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2.5 Modbus Message Framing
A Modbus message is placed by the transmitting device into a frame that has a known
beginning and ending point. This allows receiving devices to begin at the start of the
message, read the address portion and determine which device it is, and to know when
the message is completed.
Partial messages can be detected and errors can be set as a result.
Modbus RTU 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 rate that is
being used on the network (shown as T1–T2–T3–T4 in the figure below).
Another factor to consider is that each device has its own response time. This response
time can be anywhere from a few milliseconds to a few hundred milliseconds. The Host
must be configured to allow adequate time for the slowest device to respond.
The first field then transmitted is the device address.
Networked devices monitor the network bus continuously, including during the 'silent'
intervals. When the first field (the address field) is received, each device decodes 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 is 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.
10 Modbus protocol overview ç RVT communication
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