Lochinvar Knight 81 Instructions Manual page 27

8
Unit Operation
Cascade
In order to operate the boiler/water heater in Cascade with
ModBus or BACnet communications, confi gure the Leader
unit per the control modes in this manual. Connect the
remaining boilers/water heaters in the Cascade through
the normal daisy chain Cascade communications wiring.
Cascade control can then be accomplished automatically
through the Leader boiler.
Please note that with ModBus or BACnet communication
connected to only the Leader unit, only total Cascade
information can be seen through the communications link. If
you wish to see all the individual temperatures of each unit in
the Cascade, each unit will have to have a ModBus / BACnet
communication board. However, each unit can be monitored
without the need to control each one individually.
Monitoring Only
Any boiler/water heater can be equipped with the
communication board and then set up to operate with its own
internal controls. By default settings, the communication
board is a ready monitoring device for the read only variables
by polling the board.
Rate and Temperature Conversions:
Rate
When issuing a rate command the rate can be communicated
as percent modulation or a desired set point, depending on
the setting of the BMS Type in the BMS Setup Menu.
Th e proper data format for the modulation percentage is the
direct conversion to hexadecimal. Th is conversion can be
accomplished through online number based converters or
some scientifi c calculators.
For Example:
Rate % HEX
0 00
20 14
45 2D
60 3C
80 50
95 5F
100 64
To send a desired setpoint, the hexadecimal value must be
determined through linear interpolation of programmable
parameters on the BMS Setup Menu:
- BMS temperature set point at low analog input
- BMS temperature set point at high analog input
Th ese variables set the temperature values corresponding to
the minimum and maximum voltage settings of the 0-10 volt
signal. Th e defaults are as follows:
(continued)
BMS temperature set point at
BMS temperature set point at
For Example:
Send a set point of 110°F.
Th e formula to use for the interpolation is:
Rate Command =
(Desired Set point – BMS Temp at Low Analog Input) (High
(BMS Temp at High Analog Input – BMS Temp at Low
From the default values:
Desired Setpoint = 110
BMS Temp at Low Analog Input =68
BMS Temp at High Analog=158
High Voltage =10
Low Voltage = 2
[(110-69)(10-2)/(158-68)] + 2 = 5.73 Volts
5.73 Volts = 57.3% Modulation
57% = 39 Hexadecimal
A value of [00][39] in hexadecimal would be written to Holding
register 40003 to issue a command for a 110°F setpoint.
Temperature
Th e boiler/water heater passes temperature data in degrees
Celsius. Also, to accommodate decimal places the decimal
value must be divided by 10.
Here are the conversions to and from Celsius:
Example:
Outdoor temperature from remote sensor on BAS System = 80°F
Data that needs to be transmitted is 26.7 * 10 = 267
Decimal
Outlet temperature from unit sensor = 155°F
Data transmitted from unit in HEX = 2AB = 683
Decimal
ModBus and BACnet Communication Instructions
DEFAULT
VALUES
PARAMETER
Deg C
21
low analog input
82
high analog input
Voltage-Low Voltage) + Low Voltage
Analog Input)
T = (5/9) * (T -32)
c f
80°F = 26.7°C
Binary
267 100001011
155°F = 68.3°C
683 ÷ 10 = 68.3 (°C)
Binary
683 1010101011
DEFAULT
Deg F Voltages
70 2
180 10
T = (9/5) * T +32
f c
HEX
10B
HEX
2AB
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