Technical Documentation
BLR-CM Modbus
Rev. 05
2018-09
9
The application data unit, contains two additional blocks of data:
The first field specifies the "slave number", which contains the slave address.
In order to secure the transmission, the las block includes a
cyclic redundancy check
(CRC) value.
2.2.3
Serial transmission modes
The protocol defines two different encodings for the frames' data contents. The RTU- and ASCII-coding.
Attention: The BLR-CM supports the RTU-mode only!
"Remote Terminal Unit" (RTU)
At this transmission mode, every 8-bit data word contains two 4-bit hexadecimal numbers. They are transmitted as
complete byte to reach a maximum transmission density. With every data word, the following information is
transmitted:
1 Start bit
8 Data bits, "least significant bit" first
1 Parity bit (if set)
1 Stop bit for parity even or odd / 2 if parity is none to compensate missing parity bit
2.2.4
Function codes
As it was mentioned before, the data packet contains "function codes", which specify a command from the bus master
to the bus slave. The slave executes the command (if possible) and answers with the same function code to acknowledge
the command. The valid function code range is specified from 1 to 127, but only a part of it is actually used. Please refer
to the Modbus specifications for detailed Information.
If it is impossible for a slave to execute a command, it replies with an error code (exception code). The function code of
an exception package is the function code itself and the most significant bit is set by the slave to signal the error
condition to the master. Moreover, the content of the data block specify the error in more detail.
The BLR-CM supports the function codes 03
hex
(read holding register), 04
hex
(read input register) and 06
hex
(write single
register).