3 Modbus protocol description
14
3.3 Chronological sequence of communication
Character transmission time
The character transmission time (time taken to transmit one character with 8 data bits) depends on the
baud rate and the number of bits of the character (see table for data format):
Time diagram of a data request
A data request runs according to the following time diagram:
The data request and response consist of several characters (each with a start bit, 8 data bits, a parity
bit if necessary, and a stop bit) which are transmitted coherently.
NOTE!
The waiting periods t
1
and t
2
also contain the end identifier (3.5 × character transmission time), which
follows after each data request or response.
NOTE!
In the configuration of the serial interface of the device, a minimum response time can be set
(0 to 500 ms). This set time is the minimum waiting period before a response is transmitted. If a smaller
value is set, then the response time may be longer than the preset value (internal processing takes lon-
ger); the controller responds as soon as internal processing is completed. The preset time of 0 ms
Character transmission time [ms] = 1000 × number of bits/baud rate
Baud rate[Bd] Number of bits Character transmission time[ms]
115200 11 0.095
10 0.087
38400 11 0.286
10 0.260
19200 11 0.573
10 0.521
9600 11 1.146
10 1.042
t
1
The slave has to observe this waiting period before sending the response.
min.: 5 ms
typically: 5to35ms
max.: 35 ms or the minimum response time set in the configuration
t
2
The master has to observe this waiting period before starting a new data request.
35 ms
Master
Slave
Data request
Response
Data request
t
t
1
t
2
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