Design Guide | VLT® AutomationDrive FC 360
9.8 Modbus RTU Message Framing Structure
9.8.1 Introduction
The controllers are set up to communicate on the Modbus network using RTU (remote terminal unit) mode, with each byte in a telegram
containing 2 4-bit hexadecimal characters. The format for each byte is shown in the following table.
Table 40: Format for Each Byte
Start bit Data byte Stop/
parity
Stop
Table 41: Byte Details
Coding system 8-bit binary, hexadecimal 0–9, A–F.
2 hexadecimal characters contained in each 8-bit field of the
telegram.
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 Cyclic redundancy check (CRC).
9.8.2 Modbus RTU Telegram Structure
The transmitting device places a Modbus RTU telegram into a frame with a known beginning and ending point. This allows receiving
devices to begin at the start of the telegram, read the address portion, determine which device is addressed (or all devices, if the
telegram is broadcast), and to recognize when the telegram is completed. Partial telegrams are detected and errors set as a result.
Characters for transmission must be in hexadecimal 00–FF format in each field. The drive continuously monitors the network bus, also
during silent intervals. When the first field (the address field) is received, each drive or device decodes it to determine which device is
being addressed. Modbus RTU telegrams addressed to 0 are broadcast telegrams. No response is allowed for broadcast telegrams. A
typical telegram frame is shown in the following table.
Table 42: Typical Modbus RTU Telegram Structure
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
9.8.3 Start/Stop Field
Telegrams start with a silent period of at least 3.5 character intervals. The silent period 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 telegram. A new telegram can begin after
this period.
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