In Modbus communication, CRC (Cyclic Redundancy Check) is used for error checking.
The CRC code is 16-bit data generated for any data block with a data length in 8-bit unit.
For CRC code generation, the following generator polynomial is used: CRC-16 (X
16
+ X
15
+ X
2
+ + 1).
8-4-2 Required Communications Time
The time that the inverter takes to send a response after receiving a query is the sum of the silent inter-
val (3.5 characters) and the Communication Wait Time (C078).
After receiving a response from an inverter, be sure to include an interval equivalent to the silent inter-
val (3.5 characters) or more before sending the next query to the inverter.
(Example) 1 character:10 bits (1 start bit + 8 data bits + 1 stop bit)
Time required per character: 1/9,600 (bps) 10 (bit) 1,000 = 1.04 ms
Time required for 3.5 characters: 1.04 ms 3.5 = 3.64 ms
However, according to the Modbus specifications, this time is fixed to 1.75 ms for communi-
cations speeds exceeding 19.2 kbps.
8-4-3 Normal Response
If the function code included in a query is for the Loop-back Test (08 hex), Write to Coil (05 hex, 0F
hex), or Write to Holding Register(s) (06 hex,10 hex) function, the inverter returns a response with the
same content as that of the query.
If the function code included in a query is for the Read Coil Status (01 hex) or Read from Holding Reg-
ister (03 hex) function, the inverter returns a response that includes data with the same slave address
and function code.
Refer to the later section, 8-5 Explanation of Each Function Code on page 8-11.
8-4-4 Abnormal Response
⚫
Response
Slave address
Function code
Exception code
Error check (CRC-16)
If an error (except for a communications error) is found in the query content, the inverter will return
an exception response without performing any operation.
For the cause of an error, check the function code for the response. The function code for an excep-
tion response is the sum of the function code for the query and 80 hex (where the MSB is 1). For the
cause of an error, check the exception code.
To Next Page
Loading...
