8 - 8 VM600 networking manual MAVM600-NET/E
Edition 9 - February 2018
Description of Modbus RTU protocol
SETTING UP A MODBUS CONNECTION (CPUM SW VERSION 067 OR EARLIER)
8.4.5 Error handling
There are two types of error handling implemented by the Modbus server:
communication-related errors and system-related errors.
8.4.5.1 Communication errors
The VM600 Modbus server provides two error exception codes (see Figure 8-6):
• If the client (master) requests a function that is not supported by the VM600, the
exception code “01” will be returned. This code indicates an illegal function error
exception response.
• If the client (master) requests an address that is not configured in the VM600, the
exception code “02” will be returned. This code indicates an illegal data address error
exception response.
NOTE: When an error occurs, the server (slave) returns a message response that includes
an exception code. To distinguish between the message request and message
response, the MSB (most significant byte) of the function code is set to “1” in the
message response.
NOTE: Attempts to communicate with (access) an unplugged card will result in the
following communication error: data values of 0 (zero) will be returned.
Error handling
Example:
Read one register from Modbus address 01, starting from register 256 (decimal).
Request frame
Note 1:
Register 1 (decimal) has register address 0x00
Register 17 (decimal) has register address 0x10
Register 256 (decimal) has register address 0xFF and so on.
An example response if register 256 does not exist, is as follows:
Response frame
0x01 0x03 0x00 0xFF 0x00 0x01 0xXX 0xXX
Slave
address
Modbus
function
First register address
to be read = 0x00FF
(See Note 1)
1 register to be read CRC
0x01 0x83 0x02 2 bytes
Address Function Exception
code
CRC
Figure 8-6: Formats and example for error message
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