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7
Read exception status
8
Diagnostics
11
Fetch comm. event ctr.
12
Fetch comm. event log
15
Force multiple coils
Preset multiple
16
registers
17
Report slave ID
22
Mask write 4X registers
23
Read/write 4X registers
Remark: please note that for security reasons broadcast is not supported by the RVT.
3.3 Master's queries and Slave's responses
When a master device sends a query to a slave device it expects a normal response. One
of four possible events can occur from the master's query:
-
If the slave device receives the query without a communication error, and can
handle the query normally, it returns a normal response.
If the slave does not receive the query due to a communication error, no response
-
is returned. The master program will eventually process a timeout condition for
the query.
-
If the slave receives the query, but detects a communication error (parity or CRC),
no response is returned. The master program will eventually process a timeout
condition for the query.
-
If the slave receives the query without a communication error, but cannot handle
it (for example, if the request is to read a non–existent coil or register), the slave
will return an exception response informing the master of the nature of the error.
3.4 Reads and writes to Modbus addresses (functions 1, 2, 3, 4, 5, 6, 15,
16, 22, 23)
The format of a read function (read coil status (01), read input status (02), read input
registers (04), read holding registers (03)) is as follows:
QUERY
Slave address
Function
Starting data address
Quantity of points
Error check field CRC
The format of a force single coil (05) or a preset single register (06) function is as follows:
Device specific
Checks the communication system between the master
and the slave
Returns the amount of successful read/write operations on
data points
Returns log registers of communication events
0XXXX sets the status of multiple discrete outputs
4XXXX sets the value of multiple holding registers
Device specific
4XXXX and/or write of a holding register
4XXXX reads a set of holding registers and writes a set of
holding registers in one query
RESPONSE
1 byte
Slave address
Function
1 byte
2 bytes
Byte count
2 bytes
Data values
2 bytes
Error check field CRC
1 byte (echo of master's query)
1 byte (echo of master's query)
1 byte
N bytes
2 bytes
RVT communication ç Modbus function codes 15
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