Description of Each Function

Function 3 – Read Multiple Registers

Reads one or more 16 bit registers from the slave device.

 

Notes:

1. 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.

2. 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.

3. 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.

4. A request for a register that is defined in this standard as unimplemented will return the unimplemented value from the table below.

5. A request for a register that is defined as reserved in this standard will either return extended exception code 12 (Reserved register), or the 16 bit unsigned unimplemented sentinel (0xFFFF), i.e. not an exception. This option permits efficiency improvements by allowing reading across gaps in the register map. In exceptional circumstances the option to return a sentinel may be problematic for a particular page so should not be used, this will be documented in the description of the page.

6. 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.

7. The instrumentation values can return the sentinel values described in the table below to indicate a value that is over or under the measurable range, that a transducer is faulty, that the data is bad for some other reason or that the transducer is actually a digital type.

8. The sentinel value ‘high digital input’ means that the instrumentation value is high (high oil pressure, high temp, etc), similarly ‘Low digital input’ means the instrumentation value is low. They do not refer to voltage levels on the inputs.

 

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

Exception response message

Byte	Field name	Notes
0	Slave address
1	Function code +128 (131)	Top bit is set
2	Exception code	1 – Illegal function code 2 – Illegal data address 6 – Slave device busy
¾	Error check CRC

 

Unimplemented register and field values

Size of register	Value returned	Notes
1 bit flag within a register	0	No third state exists for a flag to indicate it is not valid
2 bit named digital input/output code	3	Unimplemented input/output
4 bit alarm condition codes	0xF	Unimplemented alarm
4 bit LED colour code	0xF	Unimplemented LED
16 bit unsigned, any scale	0xFFFF	The largest number
16 bit signed, any scale	0x7FFF	The largest positive number
32 bit unsigned, any scale	0xFFFFFFFF	The largest number
32 bit signed, any scale	0x7FFFFFFF	The largest positive number
ASCII strings	“                                   ”	A string of spaces (Unicode 0x0020), NULL terminators are not used
Unicode strings	“                                   ”	A string of spaces (Unicode 0x0020), NULL terminators are not used