VM600 networking manual MAVM600-NET/E 8 - 5
Edition 9 - February 2018
Description of Modbus RTU protocol
SETTING UP A MODBUS CONNECTION (CPUM SW VERSION 067 OR EARLIER)
The application of these Modbus functions to a VM600 are shown in Table 8-3.
When used to read values from a MPC4 card, Modbus functions 01 and 02 can be used
interchangeably – they both return the same result.
Similarly, when used to read values from a MPC4 card, Modbus functions 03 and 04 can be
used interchangeably. They both return the same result.
NOTE: If any unsupported Modbus function is requested, an exception code “01” will be
returned to indicate the use of an illegal function.
If a value is requested that is out of the range of what is configured in the VM600,
the exception code “02” will be returned to indicate the use of an illegal address.
8.4.4 Modbus function formats
8.4.4.1 Functions 01 and 02
Coils and discrete inputs can be read using function 01 or function 02 of the Modbus protocol.
NOTE: A maximum of 2040 discrete values (coils) can be requested at one time.
The request and response frames are shown in Figure 8-4.
Table 8-2: Modbus functions supported
Modbus
function code
Function description
01 (0x01) Read coils
02 (0x02) Read discrete inputs
03 (0x03) Read holding register
04 (0x04) Read input registers
Table 8-3: The application of the Modbus functions supported by a VM600
Modbus
function code
Use of function
01
Used to read VM600 status or configuration information (single-bit),
such as alarm, danger and OK status
02 Same as Modbus function code 01
03
Used to read dynamic values and configuration information (in 16-bit
integer format) from the VM600
04 Same as Modbus function code 03
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