Reading ASCII and RTU Modbus Devices via RS232, RS422, RS485 or TCP/IP

1. Labview Modbus Serial Example Python
2. Labview Modbus Serial Example Java
3. Modbus Programming Examples
4. Labview Modbus Serial Example Diagram
5. Labview Modbus Rtu Example

Labview Modbus Rs232 Example xAP protocol always uses broadcast for sending the messages. Suitable for monitoring circuit breakers, flow meters, PLCs etc. Modbus is a very simple protocol. Modbus Library Modbus Library. Edit edit your slave data in Excel. For example, a register name defined as 2 in a MODBUS configuration device translates to register address 1 in the Holding Registers table of the LabVIEW MODBUS library, as shown below. MODBUS Device Holding Register Name = 2 LabVIEW Holding Register Address = 1 Quantity represents how many discrete inputs to read from the slave. The Modbus message uses the register address. For example, the first register of AO Holding Register has the number 40001, but its address is 0000. The difference between these two quantities is “offset”. Each table has its own offset, respectively: 1, 10001, 30001 and 40001. The Modbus Protocol was originally developed by Modicon (now part of Schneider Electric). Modbus was developed as a communication protocol for Modicon PLC Devices. The original Modbus Protocol specification, published in 1979, describes Serial Communications where data is transmitted one bit at a time. A later update to the standard, called Modbus TCP, describes how to use Modbus. This paper focuses on interfacing of high end Programmable Logic Controller with NI-LabVIEW using Modbus RTU protocol. The aim behind doing this is to develop a platform to enable real-time.

Sorry we no longer sell software to log data from Modbus devices. The Windmill software is now only available with our hardware. This page remains to help our existing customers.

How to read data from Modbus devices over serial (RS232, RS422, RS485) and network (TCP/IP) lines, using the Windmill COMIML software as an example.

In Modbus systems, a master or client initiates queries to a slave or server (the measurement device). The slave/server responds either by supplying data or taking an action. Slaves only respond to queries from the master.

Windmill supports any mix of digital and analogue inputsand outputs. These can be distributed across any set ofModbus slave devices. You can configure the slave deviceaddress and the parameter number (register number) foreach channel individually.

The Modbus protocol defines two modes of transmission:ASCII and RTU (Remote Terminal Unit). Windmillsupports both modes.

In ASCII mode each 8-bit byte ina message is sent as two ASCII characters. It allowsintervals of up to 1 second between characters, withoutcausing an error. Messages start with a colon and endwith a Carriage Return followed by a Linefeed. The advantages of ASCII mode is that it allows intervals of up to a second to occur between characters without causing an error. ASCII mode is only used over serial (RS232, RS422 and RS485) lines.

The RTU mode uses binary coding. Each 8-bit byte in a message contains two 4-bit hexadecimal characters. Greater character density allows better data throughput than ASCII for the same baud rate. Each message is transmitted in a continuous stream. The final part of a serial RTU message is a cyclic redundancy check, CRC. This calculates its value based on all earlier bytes in the message, it then adds its 2 bytes into the message. The computer therefore knows when it has received a corrupted message and can ask the instrument to resend its data. RTU mode is used over serial and network (TCP/IP) lines

Modbus and RS232, RS422 and RS485 | Modbus and TCP/IP: Ethernet/Internet | Transferring 4-Byte Values: 32-bit Integers and Floating Points | Convert Decimal to Hexadecimal | Parsing Messages | Comments

Modbus messages sent over Serial Lines: RS232, RS422 and RS485

Each message comprises four parts: device address, function code, data, error check.

The Device or Slave Address identifies your instrument. It contains one byte of information. In ASCII it is coded with two hexadecimal characters, in RTU with one byte. Valid addresses are between 0 and 247.

The Function Code specifies the type of message. It contains one byte of information. In ASCII it is coded with two hexadecimal characters, in RTU with one byte.

Modbus Com Port Settings

ASCII
Start Bit = 1
Data Bits=7
If Parity is even or off then Stop Bits = 1
If Parity is none then Stop Bits = 2

RTU
Start Bit = 1
Data Bits=8
If Parity is even or off then Stop Bits = 1
If Parity is none then Stop Bits = 2

Modbus Serial Message Settings when using ComDebug

You can use the Windmill ComDebug software to enter Modbus settings and send test messages.

In ComDebug's Message or Terminal screen use the Prompt Grid to send commands and data to your Modbus device. If you are sending binary rather than ASCII messages, either type directly into the Hex column or use the NumLock key on the keyboard to enter decimal addresses, codes, etc. Address 1, for example, would be shown as Char 001 and Hex 01.

Modbus input registers commonly start counting at 40001; these can only be read. They are addressed with addresses starting at 0. You may need to subtract 40001 from the register number to find the starting address. (If your input register number has less than 5 digits, just subtract 1. For example, Register 1 is Address 0.) Use code 04 to read an input register.

Similarly holding register numbers start at 30001. This is really register 0001 (address 0) and is read using Modbus function code 03.

The holding register comprises two bytes and may be used to return measured values such as temperature. A holding register can be read or written to. To write a value to an analogue output holding register, use the Modbus function code 06.

If your input or holding register address starts with a 6, for example 62592, convert this number to hex. You can use the converter at the bottom of this page. 62592, for example, converts to F480 and so you would enter F4 into the Hex column of Byte 3 and 80 into the Hex column of Byte 4.

