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.

12