embedded systems - modbus - message format

Request

Byte	Meaning
1	Slave Address
2	Function Code
3	Address of first register (high)
4	Address of first register (low)
5	Total number of registers requested (high)
6	Total number of registers requested (low)
7	CRC (low)
7	CRC (high)

Response

Byte	Meaning
1	Slave Address
2	Function Code
3	Number of bytes following (not including CRC)
4	Byte 1
5	Byte 2
3 + n + 1	CRC (high)
3 + n + 2	CRC (low)

Slave address 0 is reserved for broadcasts
minimum time of 3.5 characters between messages
maximum time of 1.5 characters between bytes of a message
for baud rates of 19200 or higher, it is recommended that 750 us be used for the inter-character time out, and 1.75 ms be used for the inter-frame delay.
parity checking of individual bytes should be applied, but is not required.
frame checking (CRC) is required.
9600 bps and 19.2 Kbps are required and 19.2 is the required default Other baud rates may optionally be implemented : 1200, 2400, 4800, ... 38400 bps, 56 Kbps, 115 Kbps, ...

filename:	embedded systems - modbus - message format - RTU
filename:	embedded%20systems%20%2D%20modbus%20%2D%20message%20format%20%2D%20RTU
last edit:	May 16 2026 03:57:17 (177 days ago)
ct	= 1794244944.000000 = November 09 2026 17:22:24
ft	= 1778903837.000000 = May 16 2026 03:57:17
dt	= 15341107.000000