Technical Documentation
BLR-CM Modbus
Rev. 05
2018-09
10
2.2.5
Exception codes
If a slave is not able to execute a command, it answers with exception codes. A full list of codes can be found in the
Modbus specification. The corresponding list is not provided at this point, because the master software handles the
most exceptions automatically.
2.2.6
Master-Slave protocol
Since the master-slave protocol is used, only the bus master can initialize a transfer. The "master" starts an exchange
by sending data and the corresponding function code (=command) to the slave. Afterwards the slave executes the
command.
Usually, the unicast-mode is used to communicate within a Modbus system. Here, data packages are send to a
single slave, including its address. The valid address range is between 1 and 247. The slave executes the
command and answers by sending a acknowledge data package back to the master.
In multicast-mode all slaves on the bus are addressed in parallel. They all receive and execute the same
command, but none of them respond. The master initiates a multicast command by using "0" as slave number.
3
BLR-CM Modbus
3.1
Setup
If the device supports Modbus, an additional entry is available in the “setup” - menu of the device. In this menu the
following settings can be done:
ADDRESS: This is the device’s slave address (slave ID). The valid range is 1-247.
BAUD RATE: Here, the baud rate can be selected. The valid range is 1200 - 115200 baud.
PARITY: The parity can be selected to be none, even or odd.
While baud rate and parity must be the same for all bus devices, the address must be unique for each device.
3.2
Address space
The data in the BLR-CM is organized and accessed by addresses. Each address accesses one data word. The size of a
data word is always 16 bit.
The BLR-CM does not differ between the addresses and the function codes. There is one big address space available
and in order to access each address’s data, any valid function code can be used. Nevertheless, the data will only make
sense when interpreted the correct way!
