Modbus Protocol Overview - Rtu Transmission Mode; The Rtu Transmission Frame; The Rtu Frame Fields; Message Delineation - Siemens SIMATIC TI505 User Manual

2.2 Modbus Protocol Overview — RTU Transmission Mode
The RTU
Transmission Frame
The RTU Frame
Fields
Message
Delineation
2-4 Modbus Commands
The RTU mode determines the start and finish of a message based on a time
period of silence (no transmission) equivalent to the time it would take to
transmit 3.5 characters at the chosen baud rate. The RTU transmission
frame is shown in Figure 2-2.
Address Function
8 bits 8 bits
T1 T2 T3 . . . represents a time of 3.5 characters at given baud rate.

Figure 2-2 RTU Transmission Frame

The RTU mode of transmission is more efficient than the ASCII mode as it
only uses 8 bits for each field (except for the checksum) and the end of the
message is detected by timing. The body of the message (address, function
code, and data field) is the same with both modes; therefore, only the
message delineation and the checksum are discussed in the following
paragraphs.
The start and finish of a message is determined by timing in RTU mode.
Any time period longer than 3.5 character lengths marks the end of a
transmission. The next character after that would then mark the start of
the next message. This time period is dependent on the baud rate. Table 2-2
shows the effect of baud rate on the time delay period.
Baud Rate
19200
9600
7200
4800
3600
2400
1800
1200
600
300
200
150
110
75
50
Data
n x 8 bits

Table 2-2 RTU Mode Timing

3.5 Character Time Delay (ms)*
*Assuming longest delay with 2 stop bits & parity enabled
SIMATIC TI505/TI500 MODNIM User Manual
Checksum
16 bits
2.2
4.4
5.9
8.8
11.7
17.5
23.3
35.0
70.0
140.0
210.0
280.0
381.8
560.0
840.0
No data
T1 T2 T3