Modbus TCP driver

Reading and writing coils and registers from devices that communicate over Modbus TCP (energy analyzer, drive, gateway) using Modicon addresses; unit ID and 32-bit byte order settings.

When to use it

Use this driver if your device is a Modbus TCP server (Modbus over TCP). Addresses are written in Modicon notation: 40001 is the first holding register, and its address in the protocol is 0. Serial lines (RTU / ASCII, RS-232 / RS-485) are not supported; for serial devices a gateway that converts to Modbus TCP is required.

Prerequisites on the device side

• 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 is distinguished by its own unit ID; 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).

Setting up a 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.

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 driver.

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

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

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. [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