6
parameters (baud rate, information unit) during the configuration of
any device.
In the
MODBUS
system, transmitted messages are placed into
frames that are no related to serial transmission. These frames have
a defined beginning and end. This enables for the receiving device
to reject incomplete frames and the signalling of related errors with
them.
Taking into consideration the possibility to operate in one of these
two different transmission modes (ASCII or RTU), two frames have
been defined.
2.1. ASCII framing
In the ASCII mode each byte of information is transmitted as two
ASCII characters.
The basic feature of this mode is that it allows to long intervals
between characters within the message (to1sec) without
causing errors.
A typical message frame is shown below:
Start
Address
Function
Data
LRC
End
beginning
check
index
index
1 char :
2 chars
2 chars
n chars
2 chars
2 chars CR LF
In ASCII mode, messages start with a colon character (: -ASCII
3Ah) and end with a carriage return-line feed (CR and LF
characters). The frame information part is protected by the LRC code
(Longitudinal Redundancy Check).
2.2. RTU Framing
In RTU mode, messages start and end with an interval lasting mini-
mum 3.5 x (lasting time of a single character), in which
a silence reigns on the link.
The simplest implementation of the mentioned time interval
character is a multiple measure of the character duration time at the
Summary of Contents for RE15
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