Lika EM58 MB, Руководство пользователя

Страница: 54 / 92

ES58 • EM58 MODBUS®
The following drawing provides a description of the RTU transmission mode
state diagram. Both “Master” and “Slave” points of view are expressed in the
same drawing.
•
Transition from
Initial State
to
Idle
state needs an interval of at least
3,5 character times (time-out expiration = t
3.5
).
•
Idle
state is the normal state when neither emission nor reception is
active. In RTU mode, the communication link is declared in
Idle
state
when there is no transmission activity after a time interval equal to at
least 3.5 characters (t
3,5
).
•
A request can only be sent in
Idle
state. After sending a request, the
Master leaves the
Idle
state and cannot send a second request at the
same time.
•
When the link is in
Idle
state, each transmitted character detected on
the link is identified as the start of the frame. The link goes to Active
state. Then the end of the frame is identified when no more character is
transmitted on the link after the time interval of at least t
3.5
.
•
After detection of the end of frame, the CRC calculation and checking is
completed. Afterwards the address field is analysed to determine if the
frame is addressed to the device. If not, the frame is discarded. In order
to reduce the reception processing time the address field can be
analysed as soon as it is received without waiting the end of frame. In
this case the CRC will be calculated and checked only if the frame is
actually addressed to the Slave.
MAN ESx58x_EMx58x MB E 1.3 MODBUS® interface 54 of 92