20
The MODBUS requests sent to the MODBUS master or client are 8 bytes long as shown in Table 8.2 below.
Unit I/D Function
Starting Address No. of Registers
Requested
CRC
High Low High Low High Low
02 03 nn nn nn nn cc cc
Table 8.2: MODBUS RTU Standard 8 Byte Holding Register Read Function (03)
The data format of the MODBUS registers is nnnnH for four hexadecimal digits, ddddB for four binary digits and
ddddddddB for eight binary digits. Numbers above are, for example, a request for 2 registers only, starting from ad-
dress 02. CRC checksum is a 16 bit CRC as described in MODBUS information.
If successful, the Sigma 3 will return the message shown in Table 8.3 below.
Unit I/D Function No. of Bytes Data 1 Data N CRC
High Low High Low High Low High Low
02 03 nn nn nn nn nn nn cc cc
Table 8.3: Returned Information Structure for Holding Register Request
Register contents are shown in Tables 8.4 to 8.6 as below.
NOTE: Register number is shown in decimal but must be sent in hexadecimal form in the request.
The MODBUS register number is translated in the request such that register number 40001 is sent as hexadecimal
address 0000H, register number 40002 is sent as hexadecimal address 0001H, register number 40003 is sent as
hexadecimal address 0002H and so on.
The MODBUS registers can be read using the starting address in a block and the number of registers required up to
the number of registers available in a given block.
There are times when the Sigma 3 relay processor will not be able to respond to a request since it is busy with other
tasks and no response will be returned. For this reason it is recommended to request the maximum number of regis-
ters used by the system in a single request.
Register # Contents Format Register # Contents Format
400012 ASCII nnnnH
characters
40021 nnnnH
40002 nnnnH 40022 nnnnH
40003 nnnnH 40023 nnnnH
40004 nnnnH 40024 nnnnH
40005 nnnnH
40006 nnnnH
40007 nnnnH
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