Implementation
Almost all implementations have variations from the official standard. Different varieties may not communicate correctly between different suppliers equipment. Some of the most common variations are:
Data Types
- Floating Point IEEE
- 32 bit integer
- 8 bit data
- mixed data types
- bit fields in integers
- multipliers to change data to/from integer. 10, 100, 1000, 256 …
Protocol extensions
- 16 bit slave addresses
- 32 bit data size (1 address = 32 bits of data returned.)
- word swapped data
Modbus TCP Data Transmissions
- Messaging is supported in both peer-to-peer and Client/Server configured networks. In both cases object messages are communicated serially in the “data” portion of the Modbus protocol of each message transaction
- All object messages are sent between two nodes to execute service transactions. All service transactions are associated with “request” services and consist of a request message and a corresponding response message.
- Since both clients and servers may be constructed using devices which support only the alternate transport function codes, it is necessary that client and server devices support the alternate transport mechanism in addition to the object messaging function code if so supported.
