Woodward 2301D, Инструкция по установке и эксплуатации, Страница: 111 / 130

Manual 26247 2301D/2301D-EC Digital Control
Woodward 103
Analog Reads
Input registers are analog values that are readable from, but not writable to, the
2301D. Turbine speed is an example of an analog read value. The values of the
input registers are stored internally to the control as floating point numbers
representing engineering units (kPa or rpm). The values transmitted are integers
ranging from –32767 to +32767. Since Modbus can only handle integers, values
that have a decimal point are multiplied by a constant before being sent by
Modbus. For example, these input registers may be listed as the Modbus value
`x100’ or `x10’ under the description heading to denote the value is multiplied by
a scaling constant. This will allow transmission of decimal parts of a unit if this is
necessary for better resolution.
See the 2301D Service Mode for default communication constants and ranges.
The 2301D supports Modbus function code 4, which involves reading selected
input registers.
Analog Writes
Holding registers are analog values that are writable to the 2301D. These values
can also be read by a device performing an error check. An example of an
analog write value would be a direct speed set point as opposed to raise and
lower commands. The values of the holding registers are also stored in the
control as numbers representing engineering units (kPa or rpm). The 2301D
supports Modbus function codes 3, 6, and 16. These correspond to reading
selected holding registers, writing to a single holding register and writing to
multiple holding registers respectively. Appendix B lists the addresses and
descriptions of all Boolean and analog reads and writes.
Modbus Scale Factors
Modbus has two limitations:

only integers can be sent across

the value is limited between –32767 and +32767.
These limitations can be overcome by scaling the value before it is sent across
the Modbus. The default scale factor for analog values is 1. The scale factor can
be changed in the Watch Window (INTFC.COM) between 1 and 1000. These
scaled parameters and their scale factor are available through the Modbus.
Values that require a decimal point must be multiplied by the scale factor (10,
100 or 1000) before being sent across the Modbus. The value sent is then
divided by the scale factor in the Master. The Scale Factor adjusts all associated
analog reads accordingly. The Multiplier (scale factor) can be changed in the
Watch Window.
Shutdown Command through Modbus
Two different types of shutdown commands (emergency and controlled) can be
issued through Modbus. The Emergency Shutdown command instantly takes the
actuator current to zero and activates the shutdown relay. The Controlled
Shutdown function is used to stop the engine in a controlled manner, as opposed
to an Emergency Trip. When a STOP command (controlled shutdown) is issued
the shutdown relay is not activated.