Destination Register Length Justification Options Explained; Source Register Address And Source Register Type Explained; Figure 5-30. Bit Justification Notation - ABB TPU2000 Technical Manual

TPU2000/2000R Modbus/Modbus Plus/ Modbus TCP/IP Automation Guide
EQUATION 3:
Unipolar
EQUATION 4:
Negative Unipolar
NOTE: for the above equations "N" = the amount of bits selected for scaling (i.e. 16, 12, 8, 4, 2)

Destination Register Length Justification Options Explained

Modbus has one definition, but its definition has been interpreted differently by various protocol implementers.
This presents a special challenge to the automation engineer. For example, some host device implementations
count the first address as address zero whereas other implementers count the first address as address 1 and
internally shift the address to offset it by 1 to account for the baseline format.
Another interpretation has been that of most significant bit and least significant bit justification. Two selections are
possible for the query DESTINATION BIT JUSTIFICATION. Selections as per Table 5-10 and Figure 5-30 are
MSB and LSB. Figure 5-30 illustrates the bit definition and bit padding for the DESTINATION BIT
JUSTIFICATION field selection and DESTINATION REGISTER SIZE query.
16
BIT
12
BIT
8
BIT
4
BIT
2
BIT
An investigation of Figure 5-30 illustrates that register justification shifts the data to the left of the right of the
register. If the reported data for example is to be reported as 1 after scaling, the internal Modbus presentation to
the host shall be 0001 hex in 12 bit MSB justification format and 0010 in the 12 bit LSB justification format. In both
cases Bit 12 is set to represent the number 1, however the reported data to the host is shifted accordingly
depending upon the hosts interpretation of the Modbus data.

Source Register Address and Source Register Type Explained

Table 5-11 lists the source addresses of each of the TPU quantities which may be mapped to the User Definable
Registers. The addresses are actually the MODBUS addresses from the TPU Modbus Address map. One may
consult the TPU 2000/2000R Automation Guide for the exact addresses. For example, if one wished to map the
Voltage a to neutral value from its Modbus address at Register 40265, the entry within the SOURCE REGISTER
query would be 265. The leading 40 designation (or 4X as some refer to it as) is not required. However, for ease
of configuration, the pointer addresses are given to the user in Table 5-11.
N
(0 to 2
N
(0 to 2
MSB Justification
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16
0 0 0 0 1 2 3 4 5 6 7 8 9 10 11 12
0 0 0 0 0 0 0 0 1 2 3 4 5 6 7 8
0 0 0 0 0 0 0 0 0 0 0 0 1 2 3 4
0 0 0 0 0 0 0 0 0 0 0 0 0 0 1 2
NOTE : Bit designated as a 1 is the words most significant bit

Figure 5-30. Bit Justification Notation

N
-1) where 0 = 0 and 2 -1 = +FS
N
-1) where 0 = 0 and 2 -1 = -FS
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16
1 2 3 4 5 6 7 8 9 10 11 12 0 0 0 0
1 2 3 4 5 6 7 8 0 0 0 0 0 0 0 0
1 2 3 4 0 0 0 0 0 0 0 0 0 0 0 0
1 2 0 0 0 0 0 0 0 0 0 0 0 0 0 0
whereas the highest bit number is the least significant bit.
0 indicates a padded bit.
LSB Justification
80