10. MODBUS COMMUNICATION DETAIL
10.1 Overview
When Vibration Monitor are setup to communicate on a Modbus network using RTU (Remote
Terminal Unit) mode, each 8–bit byte in a message contains two 4–bit Hexadecimal characters. The
main advantage of this mode is that it‟s greater Character density allows better data throughput than
ASCII for the same baud rate.
Each message must be transmitted in a continuous stream. The format for each byte in RTU mode is:
Coding System: 8–bit binary, hexadecimal 0–9, A–F
Two hexadecimal characters contained in each
8–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)
o In RTU mode, messages start with a silent interval of at least 3.5 character times.
o 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.
Table 35 Modbus Communication frame format
Figure 21 The Query-Response Cycle
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