stephane
Read single register of more than 2 bytes
Hi,
I am trying to read a modbus device but it doesn't follow modbus standard.
Request: 01 03 07 EE 00 02 A5 4A
| Part of Data Package | Description | Value |
|---|---|---|
| 01 | Slave address | 0x01 (1) |
| 03 | Function code | 0x03 (3) - Read Holding Registers |
| 07 EE | Starting address | Physical: 0x07EE (2030)Logical: 0x07EF (2031) |
| 00 02 | Quantity | 0x0002 (2) |
| A5 4A | CRC | 0xA54A (42314) |
Response: 01 03 0c 43 3f 00 00 20 4c 49 54 48 49 55 4d d0 05
| Part of Data Package | Description | Value |
|---|---|---|
| 01 | Slave address | 0x01 (1) |
| 03 | Function code | 0x03 (3) - Read Holding Registers |
| 0C | Byte count | 0x0C (12) |
| 43 3F 00 00 20 4C 49 54 48 49 55 4D | Register value | 0x433F (17215), 0x0000 (0), 0x204C (8268), 0x4954 (18772), 0x4849 (18505), 0x554D (21837) |
| D0 05 | CRC | 0xD005 (53253) |
Here I am reading only 2 registers but I am getting 12 byte count.
I know that register 2030 has 4 bytes of data and 2031 has 8 bytes of data.
How do I handle this situation? I know length of each register that I am going to read but libmodbus computes response length based on request ( request for 2 bytes -> response will be of 9 bytes) but in my case it is 17 bytes in total.
I have a similar issue https://github.com/stephane/libmodbus/issues/648 - just a little easier as my device (Dossena MV3DL 3-phase electricity meter) has all readable registers of the same non-standard length (3 bytes instead of 2). Since such strange devices seem to be more widespread (I thought mine was the only one, now there are two!) it might be a good idea to support them in some way. I hope to test modbus_send_raw_request soon, to see if it can be made to work in my setup (with MikroTik KNOT used as Modbus TCP to RTU gateway, and a few other more standard-compliant devices on the shared RS485 bus).
