17
Temperature
The Crest boiler 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:
T
c
= (5/9) * (T
f
-32) T
f
= (9/5) * T
c
+32
Example:
Outdoor temperature from remote sensor on BAS System = 80°F
80°F = 26.7°C
Data that needs to be transmitted is 26.7 * 10 = 267
Decimal
Binary
HEX
267
100001011
10B
Outlet temperature from unit sensor = 155°F
155°F = 68.3°C
Data transmitted from unit in HEX = 2AB = 683
683 ÷ 10 = 68.3 (°C)
Decimal
Binary
HEX
683
1010101011
2AB
Rate and Temperature Conversions:
Rate
When issuing a rate command the rate can be communicated
as percent modulation or a desired setpoint, depending on
the setting of the BMS Type in the BMS Setup Menu.
The proper data format for the modulation percentage is the
direct conversion to hexadecimal. This conversion can be
accomplished through online number based converters or
some scientifi c calculators.
For Example:
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
These variables set the temperature values corresponding to
the minimum and maximum voltage settings of the 0-10 volt
signal. The defaults are as follows:
Rate %
HEX
0
00
20
14
45
2D
60
3C
80
50
95
5F
100
64
PARAMETER
DEFAULT
VALUES
DEFAULT
Deg C Deg F
Voltages
BMS temperature setpoint at
low analog input
21
69.8
2
BMS temperature setpoint at
high analog input
82
179.6
10
For Example:
Send a setpoint of 110°F.
The formula to use for the interpolation is:
Rate Command =
(Desired Setpoint – BMS Temp at Low Analog Input) (High
Voltage-Low Voltage) + Low Voltage
(BMS Temp at High Analog Input – BMS Temp at Low
Analog Input)
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
[(110-69.8)(10-2)/(179.6-69.8)] + 2 = 4.92 Volts
(4.92/10) x 100 = 49.2
49 = 31 Hexadecimal
A value of [00][31] in hexadecimal would be written to Holding
register 40003 to issue a command for a 110°F setpoint.
5
Unit Operation
(continued)
ModBus Instructions