Function Codes Supported; Exception Responses And Exception Codes - General Monitors IR400 Manual

1.2 Function Codes Supported

Function Code 03 (Read Holding Registers) will be used to read status from the slave
unit.
Function Code 06 (Preset Single Register) will be used to write a command to the slave
unit.

1.2.1 Exception Responses and Exception Codes

1.2.1.1 Exception Response
In a normal communications query and response, the master device sends a query to the
IR400 and the IR400 receives the query without a communications error and handles the
query normally within the master device's allowable timeout. The IR400 then returns a
normal response to the master. An abnormal communications produces one of four
possible events:
4. If the IR400 does not receive the query due to a communications error, then no
response is returned from the IR400 and the master device will eventually
process a timeout condition for the query.
5. If the IR400 receives the query, but detects a communication error (CRC, etc.),
then no response is returned from the IR400 and the master device will
eventually process a timeout condition for the query.
6. If the IR400 receives the query without a communications error, but cannot
process the response to the master within the master's timeout setting, then no
response is returned from the IR400 and the master device will eventually
process a timeout condition for the query. In order to prevent this condition from
occurring, the maximum response time for the IR400 is 200 milliseconds.
Therefore the MASTER'S Timeout Setting should be set to 200 milliseconds or
greater.
7. If the IR400 receives the query without a communications error, but cannot
process it due to reading or writing to a non-existent IR400 command register,
then the IR400 will return an exception response message informing the master
of the error.
The exception response message (ref. No. 4 above) has two fields that differentiate it
from a normal response:
Byte Modbus
1st Slave Address
2nd Function Code
3rd Exception Code
4th CRC Hi
5th CRC Lo
Table 6: IR400 Exception Response Message Exception Code
Exception Code Field: In a normal response, the IR400 returns data and status in the
data field, which was requested in the query from the master. In an exception response,
the IR400 returns an exception code in the data field, which describes the IR400
Range Referenced to IR400
1-247* (Dec) IR400 ID (Address)
83 or 86 (Hex) MSB is set with Function Code
01 - 06 (Hex) Appropriate Exception Code (See Below)
00-FF (Hex) CRC Hi Byte
00-FF (Hex) CRC Lo Byte
3
Model IR400 MODBUS