E 15
RS485 Modbus
1) Internal structure:
1.1) Presentation:
The device is divided in two cells. Each cell has a specific function while keeping a continuous exchange of pieces of
information with the second cell. The first cell is in charge of the measure, analysis and conversion function.
The second cell is in charge of the communication function. The information exchange is continuous and automatic.
1.2) Measure function:
The measure cell runs the acquisition of the different signals and calculates all the values with regards to the configura-
tion of the device.
It also runs all the output functions (analogical, alarm, meter, RS 232). All measured or calculated parameters are stored
in the system memory and are constantly refreshed.
1.3) Communication function:
The communication cell runs the RS485 communication interface in the MODBUS/JBUS protocol. It analyzes the re-
quests of the main station and answers if the device is addessed. It draws all these data from the system memory that
can be continuously accessed.
1.4) System memory:
Each cell can continuously access the system memory. The latter has a dual access, which allows a reading/writing of
the data whitout any possible internal conflicts.
2) Communication:
The type of protocol used is: MODBUS/JBUS in RTU mode. The communication has neither header nor delimitator of
frame. The detection of the start of frame is made by a silence whose time is at least equal to the transmission of 3.5
bytes. It implies that a frame received can be processed only after a time equal to the silence given before. The time of
this silence is directly linked to the speed of transmission of the system:
Ex:
Speed 9600 bauds - no parity (10 bits/byte)
Silence = (3.5 x 10) / 9600 = 3.64 ms
The device starts to process the frame 3.64 ms after receiving the last byte.
The time separating two bytes from a same frame must be inferior to a silence. If the user does not comply with this con-
dition, the second byte will be considered as the first one of a new frame.
The interval of time separating the end of reception of the last byte of the question frame and the end of emission of the
first byte of the answer frame (detection of frame of the main station) constitutes the answer time of the device.
This answer time Trep includes:
- the silence (time of 3.5 bytes) Ts,
- the processing of the frame Tt,
- the emission of the first byte Te1.
