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8–bit byte in a message contains two 4–bit hexadecimal characters. The main advantage of this mode is that its
greater character density allows better data throughput than ASCII for the same baud rate. Each message must be
transmitted in a continuous stream.
The format for each byte in RTU mode is:
Error Check Field:
Cyclical Redundancy Check (CRC)
Coding System:
8–bit binary, hexadecimal 0–9, A–F
Two hexadecimal characters contained in each
8–bit field of the message
Bits per Byte:
1 start bit
8 data bits, least significant bit sent first
1 bit for even/odd parity; no bit for no parity
1 stop bit if parity is used; 2 bits if no parity
Error Check Field:
Cyclical Redundancy Check (CRC)
6.1.2RTU Framing
In RTU mode, messages start with a silent interval of at least 3.5 character times. This is most easily implemented
as a multiple of character times at the baud ratethat is being used on the network (shown as T1–T2–T3–T4 in the
figure below).The first field then transmitted is the device address.
The allowable characters transmitted for all fields are hexadecimal 0–9, A–F.Networked devices monitor the
network bus continuously, including during the ‗silent‘ intervals. When the first field (the address field) is received,
each devicede codes it to find out if it is the addressed device.
Following the last transmitted character, a similar interval of at least 3.5 character times marks the end of the
message. A new message can begin after this interval .The entire message frame must be transmitted as a
continuous stream. If a silent interval of more than 1.5 character times occurs before completion of the frame, the
receiving device flushes the incomplete message and assumes that the next byte will be the address field of a new
message.
Similarly, if a new message begins earlier than 3.5 character times following a previous message, the receiving
device will consider it a continuation of the previous message. This will set an error, as the value in the final CRC
field will not be valid for the combined messages. A typical message frame is shown below.
START
ADDRESS
FUNCTION
DATA
CRC
CHECK
END
T1–T2–T3–T4
8 BITS
8 BITS
n x 8 BITS
16 BITS
T1–T2–T3–T4
6.1.3CRC Checking
In RTU mode, messages include an error–checking field that is based on a Cyclical Redundancy Check (CRC)
method. The CRC field checks the contents of the entire message. It is applied regardless of any parity check
method used for the individual characters of the message.
The CRC field is two bytes, containing a 16–bit binary value. The CRC value is calculated by the transmitting
device, which appends the CRC to the message. The receiving device recalculates a CRC during receipt of the
message, and compares the calculated value to the actual value it received in the CRC field. If the two values are
not equal, an error results.
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