Modbus Read (MRX) Instruction

Mnemonic (Keyboard Shortcut) = MRX

Icon / Button =

Purpose

Perform Read requests from the CPU to Modbus TCP Ethernet and Modbus RTU Serial Slave devices.

Instruction Parameters

Parameter	Parameter Type	Requirements	Description
Count Up	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.
Ethernet Port	Selectable Option with Drop-down Selection	Must Select Ethernet Port or Serial Port	CPU External Ethernet Port that will send Modbus Read requests.
IP Address	Constant	Must Have when Ethernet Port Selected	IP Address of the Modbus TCP server that the Request will be sent to. Enabled when Ethernet Port is selected.
TCP Port Number	Constant		Default is 502 (specified by Modbus TCP specification). Some devices will require a different value. Enabled when Ethernet Port is selected.
Slave Node Number (Ethernet)	Constant		Typically used for Modbus TCP to Modbus RTU bridges but some server devices may require different values. Ethernet Slave Node range is 0 to 255, with a default of 255.
Serial Port	Selectable Option with Drop-down Selection	Must Select Ethernet Port or Serial Port	RS232 or RS485 CPU Port or P3-RS/RX Remote Slave Port that will perform Modbus Read Requests.
Slave Node Number (Serial)	Constant	Must Have when Serial Port Selected	Number assigned to the Slave device. Slave Node range is 1 to 247, with a default of 1.
Use Structure	Checkbox	Optional	Enables the use of Structure.
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	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.
Automatic Polling	Checkbox	Optional	The CPU will send out a request at the specified Poll Rate + Offset Time Value (msec).

Instruction Parameters (continued)

Parameter	Parameter Type	Requirements	Description
Skip Execution if Buffer is Greater Than	Checkbox	Optional	Used with Automatic Polling, allows the user to back-off an instruction if the buffer is getting too full (for CPU ports only). Set by a percentage. More details in the Communications topic.
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	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	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: 1:Read Coils 2:Read Input Bits 3:Read Holding Registers 4:Read Input Registers
(Tagname Mapping) Non- Array	Selectable Option	Must Select Array or Non-Array	Allows the user to specify non sequential Tags into which the data can be Read in the Modbus Request. The number of Tags entered will determine the size of the Request.
Tag	Numerical Tag / Boolean Tag	Must Have when Non-Array Selected	Specifies the Tags that will Receive the response . 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.”
(Tagname Mapping) Array	Selectable Option	Must Select Array or Non-Array	Allows the user to select the starting Tag and sequential number of Tags the data will be Read into in the Modbus Request.
Array Name	Numerical Tag	Must Have when Array is Selected	Specifies the Array that will Receive the response.
Start Index	Constant		Starting position of the specific Array.
End Index	Constant		Ending position of the specific Array: 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
String Name	String Tag	Must Have when String is Selected	Specifies the Tag that contains the ASCII string.
Number of Characters	Constant	Must Have when String is Selected	Enter even number of characters, max 128.
Byte Swap	Checkbox	Optional	Every Two Characters will be Swapped when data is Read.

Note: Communications using this instruction are performed through the External Ethernet Port on the CPU (upper (P3) or forward (P2) Ethernet Port), the RS232 Port on the CPU (RJ12 Port), the RS485 Port on the CPU (3-pin Terminal Block). Uses Modbus based protocol exclusive to the P1-Series CPUs.

Note: The Modbus Read instruction can be used with the GS-EDRV100 (GS1, 2, 3) or the Modbus TCP communication option module (GS4, 20) if connected to the External Ethernet Port. See more details in the Communications topic.

Note: Tag Values are updated immediately as each Ladder Rung is executed, top to bottom. However, Tag Values representing physical Outputs are only applied to the physical Output after the END statement of the last Task to be scanned is reached. Outputs in Remote Base Groups have additional limitations regarding Update Intervals.

Note:During a runtime transfer any Modbus communications instruction timeout bit will occur during transfer and resume once complete.

Instruction Configuration

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

Parameter Configuration Table

Modbus Read

Instruction

(MRX)

Notes
* or or
These icons indicate to use 1D and/or 2D Array data types of respective Data Type column.

Ethernet Port

Select if Ethernet Port is used.

IP Address

Enter an IP Address.

TCP Port Number

Enter TCP Port Number. Default is 502.

Slave Node Number

Enter a Slave Node Number. Default is 255 for Ethernet and 1 for Serial.

Serial Port

Select if Serial Port is used.

Automatic Polling

Select and enter Polling Time and Offset Time (msec).

Skip Execution

Select and enter % Value.

Word Swap

Select if Word Swap is desired.

Map 16 bit data to 32 bit

Select if 16 to 32 bit conversion is desired.

Slave Modbus Starting Address

Enter a Starting Address for Slave Modbus.

Modbus Decimal Addressing

Select desired Addressing.

Zero Based Modbus Addressing

Select desired Addressing.

Modbus Function Code

Select desired Function Code.

Non-Array

Select if Non-Array Mapping is desired.

Tag

Enter desired Tag(s) to be Mapped.

Array

Select if Array Mapping is desired.

Array Name *

Enter Tagname of Array to be Mapped.

Starting Index

Enter the Starting Point of Mapped Array.

End Index

Enter the Ending Point of Mapped Array.

String

Select if String Mapping is desired.

String Name

Enter the desired Tagname to be Mapped for this Project.

Number of Characters

Enter the number of Characters to be Read.

Byte Swap

Select if Byte Swap is desired.

Use Structure

Select if Structures are used.

In Progress

Select a BooleanTag for In Progress.

Complete

Select a BooleanTag for Complete.

Success

Select a BooleanTag for Success.

Error

Select a BooleanTag for Error.

Timeout

Select a BooleanTag 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 = 10 msec.

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 that 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 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.

Application Example

In the following example, the CPU Ethernet Port is performing a Modbus TCP Read of four consecutive status coils from a Modbus TCP Slave device.