T1023-07C HIER466513P111 Logic input module

Similarly, if a new message begins earlier than 3.5 character times following a previous message, the receiving device will consider it a continuation of the previous message. This will set an error, as the value in the final CRC field will not be valid for the combined messages. A typical message frame is shown below.

When the slave responds to the master, it uses the function code field to indicate either a normal (error-free) response or that some kind of error occurred (called an exception response). For a normal response, the slave simply echoes the original function code. For an exception response, the slave returns a code that is equivalent to the original function code with its most significant bit set to a logic 1.

How the Address Field is Handled 

The address field of a message frame contains eight bits (RTU). Valid slave device addresses are in the range of 0 – 247 decimal. The individual slave devices are assigned addresses in the range of 1 – 247. A master addresses a slave by placing the slave address in the address field of the message. When the slave sends its response, it places its own address in this address field of the response to let the master know which slave is responding. Address 0 is used for the broadcast address, which all slave devices recognise. When Modbus protocol is used on higher level networks, broadcasts may not be allowed, or may be replaced by other methods. For example, Modbus Plus uses a shared global database that can be updated with each token rotation.

How the Function Field is Handled

 The function code field of a message frame contains eight bits (RTU). Valid codes are in the range of 1 – 255 decimal. Of these, some codes are applicable to all Modicon controllers, while some codes apply only to certain models, and others are reserved for future use. The NMBA01 supports the function codes 3, 6, and 16 (0x03, 0x06, and 0x10 Hex) only. When a message is sent from a master to a slave device the function code field tells the slave what kind of action to perform. Example is to read a group of outputs.