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3.1 Modbus Settings
Settings relating to the Modbus/TCP feature can be configured in the Remote Communication
Configuration Menu.
The ‘Verbose Logging’ setting is used to generate an Event log entry for every Modbus message into and
out of the controller. The information may be used in conjunction with an HMI log when
troubleshooting communications during commissioning. However, the additional logging adds a
significant load on the controller’s systems. Therefore, this feature should only be enabled temporarily
while actively troubleshooting communications. The setting automatically reverts to Disabled when the
controller is restarted.
Detailed information on the Remote Communication Configuration Menu can be found in the controller
Instruction Manual.
4.0 MODBUS/TCP DRIVER
4.1 Modbus/TCP Protocol
The Modbus protocol, as well as the Modbus/TCP variant, is well documented in the specifications
which are available at http://www.modbus.org
, a website established by the Modbus Organization for
supporting and organizing the Modbus protocol.
The Modbus/TCP Application Data Unit (ADU) consists of 2 distinct sections:
Modbus Application Protocol (MBAP)
Header
Protocol Data Unit (PDU)
The total size of the ADU is between 9 and 263 bytes, depending on the function code and number of
data addresses requested.
4.1.1 MBAP Header
The Modbus/TCP extension adds 7 additional bytes to the original Modbus protocol, which allows for
the transport over the TCP/IP layers. These 7 bytes make up the MBAP Header:
Modbus Application Protocol (MBAP) Header
Transaction Identifier 2 bytes
Identification of Request/Response transaction.
Copied from request to response
Number of following bytes (includes the Unit Identifier and PDU)
Unit Identifier 1 byte
Identification of Remote Device.
Can be used for broadcasting (unsupported).
The Unit Identifier has a special consideration in the Modbus/TCP implementation. If the value is 0, then
the request is a broadcast message and the packet will be processed, but no response will be generated.
If the value is non-zero, the packet will be processed and a response will be returned.
Normally the Slave ID, which is not present in the Modbus/TCP protocol, will be set in the HMI client
software to 1. The broadcast Unit Identifier address is not supported by the controller; all supported
function codes require a response message at all times.
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