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8
ED 15121
Example Data Point: Chiller On/Off
This output data point indicates the current state of the chiller. The OFF state is represented by state = FALSE and value
= 0. The other discrete states are represented by state = TRUE and value > 0.
Data Type
Holding Register
Measurement
Units
Valid Range
RO Holding Register
8
Chiller State
N/A
0=Off
1=On
Data Type
Data is represented as either single-bit elements or 16-bit elements. A single-bit element is referred to as a Discrete Input
when it refers to read-only data and as a Coil when it refers to read-write data. A 16-bit element is referred to as a Input
Register when it refers to read-only data and as an Holding Register when it refers to read-write data.
All of the Modbus registers defined in the MicroTech III Chiller Unit Controller are 16-bit Holding Registers. Some are
read only (RO) and some are read-write (RW).
Holding Register
There are up to 65,536 elements of each data type in a Modbus device. Data elements are numbered from 1 to 65,536 in
each type. Data elements are addressed with an index in the range from 0 to 65,535. The index is
not
the address of the
data element in the unit controller memory. The index is used in Modbus PDUs to specify the location the data in the
unit controller. This means, for example, that data element number 1 (ie, holding register 40001) is addressed using
index 0 in the PDU.
In addition, the function code field portion of the message already specifies a ‘holding register’ operation. Therefore the
‘4xxxx’ reference is implicit. This document follows this assumption and has published the holding registers without
the implicit 4xxxx. For example, Holding Register 8 is actually Holding Register 40008.
Valid Range
Some properties are standard data types and some are enumerated sets. If the property value represents a range of values,
e.g., temperature or pressure, a range of values is given. If the property value is an enumerated set, all enumerated values
and corresponding meaning are given.
Configuring the Unit Controller
The MicroTech III Chiller Unit Controller and the Modbus Communication Module ship with default parameter values.
Default values may be changed with the unit keypad or via the network. Parameters must be adjusted to accommodate
your particular network. See the appropriate Operation Manual for default values and keypad operating instructions. See
the MicroTech III Modbus Communication Module Installation Manual, IM 969, for details regarding network
parameters available via the unit controller keypad/display. All documents are available on www.mcquay.com.