can be done to ensure reliable detection of status bits:
1. Do not use Automatic Poll and instead use interlocking logic to detect status completion of an instruction before enabling that instruction again. This is the most reliable method.
2. Ensure that the Automatic Poll time is long enough to prevent queueing of that same instruction.
See Communications Modbus Functionality help topic for more details.