Information Technology Reference
In-Depth Information
'0110 1111' '0001 0011' '1100 1100' '1011 0000'
which allows for a faster transmission of values, and they can thus be decoded quicker
than the ASCII mode. RTU will obviously be twice as fast as the ASCII method. It also
allows continuous bit streams to be transmitted. In this mode, a start bit is transmitted,
followed by an 8-bit binary value, an optional parity bit and then two stop bits.
In summary, the modes are:
ASC I
RTU
Coding
Hexadecimal characters
8-bit binary
t t its
1
1
No of bits/character
7
8
Parity
Optional
Optional
Stop bits
1 or 2
1 or 2
Error checking
LRC
CRC
18.1.3 Modbus message frame
The Modbus message has different formats, depending on the transmission mode. These are:
•
ASCII framing - a colon ASCII character (:, or 3Ah) starts the message and the carriage
return-line feed sequence ends the message (CRLF, or 0Dh and 0Ah). The characters
within the message will then be '0' to '9' or 'A' to 'F'. On a network, devices continu-
ally listen for the colon character. The field after this is the address field. The maximum
interval between characters is one second. Figure 18.2 shows the standard format.
•
RTU framing - messages start with a silent interval of at least 3.5 character times. After
this, the device address is transmitted. All devices on the network listen to the bus, and
wait for a silent period, which must be at least 3.5 characters since the last message. It
then transmits the message as a continuous stream. The first eight bits are the target ad-
dress. Errors occur if there is a silent period of more than 1.5 character times or if a de-
vice transmits its message before 3.5 character delays after the previous message. Figure
18.2 shows the standard format.
Start
(:)
End
(CR-LF)
Address
Function
DATA
LRC Check
ASCII
frame
Number of
characters:
1
2
2
n
2
2
RTU
frame
Start
Address
Function
DATA
LRC Check
End
Time:
3.5 characters
8 bits
8 bits
n x 8 bits
16 bits
3.5 characters
Figure 18.2
ASCII and RTU message frame
