E.3
Date Code 20170814
Instruction Manual
SEL-700G Relay
Modbus RTU Communications
Communications Protocol
Modbus Exception
Responses
The SEL-700G 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-700G 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-700G, 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 Table 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-700G coil addresses start at 0 (for example, 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-700G 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.
3
Illegal Data Value
The received command contains a value that is out of
range.
4
Device Error
The SEL-700G 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 perform
the action specified by a query (that is, it cannot write
to a read-only register).
6
Busy
The device is unable to process the command at this
time, because of a busy resource.
Table E.4
01h Read Discrete Output Coil Status Command
(Sheet 1 of 2)
Bytes
Field
Requests from the master must have the following format:
1 byte
Slave Address
1 byte
Function Code (01h)
2 bytes
Address of the first bit
2 bytes
Number of bits to read
2 bytes
CRC-16
Summary of Contents for SEL-700G Series
Page 14: ...This page intentionally left blank ...
Page 22: ...This page intentionally left blank ...
Page 32: ...This page intentionally left blank ...
Page 52: ...This page intentionally left blank ...
Page 106: ...This page intentionally left blank ...
Page 510: ...This page intentionally left blank ...
Page 560: ...This page intentionally left blank ...
Page 578: ...This page intentionally left blank ...
Page 588: ...This page intentionally left blank ...
Page 604: ...This page intentionally left blank ...
Page 634: ...This page intentionally left blank ...
Page 738: ...This page intentionally left blank ...
Page 802: ...This page intentionally left blank ...