To Read a Single Modbus Register

Byte 1 =	(slave) device address
Byte 2 =	Modbus function code, eg: 03 (read holding register) or 04 (read input registers)
Byte 3 =	msb of register: starting address
Byte 4 =	lsb of register: starting address
Byte 5 =	msb of number of bytes to read:normally 0
Byte 6 =	lsb of number of bytes to read:for example 2
Byte 7 =	CRC: use the CRC menu

Entering Modbus serial settings into ComDebug:
Reading Registers from Device 1

The Serial Reply Comprises

To Write to a Register over Serial Communications

The Serial Reply Comprises

(In ComDebug you use the Outputs menu to choose the correct format in which to send data values. For floating point and 32-bit values see Transferring 4-Byte Values below.)

Modbus messages sent over TCP/IP: Ethernet/Internet

Modbus TCP/IP Port Settings

This sections tells you how to communicate with a Modbus device over TCP/IP using the free ComDebug program. However, the settings apply to other software as well.

In ComDebug, edit an existing instrument file (or create a new one) and choose TCPIP Communications.

The master and slave must use the same port, usually port 502.

Labview Modbus Serial Example Python

Configuring the free Windmill ComDebug software to log from a TCP/IP device
For full details on what to enter here see the Understanding TCP/IP page.

Using ComDebug to send a message via TCP/IP

Use the prompt grid in ComDebug's Terminal screen to send commands and data to your Modbus device. Either type directly into the Hex column or use the NumLock key on the keyboard to enter decimal addresses, codes, etc. Address 1, for example, would be shown as Char 001 and Hex 01.

Each message comprises three parts: modbus application header, function code and data. When more than one byte specifies an item, the most significant byte is sent first.

The Modbus Application Header (MBAP) is a 7-byte header at the start of the message.
The Function Code specifies the type of message. It contains one byte of information (Byte 8).

Bytes 1 & 2	are a transaction identifier,usually 0 0, but may be differentdepending on how the system hasbeen set up.
Bytes 3 & 4	are a protocol identifier,always 0 0.
Bytes 5 & 6	identify the number of bytesto follow. Byte 5 is always 0 asmessages are shorter than 256 bytes
Byte 7	identifies a unit. It is used when the Modbus device is actually several devices behind a gateway or bridge, and specifies the Slave address of one of those devices.
Byte 8	is the Modbus function code, eg: 03 (read holding register) 04 (read input registers) 06 (write holding register)
Byte 9	is the msb of register: starting address
Byte 10	is the lsb of register: starting address
Byte 11	is the msb of number of bytes to read or write:normally 0
Byte 12	is the lsb of number of bytes to read or write:for example 2

As with serial Modbus communication, the function code 03 reads the holding register. This comprises two bytes. You can read or write to a holding register. The function code 04 reads an input register; this can only be read. It may be used to return measured analogue input values such as temperature. The function code 06 writes an output value to a holding register.

(In ComDebug you use the Outputs menu to choose the correct format in which to write data values to the holding register with Modbus command 06. For floating point and 32-bit values see Transferring 4-Byte Values below.)

Modbus input registers commonly start counting at 40001. They are addressed with addresses starting at 0. You need to subtract 40001 from the register number to find the starting address. If your register address starts with a 6, for example 62592, convert this number to hex. You can use the converter at the bottom of this page. 62592, for example, converts to F480 and so you would enter F4 into the Hex column of Byte 3 and 80 into the Hex column of Byte 4. Similarly holding register numbers start at 30001. This is really register 0001 (address 0) and is accessed using Modbus function code 03.

The Reply to a Read Input Register Request (TCP/IP) Comprises

Transferring 4-Byte Values: 32-bit Integers and Floating Point Numbers

The above discussions comply to the Modbus standard which uses 2-byte register values, with the most significant byte preceding the least significant byte. There is no guidance in the standard for 4-byte values - signed and unsigned 32-bit values and floating point numbers. Manufacturers of Modbus devices can therefore choose for themselves in which order the bytes need transmitting.

If we think of a 4-byte number as comprising bytes A, B, C and D with A as the most significant byte and D as the least significant, there are four ways a manufacturer could order the byte transfer.

Standard Sequence:	ABCD
Byte Swap Sequence:	BADC
Word Swap Sequence:	CDAB
Byte and Word Swap Sequence:	DCBA

In ComDebug use the Outputs menu to choose in which format to send data values.

Convert Decimal to Hexadecimal

Parsing Messages

There are two steps to parsing the data:

1. Locate the data in which you are interested
2. Extract the data

Locating the Data

You can locate the data by:

Labview Modbus Serial Example Java

1. Searching for specific characters in the messagestring. The next action occurs after all the specified characters
2. Ignoring characters until one of the ones in whichyou are interested appears. The next action occurs on any one of the specified characters
3. Ignoring a number of characters in the messagestring.

At first glance methods 1 and 2 seem to do the same job.However, when you search for characters the next actionoccurs AFTER the searched for characters. When youignore characters the next action occurs ON thespecified character. So if you searched for a + sign youwould not be able to extract it, but if you ignoredall characters until the + sign you would. Also, a searchwill look for the entire string specified (eg 'abc'),whilst ignoring characters will stop at any of thecharacters specified (eg 'a' or 'b' or 'c'). Use the NonPrint menu to enter characters like carriage returns.

Be aware that with instruments that constantly send dataWindmill will not know where the beginning of the messageis. So it's no use starting by ignoring a number of characters in these cases..

Extracting the Data

Once you've located the data you need to extract it. There are several ways to do this, the two most commonly used are:

1. Extract until a specified character occurs. The next action occurs on this character.
2. Extract the next so many characters.

Modbus Programming Examples

Extract until means COMIML or ComDebug will extract values until it finds any one of the characters specified. The next command then acts ON this character: it will not be extracted.

For full details of all the extract options, see the ComDebug Help menu.

Labview Modbus Serial Example Diagram

For more details see Parsing Messages.

Related Topics

Labview Modbus Rtu Example

Need more help? See Getting Started with ComDebug and COMIML.