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12
A Note About Modbus Communication Formats
According to the Modicon Modbus Protocol Reference Guide
, the specifications for
both Modbus ASCII and RTU communication modes are as follows:
ASCII Mode
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)
RTU Mode
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)
According to the Modicon Modbus specification, therefore, the number of bits per
byte in ASCII mode is 1 start + 7 data + 1 parity + 1 stop (if parity is used), or 1 start
+ 7 data + 2 stop (if parity is not used). The ASCII data frame is therefore fixed at 10
bits per byte. Similarly, the number of bits per byte in RTU mode is 1 start + 8 data +
1 parity + 1 stop (if parity is used), or 1 start + 8 data + 2 stop (if parity is not used),
resulting in an RTU data frame that is fixed at 11 bits per byte.
In addition to these standard specified data frame sizes, the MDB-100 interface
offers an optional configuration of providing for only 1 stop bit when no parity is
selected in the RTU communication mode. As indicated on the previous page, by
setting SW5 and SW6 both to “ON”, the RTU data frame size is modified to consist of
1 start + 8 data + 1 stop bit = 10 bits per byte. Please note that this setting is only
valid when RTU mode is selected; if SW5 and SW6 are both set to “ON” when ASCII
mode is selected, the MDB-100 will halt and will not communicate with the Modbus
network.