Modbus drivers (TCP, RTU, ASCII)

Reading and writing coils and registers with Modicon addresses in Modbus TCP (Ethernet), Modbus RTU and Modbus ASCII (RS-485 / RS-232 serial line) devices; unit ID, byte order and serial port settings.

When to use it

There are three Modbus drivers. Addresses, data types, byte order and read / write rules are the same in all three; only the transport differs:

Addresses are written in Modicon notation: 40001 is the first holding register; its address in the protocol is 0.

Prerequisites on the device side

• For Modbus TCP, the Modbus TCP server must be enabled on the device (default port 502).
• Find out the device's unit ID (Unit ID, slave address) and its address map. Each device behind a gateway or on a serial line is distinguished by its own address; for a single device it is usually 1.
• Does the address in the device documentation give a 1-based Modicon address (40001) or a 0-based protocol address (0)? If it is the latter, enter the table digit and documentation address + 1 in Studio: a holding register with protocol address 100 becomes 40101.
• Find out the order in which 32-bit values are held in the two registers (32-bit byte order below).
• For a serial line, also find out the device's baud rate, parity, data bits and stop bits.

Setting up a Modbus TCP connection

1. Add a connection

   In the Connections section: Add, driver Modbus TCP. Enter the IP address; port 502 is filled in.

2. Unit ID and byte order

   

   Unit ID 0 – 255 (default 1). Options of 32-bit byte order: ABCD — big-endian (default), CDAB — word swapped, BADC — byte swapped, DCBA — little-endian. 16-bit values are always big-endian, as in the Modbus standard.

3. Add a tag

   Enter the Modicon address as the tag address. The address is validated against the data type of the tag (Tag addresses).

4. Test the byte order

   If the value of a 32-bit Real or DInt tag differs greatly from what is expected (e.g. a very large or very small number), change the byte order. To do this, store a known value (e.g. 1.0) in the device and try the options until the correct value appears.

Modbus RTU and ASCII (serial line)

The serial drivers use a COM port of the Live computer: the computer's own serial port or a USB – RS-485 / RS-232 converter. The Live computer is the master of the line; the devices answer the requests.

1. Find the COM port

   In Windows, look up the port number of the converter under Device Manager›Ports (COM and LPT) (e.g. COM3).

2. Add a connection

   Add, driver Modbus RTU (serial) or Modbus ASCII (serial). Enter the serial port in the IP address / computer name cell (COM3; com 3 is also corrected to COM3). The Port cell is locked.

3. Device address and serial settings

   Device address (slave ID) 1 – 247 (default 1). Speed (baud) 1200 – 115200 (default 9600), Parity None (N), Even (E) (default) or Odd (O), Data bits 7 or 8, Stop bits 1 or 2. The default setting is 9600 8E1 for RTU and 9600 7E1 for ASCII. With RTU the data bits must be 8. All values must match the setting in the device.

   1234

   1. Address — for a serial driver, the COM port (COM3); the port field is locked.
   2. Device address (slave ID) — 1 – 247; different for each device on the line.
   3. Speed, parity, data bits, stop bits — the same as on the device and on all connections on the same port.
   4. Target — port · serial settings · device address.

4. Set the timeout

   Timeout (ms) is the time allowed for the device to start responding; Live adds the transmission time of the request and the response on the line. Long blocks at low speed therefore do not time out.

5. Add a tag

   Addresses and types are the same as for Modbus TCP (tables below).

Several devices on one line

Create a separate connection for each device on the same RS-485 line: their address is the same COM port, their Device address values differ. Live opens the port once and shares it among the connections; only one query is on the line at a time, and requests are sent one after another. Therefore, the more devices on a line, the longer the read interval of each device.

Connections that use the same port must have the same Modbus type (RTU or ASCII), speed, parity, data bits and stop bits. If they differ, Studio reports an error in Problems; in Live the second connection is not established and shows the error message. A connection with Enabled off does not open the port and is not checked.

• RTU frames are checked with a CRC; between two frames the line is kept silent for at least 3.5 character times (1.75 ms above 19200 baud). ASCII frames are checked with an LRC.
• When a corrupt frame arrives, the remaining bytes on the line are discarded and the connection is re-established. If the USB converter is unplugged, all connections on the same port drop and Live tries to reopen the port.
• Online›Test connections… does not test serial connections; it counts them as skipped. A serial connection can only be tried in Live.

Address ranges and register usage

The first digit gives the table; the remaining four or five digits give the sequence number (1 – 65536). Each type occupies the following number of registers; the address of the tag is the first register:

No bit number is written in coil and discrete input addresses; if Bool is given for a register address, the bit number is mandatory. The Byte and SInt types do not exist in the Modbus drivers.

Read and write

• Nearby tags in the same table are read in a single request: at most 125 registers or 2000 coils; gaps of at most 8 registers (64 for coils) between them are merged. If the device rejects a read with gaps, the driver from then on merges only adjacent tags.
• If the device rejects a request, the tags in that block are read one by one; only the problematic tag gets an error.
• Writing is done with a write single coil request for a coil, a write single register request for a single register, and a write multiple registers request for multi-register values.

Driver reference

The parameters, address formats and examples of Modbus ASCII are the same as for Modbus RTU; the driver ID is modbus-ascii and the default for Data bits is 7.

Modbus TCP

Addresses are written in Modicon notation: 40001 is the first holding register (protocol address 0). 32-bit types use two consecutive registers.

Driver ID

modbus (name in the project file)

Connection

An IP address / computer name and a port are required (default port 502)

Data types

Bool Int UInt DInt UDInt Real LReal String

Connection parameters

Appears in the Inspector window when a connection is selected.

Address formats

Sample addresses

Modbus RTU (serial)

Modbus RTU over an RS-485 / RS-232 serial port. The connection's address is the serial port (e.g. COM3). A separate connection is opened for each device on the same line; connections that use the same port must have the same serial port settings. Tag addresses are the same as in Modbus TCP: 40001 is the first holding register.

Driver ID

modbus-rtu (name in the project file)

Connection

The COM port is entered in the address column (e.g. COM3); speed, parity, data bits and stop bits are connection parameters

Data types

Bool Int UInt DInt UDInt Real LReal String

Same port

Connections with the same settings share a COM port (several devices on an RS-485 line); a second connection with different settings gives an error.

Connection parameters

Appears in the Inspector window when a connection is selected.

Address formats

Sample addresses

Common errors

The marker at the start of each line tells where the message will appear in Live: [connection] in the Connections section of the system menu; [write] in the notification window of the operator who performed the write; [Studio] in the address cell in Studio or in the Problems list. [tag] is a tag error during reading: the tag is left with bad quality, the text of the message is not shown in Live in this version (Tag errors); read the text to understand the cause.

Tips and limits