Chapter 8 Modbus Communication
8-2
3) Address area
(1) PLC option card supports 0 to 31.
4) Function code area
(1) PLC option card supports only 01, 02, 03, 04, 05, 06, 15, and 16 among Modicon products’
function code.
(2) If the response format is (ACK), it uses the same function code.
(3) If the response format is Confimr-(NCK), it returns as it sets the 8
th
bit of function code as 1.
Ex) If function code is 03,
- Only function code is written here because only function codes are different.
[Request]
0000
0011
(H03)
[]
0000
0011
(H03)
[Confirm-]
1
000 0011 (H83)
5) Data area
(1) It sends data, using ASCII data (ASCII mode) or hex (RTU mode).
(2) Data is changed according to each function code.
(3) Response frame uses data area as response data or error code.
6) LRC Check/CRC Check area
(1) LRC (Longitudinal Redundancy Check): It works in ASCII mode. It takes 2 complement from sum
of frame except header or tail to change into ASCII code,
(2) CRC (Cyclical Redundancy Check): It works in RTU mode. It uses 2-byte CRC check rules.
It returns as it sets the 8th bit of
function code of request frame.
1) All numerical data can use hexadecimal, decimal, and binary type. If we convert decimal 7
and 10 into each type:
Hexadecimal: H07, H0A or 16#07, 16#0A
Decimal: 7, 10
Binary: 2#0111, 2#1010
Remark
