E.3
Date Code 20170927 Instruction Manual SEL-751 Relay
Modbus RTU Communications
Communications Protocol
Modbus Exception
Responses
The SEL-751 sends an exception code under the conditions described in
Table E.3.
In the event that any of the errors listed in Table E.3 occur, the relay assembles
a response message that includes the exception code in the data field. The
relay sets the most significant bit in the function code field to indicate to the
master that the data field contains an error code, instead of the necessary data.
Cyclical
Redundancy Check
The SEL-751 calculates a 2-byte CRC value through use of the device
address, function code, and data region. It appends this value to the end of
every Modbus response. When the master device receives the response, it
recalculates the CRC. If the calculated CRC matches the CRC sent by the
SEL-751, the master device uses the data received. If there is no match, the
check fails and the message is ignored. The devices use a similar process
when the master sends queries.
01h Read Discrete
Output Coil Status
Command
Use function code 01h to read the On/Off status of the selected bits (coils) (see
the Modbus Register Map shown in Ta b le E .14 ). You can read the status of as
many as 2000 bits per query, using the fields shown in Table E.4. Note that the
SEL-751 coil addresses start at 0 (e.g., Coil 1 is located at address zero). The
coil status is packed one coil per bit of the data field. The Least Significant Bit
(LSB) of the first data byte contains the starting coil address in the query. The
other coils follow towards the high order end of this byte and from low order
to high order in subsequent bytes.
Table E.3 SEL-751 Modbus Exception Codes
Exception
Code
Error Type Description
1 Illegal Function Code The received function code is either undefined or
unsupported.
2 Illegal Data Address The received command contains an unsupported
address in the data field (i.e., cannot write to a read-
only register, cannot write because settings are
locked, etc.).
3 Illegal Data Value The received command contains a value that is out
of range.
4 Device Error The SEL-751 is in the wrong state for the function a
query specifies.
This also stands for Service Failure for DeviceNet
interface applications. The relay is unable to per-
form the action specified by a query (i.e., cannot
write to a read-only register, cannot write because
settings are locked, etc.).
6 Busy The device is unable to process the command at this
time, because of a busy resource.
To Next Page
Loading...
