7-10
Chapter 7. Modbus RTU Communication Functions
z
Time specifications for instruction and response messages
For the timing of the transmission of instruction messages from the master station
and response messages from the slave station, take the following into account.
z
z
Response monitoring time
The response time from when the master station finishes sending an instruction
message until it begins to receive a response message from the slave station is 2000
ms or less. (No. 1 in the diagram)
Therefore, set the response monitoring time to 2000 ms or longer.
Generally, if response monitoring times out, the instruction message is resent.
z
z
Transmission start time
The master station needs to wait for 10 ms or longer from when it finishes receiving
the response message until it begins sending the next instruction message (whether
to the same slave station or a different slave station). (No. 2 in the diagram)
Instruction
message
Transmission line
Response
message
Instruction
message
Response
message
(1) Master station send end time − Slave station send start time = 2000 ms or less
(2) Slave station send end time − Master station send start time = 10 ms or more
(1)
(2)
z
Specifications of RS-485 driver control timing
If the master station directly controls data transmission/reception through 3-wire
RS-485, carefully check the timing shown below.
(Enabled)
(Enabled)
(Disabled)
(Disabled)
Master station
driver control
Slave station
driver control
Transmission line
(Instruction message)
Valid data
(1) Master station send end time − Driver disabled time = 0.5 ms or less
(2) Slave station receive end time − Driver enabled time = Communication minimum response time
(3) Slave station send end time − Driver disabled time = 10 ms or less
(4) Master station receive end time − Driver enabled time = 10 ms or more
Master station send end time
Slave station send end time
(Response message)
(1)
(4)
(2)
(3)
Valid data
7 - 5
Send/Receive Timing
