8. Modbus protocol
8. 3. Modbus functions available
The following table indicates which Modbus functions are managed by the Altivar 71 and specifies their limits. The "read" and "write" functions are defined from the point of view of the master.
Code | Modbus name | Description | Broadcast | Size of data |
3 = 16#03 | Read Holding Registers | Read N output words | NO | 63 words, max. |
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4 = 16#04 | Read Input Registers | Read N input words | NO | 63 words, max. |
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6 = 16#06 | Write Single Register | Write one output word | YES |
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8 = 16#08 | Diagnostics | Diagnostics | NO |
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11 = 16#0B | Get Comm Event Counter | Read counter | NO |
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16 = 16#10 | Write Multiple Registers | Write N output words | YES | 61 words, max. |
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23 = 16#17 | Read/Write Multiple Registers | Read/write N words | NO | 20/20 words max. |
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43 = 16#2B | Read Device Identification | Identification | NO |
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8. 4. Read Holding/Input Registers (3/4)
Functions 3 and 4 access all the drive registers that make no distinction between the "Holding" or "Input" types.
Request: |
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Slave no. | 03/04 | No. of first word |
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| Hi |
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1 byte | 1 byte | 2 bytes |
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Response: |
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Slave no. | 03/04 | Number of |
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| Hi |
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1 byte | 1 byte | 1 byte |
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Example: Use function 3 to read 4 logic address words 3102 to 3105 (16#0C1E to 16#0C21) on slave 2, where:
•SFr = Switching frequency = 4 kHz (logic address 3102 = 16#0028)
•tFr = Maximum output frequency = 60 Hz (logic address 3103 = 16#0258)
•HSP = High speed = 50 Hz (logic address 3104 = 16#01F4)
•LSP = Low speed = 0 Hz (logic address 3105 = 16#0000)
Request:
02
03
0C1E
0004
276C
Response:
02 | 03 | 08 | 0028 | 0258 | 01F4 | 0000 | 52B0 |
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| Value of: |
| 3102 | 3103 | 3104 | 3105 |
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Parameter code: | SFr | tFr | HSP | LSP |
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