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366 Unidrive SP Advanced User Guide
www.controltechniques.com Issue Number: 7
7.2.9 Extended data types
Standard MODBUS registers are 16bit and the standard mapping maps
a single #X.Y parameter to a single MODBUS register. To support 32bit
data types (integer and float) the MODBUS multiple read and write
services are used to transfer a contiguous array of 16bit registers.
Slave devices typically contain a mixed set of 16bit and 32bit registers.
To permit the master to select the desired 16bit or 32bit access the top
two bits of the register address are used to indicate the selected data
type.
The selection is applied for the whole block access.
The 2bit type field selects the data type according to the table below:
If a 32bit data type is selected then the slave uses two consecutive 16bit
MODBUS registers (in 'big endian'). The master must also set the
correct 'number of 16bit registers'.
Example, read #20.1 through #20.4 as 32bit parameters using FC03
from node 8:
Table 7-11 Master request
Table 7-12 Slave response
Reads when actual parameter type is different from selected
The slave will send the least significant word of a 32 bit parameter if that
parameter is read as part of a 16 bit access.
The slave will sign extent the least significant word if a 16 bit parameter
is accessed as a 32 bit parameter. The number of 16 bit registers must
be even during a 32 bit access.
Example, If #20.1 is a 32 bit parameter with a value of 0x12345678,
#20.2 is a 16 bit parameter with a value of 0xABCD, and #20.2 is a 16 bit
parameter with a value of 0x0123.
NOTE
bit 15
TYP1
bits 0 - 13
Type select Parameter address
X x 100+Y-1
bit 14
TYP0
Type field
bits 15-14
Selected data
type
Comments
00 INT16 backward compatible
01 INT32
10 Float32
IEEE794 standard
Not supported on all slaves
11 Reserved
Byte Value Description
0 0x08 Slave destination node address
1 0x03 FC03 multiple read
20x40
Start register address #20.01
(0x4000 + 201 - 1) = 16584 = 0x40C8
30xC8
4 0x00 Number of 16bit registers to read
#20.1 through #20.4 is 4x32bit registers = 8x16bit
registers
50x08
6 CRC LSB
7
CRC
MSB
Byte Value Description
0 0x08 Slave destination node address
1 0x03 FC03 multiple read
20x10
Length of data (bytes) = 4x32bit registers =
16bytes
3-6 #20.01 data
7-10 #20.02 data
11-14 #20.03 data
15-18 #20.04 data
19 CRC LSB
20 CRC MSB
Read
Start
register
address
Number
of 16bit
registers
Response Comments
#20.1 200 1 0x5678
Standard 16 bit access
to a 32bit register will
return low 16bit word of
truncated data
#20.1 16584 2 0x12345678 Full 32bit access
#20.1 16584 1 Exception 2
Number of words must
be even for 32bit access
#20.2 201 1 0xABCD
Standard 16 bit access
to a 32bit register will
return low 16bit word of
data
#20.2 16585 2 0xFFFFABCD
32bit access to a 16bit
register will return 32bit
sign extended data
#20.3 16586 2 0x00000123
32bit access to a 16bit
register will return 32bit
sign extended data
#20.1-
#20.2
200 2
0x5678,
0xABCD
Standard 16 bit access
to a 32bit register will
return low 16bit word of
truncated data
#20.1-
#20.2
16584 4
0x12345678,
0xFFFFABCD
Full 32bit access
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