Varec 2920 - Status Bits; Exception Responses

2920 Float & Tape Transmitter MODBUSAppendix A — MODBUS Implementation
156 Installation and Operations Manual
Status Bits
In the 2920 FTT 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 2920
FTT is shows as follows.
Example of Status Bit Data
Start Register 0002
Number of Registers 0004
This reads Alarm Status bit 2 through 5. The response data is 03.
Bit #2: High Advisory Alarm ON
Bit #3: High Critical Alarm ON
Bit #4: Critical Low Temp OFF
Bit #5: Advisory Low Temp OFF
Exception Responses
The exception responses returned by the 2920 FTT are listed below:
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 C8 09
2920 FTT Response
Address Function
Code
Byte Count Data Error
Check
01 02 01 03 E1 89
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