Appendix B – Modbus Protocol
B-16
NMBA-01 Installation and Start-up Guide
Response
The normal response returns the slave address, function code, starting
address, and quantity of registers preset.
Here is an example of a response to the query shown above.
Figure B-10 Preset Multiple Registers - Response
Exception Responses
Except for broadcast messages, 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:
1. If the slave device receives the query without a communication
error, and can handle the query normally, it returns a normal
response.
2. If the slave does not receive the query due to a communication
error, no response is returned. The master program will eventually
process a time-out condition for the query.
3. If the slave receives the query, but detects a communication error
(parity, LRC, or CRC), no response is returned. The master program
will eventually process a time-out condition for the query.
4. 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. The exception
response message has two fields that differentiate it from a normal
response:
RESPONSE
Field Name
Example
(Hex)
Slave Address
11
Function
10
Starting Address Hi
00
Starting Address Lo
01
No. of Registers Hi
00
No. of Registers Lo
01
Error Check CRC
CRC (16-Bits)
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