Varec 2920 - Coded Data; Packed Bit Data

MODBUSAppendix A — MODBUS Implementation
Varec, Inc. 153
If a variable goes out-of-bounds (outside the minimum or maximum scale points), a value
equal to the maximum MODBUS integer + 1 is returned. Writing to dynamically calculated
parameters has no effect on those parameters.
Coded data
Coded data represents a table look-up value. Data written to these registers must be a valid
table entry. Invalid data may cause a Configuration Error to occur. The following example
shows coded data at data address 20 (0014 Hex) representing level units. The value 44
(002C Hex) returned from the 2920 FTT corresponds to level units = Feet.
• Hexadecimal representation 0001
• Level unit table representation: FT
Packed Bit Data
Packed bits represent 16 individual status bits packed into one register. The status bits have
been packed this way for systems that prefer handling only register information. These bits
may also be read or written individually using a bit command. The bits within the packed
registers are grouped by data or function type. The following communication example of
packed bits shows alarm status information at data address 5 (0005 Hex) returned by the 2920
FTT.
Note Bit #0 is ON indicating a critical low level Alarm
Host Request
Address Function
Code
Start Reg
H
Start Reg
L
# of Reg
H
# of Reg Error
Check
01 03 00 14 00 01 C4 0E
2920 FTT Response
Address Function
Code
Byte Count Data
MSB
Data
LSB
Error
Check
01 03 02 00 2C B9 99
Host Request
Address Function
Code
Start Reg
H
Start Reg
L
# of Reg
H
# of Reg
L
Error
Check
01 03 00 05 00 01 94 0B
2920 FTT Response
Address Function
code
Byte Count Data
MSB
Data
LSB
Error
Check
01 03 02 00 01 79 84