15
Write Multiple Coils
16
Write Multiple Holding Registers
Modbus Exception (error) Codes
When a Modbus slave recognizes a packet, but determines that there is an error in the
request, it will return an exception code reply instead of a data reply. The exception
reply consists of the slave address or unit number, a copy of the function code with the
high bit set, and an exception code. If the function code was 3, for example, the
function code in the exception reply will be 0x83. The exception codes will be one of
the following:
1
Illegal Function
The function code received in the query is not recognized
by the slave or is not allowed by the slave.
2
Illegal Data
Address
The data address (register number) received in the query is
not an allowed address for the slave, i.e., the register does
not exist. If multiple registers were requested, at least one
was not permitted.
3
Illegal Data
Value
The value contained in the query's data field is not
acceptable to the slave.
4
Slave Device
Failure
An unrecoverable error occurred.
6
Slave Device
Busy
The slave is engaged in processing a long-duration
command. The master should try again later.
10
(hex
0A)
Gateway Path
Unavailable
Gateway could not establish communication with target
device.
11
(hex
0B)
Gateway Target
Device Failed
to Respond
Specialized use in conjunction with gateways, indicates no
response was received from the target device.
17
(hex
11)
Gateway Target
Device Failed
to Respond
No response from slave, request timed out.
Modicon convention notation for Modbus registers
Modbus was originally developed by Gould-Modicon, which is presently Schneider
Electric. The notation originally used by Modicon is still often used today, even though
considered obsolete by present Modbus standards. The advantage in using the
Modicon notation is that two pieces of information are included in a single number: (a)
The register type; (b) The register number. A register number offset defines the type.
The types of registers referenced in Modbus devices, and supported by Babel Buster
gateways, include the following:
• Coil (Discrete Output)
• Discrete Input
• Input Register
• Holding Register
B. Modbus Reference Information
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