Manual LogoTronic Hub OTC
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7.1.2.1 Uni-/Broadcast transmission principles
In
unicast mode
, the client addresses an individual device which, after receiving the message, processes
it and generates a response. The device address can vary from 1 to 247. A message always consists of a
request and a response. If no response is received within a specified time, timeout is detected.
In
broadcast mode
, the client sends a written command (request) to all participants on the bus, but they
do not generate a response. Address 0 is reserved for broadcast messages.
7.1.2.2 Function codes
The function code in a Modbus protocol message defines the action to be performed by the server. The
function codes are stored in an assignment table.
Assignment table:
The assignment table contains the function code, the data type, the register and information about the
register. With the information, a client can read out a register of a server via an RTU telegram.
7.1.2.3 Data/parameter scaling
Due to the limitations for integer data in the Modbus protocol, it is necessary to convert the parameters
before transmitting them. This is achieved by scaling, where a parameter containing a position after a
decimal separator is multiplied by a factor so that a fractional value is no longer required. The scaling
factor to be applied can be taken from a corresponding table.
7.1.2.4 CRC
The CRC is a 16-bit value that is appended to the message. It is used to determine whether the
transmission of a message was detected without errors. Together with the parity check, all possible
transmission errors should be detected. If a parity error is detected when a message is received, no reply
message is generated by the device.
7.1.2.5 Start/end
The end identification of a message is specified as a rest situation on the Modbus with a length of 3.5
characters. After this time has elapsed, the earliest a server will begin its response or the earliest a client
will send out a new message.
The evaluation of a message can already begin when it is detected that the rest condition on the Modbus
has occurred for more than 1.5 characters. However, a response is started after 3.5 characters at the
earliest.
7.1.3 Transmission parameters
Setting options for parity/
stop bit*
The parity bit can be used to check whether
a single error has occurred within a byte
during transmission.
No parity and 1 stop bit,
Same parity and 1 stop bit,
Unequal parity and 1 stop bit
Setting options for baud
rate*
The baud rate is a measure of the
transmission speed.
e.g. 2400, 4800, 9600, 19200, 38400,
57600, 115200 bits/s
* Factory setting options
Note:
The transmission parameters must be set the same for all devices participating in a bus.
