Rappel sur la requête 23 (0x17) Read/Write Multiple registers
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- This function code performs a combination of one read operation and one write operation in a single MODBUS transaction. The write operation is performed before the read.
- Holding registers are addressed starting at zero. Therefore holding registers 1-16 are addressed in the PDU as 0-15.
- The request specifies the starting address and number of holding registers to be read as well as the starting address, number of holding registers, and the data to be written. The byte count specifies the number of bytes to follow in the write data field.
- The normal response contains the data from the group of registers that were read. The byte count field specifies the quantity of bytes to follow in the read data field.
| Request | ||
| Function code | 1 Byte | 0x17 |
| Read Starting Address | 2 Bytes | 0x0000 to 0xFFFF |
| Quantity to Read | 2 Bytes | 0x0001 to 0x007D |
| Write Starting Address | 2 Bytes | 0x0000 to 0xFFFF |
| Quantity to Write | 2 Bytes | 0x0001 to 0X0079 |
| Write Byte Count | 1 Byte | 2 x N* |
| Write Registers Value | N*x | 2 Bytes |
| *N = Quantity to Write | ||
| Response | ||
| Function code | 1 Byte | 0x17 |
| Byte Count | 1 Byte | 2 x N'* |
| Read Registers value | N'* x 2 Bytes | |
| *N' = Quantity to Read |