3-2
MODBUS RTV PROTOCAL DESCRIPTION
3.2.1 RTU Mode
The Phoenix AC drive are setup to communicate on a
Modbus network using RTU (Remote Terminal Unit)
mode, each eight-bit byte in a message contains two
four-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.
Coding System
Eight-bit binary, hexadecimal 0…9, A…F
Two hexadecimal characters contained in each eight-
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)
3.2.2 ASCII Mode
The Phoenix AC drive does not incorporate ASCII
(American Standard Code for Information
Interchange) mode. But, if a device is using ASCII
mode, each eight-bit byte in a message is sent as two
ASCII characters. The main advantage of this mode is
that it allows time intervals of up to one second to
occur between characters without causing an error.
Coding System
Hexadecimal, ASCII characters 0… 9, A…F
One hexadecimal character contained in each ASCII
character of the message.
Bits per Byte
1 start bit
7 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
Longitudinal Redundancy Check (LRC)
3.3 Modbus Message Framing
In either of the two serial transmission modes (ASCII
or RTU), 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 is
addressed (or all devices, if the message is broadcast),
and to know when the message is completed. Partial
messages can be detected and errors can be set as a
result.
3.3.1 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). 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 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 that 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
C R C
CHECK
E N D
T1-T2-T3-T4
8 BITS
8 BITS
NX 8BITS
16 BITS
T1-T2-T3-T4
Содержание DS Series
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