Lenze E94P PositionServo Modbus - Manual (Page 27)

27 P94MOD01C
Protocol Implementation
5.7 Exception Responses
If an invalid message is received, the drive will respond with a Modbus Exception as per the “Modbus application
Protocol specification V1.1”, i.e. the exception function code = the request function code + 0x80 (an exception
code is provided to indicate the reason of the error).
Table 9: Exception Codes
Code V1.1 Specification Description
0x01 Illegal Function function not supported by PositionServo
0x02 Illegal data address requested address is not a valid register address
0x03 Illegal Data Value set value not valid for specific variable
5.8 Modbus Message Frame
The Modbus protocol defines a simple protocol data unit (PDU) independent of the underlying communication
layers. There are additional application data unit (ADU) fields introduced by the network layer.
ADU for Modbus RTU
PDU
Address Function Code Data Error
Figure 18: Modbus RTU Frame EIA-485
ADU for Modbus TCP/IP
PDU
Length Unit Identifier Function Code Data Transaction Protocol Identifier
MBAP
Figure 19: Modbus TCP/IP Request/Response
5.8.1 PDU Function Code
• Size = 1 byte
• The function code indicates what kind of action to perform.
• The function code (depending upon the function) is normally followed by a data field that contains request
and response parameters.
5.8.2 PDU Data
The data field of messages sent from a master/client to slave/server device contains additional information that
the slave/server uses to take the action defined by the function code. This can include items like discrete and
register addresses, the quantity of the items to be handled, and the count of the actual data bytes in the field.
The data field may be nonexistent (of zero length) in certain kinds of requests. In this case the slave/server does
not require any additional information. The function code alone specifies the action.