Varec Network Adapter 4000, Installation And Operation Manual, Page: 65 / 80

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4000 Modbus Implementation
9.7 Status Bits
In the ATT implementation, status bits contain alarms, commands, and status
information. The state of a Modbus status bit is defined as either ON (true) or OFF (false).
The ON state is represented by a "1". The status bits may be read with function code 1
or 2. They may be written one at a time with function code 5 or multiple bits may be
written with function code 15. An example of a read message for bits 2 through 6 as
returned by the ATT is shows as follows.
9.7.1 Status Bit Data
Binary representation 1 0010 (bits 6-2)
Bit #2: High Advisory Alarm OFF
Bit #3: High Critical Alarm ON
Bit #4: Unauthorized Mass Movement OFF
Bit #5: Standard Density Alarm OFF
Bit #6: Critical Zone Alarm ON
9.8 Exception Responses
The exception responses returned by the ATT are listed below:
In addition, messages that are received with a parity error, checksum error, or message
format error will be ignored.
Host Request
Address
Function
Code
Start Reg
H Start Reg
L # of Reg
H # of Reg
L Error
Check
01 02 00 02 00 04 XX
ATT Response
Address
Function
code
Byte
Count
Data Error
Check
01 02 01 12 XX
Exception Response Reason
01 Illegal Function Message is not allowed
02 Illegal Data Address Data address (bit or register) requested is not defined
03 Illegal Data Value Data value being written is out of range
04 Busy During power-up