55xGenComm1_29 www.wellfut.com 0755-27153030 0755-27732300
10
Modbus Functions Used
This standard only uses Modbus functions 3 and 16 as recommended in the Open Modbus/TCP
Specification draft 2.
NOTE:- Any other functions may be implemented if required, for example for configuration of
the slave device. These are not defined in this standard.
Any device which requires other functions to be implemented in order to perform a task, which can
be performed, by function 3 or 16 is deemed to be non-compliant with this standard.
Function 3 - Read Multiple Registers
Reads one or more 16 bit registers from the slave device, complying to the following rules below.
The limit of 125 registers is to comply with the Modbus specification, which requires that a
message must not exceed 256 bytes including all fields.
Any request for a register that is defined in this standard must return a normal response, if the
register is not implemented by a particular product it must return the unimplemented value from the
table below to indicate this fact, it must not return an exception.
A request for a register that is defined in this standard as contain some unimplemented bits must
return a normal response, the unimplemented bits will contain the unimplemented value from the
table below.
A request for a register that is defined in this standard as unimplemented will return the
unimplemented value from the table below.
A request for a register that is defined as reserved in this standard will return extended exception
code 12 (Reserved register).
A read from a multi-register value such as a 32-bit value or a string must be performed by a single
message, not by multiple ones. This avoids the possibility of a value being partly current and partly
old data.
Query message
Byte Field name Notes
0 Slave address
1 Function code (3)
2 First register address - high byte 16 bit register address
3 First register address - low byte
4 Number of registers to read - high byte 16 bit number of registers, must be in the
range 1 to 125
5 Number of registers to read - low byte
6/7 Error check CRC
Normal response message
Byte Field name Notes
0 Slave address
1 Function code (3)
2 Byte count (n) 8 bit even number in the range 2 to 250 (number of
registers *2)
3 First register - high byte 16 bit register
4 First register - low byte
…
1+n Last register - high byte 16 bit register
2+n Last register - low byte
3+n/4+
n
Error check CRC
To Next Page
Loading...
Need help?
General
