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 zero are broadcast messages. No response is permitted for broadcast messages. A typical message frame is shown below.

Typical Modbus RTU Message Structure

Start

T1-T2-T3-T4

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 address 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 message. 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.

7-14

Data byte

8-bit binary, hexadecimal 0-9, A-F. Two hexadecimal characters contained in each 8-bit field of the

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