105
12. MODBUS VARIABLES. RS4851 COMMUNI
CATION. MODBUS WITH THE BMS SYSTEM
The controller has implementations of Modbus RTU proto-
col. In order to perform network coupling it is necessary to
connect RS-485 buses to the RS485-1 port on the controller
bar. The modbus address is set in the programming device
menu (MAC address, see point 12).
Default communication parameters:
• transmission rate of 9600 bps (can be changed from the
built-in or external HMI)
• 8 bits of frame
• 2 stop bits
• Lack of parity
All variables are 32-bit Holding Register-type values Mod
-
bus registers are 16-bit coded and that is why one 32-bit
variable takes two 16-bit variables. Use Modbus 0x03 com-
mand to read variables, 0x06 to write 16-bit single variable
and 0x10 to write many variables.
12.1 Variable representation
The table below shows all variables of the control system.
The variables have go several number representations:
• Multistate – specied integral values of variable corre-
spond to described conditions
• Decimal – 32-bit variable value is considered as a integer
type with a symbol
• Fixed – a xed type in which the 8 least signicant bits
are intended for a fractional part, while the remaining 24
bits are the total part with a symbol.
Therefore, the accuracy of Fixed value is 1/256. To scale the
value represented by Fixed to the target (proper) value,
multiply it by 1/256 = 0.00390625.
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