Danfoss VLT Series Design Manual page 148

7. RS-485 Installation and Set-up
Coding System
Bits Per Byte
Error Check Field
7.8.2. Modbus RTU Message Structure
The transmitting device places a Modbus RTU message into a frame with a known beginning and
ending point. This allows receiving devices to begin at the start of the message, read the address
portion, determine which device is addressed (or all devices, if the message is broadcast), and to
recognize when the message is completed. Partial messages are detected, and errors are set as
a result. Characters for transmission must be in hexadecimal 00 to FF format in each field. The
adjustable frequency drive continuously monitors the network bus, also during 'silent' intervals.
When the first field (the address field) is received, each adjustable frequency drive or device
decodes it to determine which device is being addressed. Modbus RTU messages addressed to
7
zero are broadcast messages. No response is permitted for broadcast messages. A typical message
frame is shown below.
Start
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7.8.3. Start / Stop Field
Messages start with a silent period of at least 3.5 character intervals. This is implemented as a
multiple of character intervals at the selected network baud rate (shown as Start T1-T2-T3-T4).
The first field to be transmitted is the device address. Following the last transmitted character, a
similar period of at least 3.5 character intervals marks the end of the message. A new message
can begin after this period. The entire message frame must be transmitted as a continuous stream.
If a silent period of more than 1.5 character intervals occurs before completion of the frame, the
receiving device flushes the incomplete message and assumes that the next byte will be the ad-
dress field of a new message. Similarly, if a new message begins prior to 3.5 character intervals
after a previous message, the receiving device will consider it a continuation of the previous mes-
sage. This will cause a time-out (no response from the slave), since the value in the final CRC field
will not be valid for the combined messages.
7.8.4. Address Field
The address field of a message frame contains 8 bits. Valid slave device addresses are in the range
of 0–247 decimal. The individual slave devices are assigned addresses in the range of 1–247. (0
is reserved for broadcast mode, which all slaves recognize.) A master addresses a slave by placing
the slave address in the address field of the message. When the slave sends its response, it places
its own address in this address field to let the master know which slave is responding.
148
8-bit binary, hexadecimal 0-9, A-F. Two hexadecimal characters
contained in each 8-bit field of the message
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
Cyclical Redundancy Check (CRC)
Address Function
8 bits
Typical Modbus RTU Message Structure
MG.11.B4.22 - VLT ®
VLT
Data CRC check
N x 8 bits 16 bits
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® HVAC Drive Design Guide
End
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