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FORM 150.73-NM2
ISSUE DATE: 09/07/2020
CHECKING INPUTS AND OUTPUTS
Digital Inputs
Refer to the unit wiring diagram. All digital inputs are
connected to J13-1 of the I/O board. The term "digital"
refers to two states – either on or off. As an example,
when the flow switch is closed, 30 volts DC will be ap-
plied to J13, pin 5 (J13-5) of the I/O board. If the flow
switch is open, 0 volts DC will then be present at J13-5.
Pin 1 of J13 is an unregulated 30VDC source used
to supply the DC voltage to the various user contacts,
unit switch, flow switch, etc. This DC source is factory
wired to XTBC1, terminal 13. Any switch or contact
used as a digital input would be connected to this ter-
minal, with the other end connecting to its respective
digital input on the microboard. Any time a switch or
contact is closed, 30VDC would be applied to that par-
ticular digital input. Any time a switch or contact is
open, 0VDC would be applied to that particular digital
input.
Typically, voltages of 24 – 36VDC could be measured
for the DC voltage on the digital inputs. This voltage is
in reference to ground. The unit case should be suffi-
cient as a reference point when measuring digital input
voltages.
Analog Inputs – Temperature
Refer to the unit wiring diagram. Temperature inputs
are connected to the microboard on plug J6. These ana-
log inputs represent varying DC signals corresponding
to varying temperatures. All voltages are in reference
to the unit case (ground). Following are the connec-
tions for the temperature sensing inputs:
Outside Air Sensor
J6-6 = +5VDC regulated supply to sensor.
J6-9 = VDC
input
signal
See Table 30 for voltage readings that
correspond to specific outdoor temperatures.
J6-3 = drain (shield connection = 0VDC) Return
JOHNSON CONTROLS
to
the
microboard.
SECTION 9 – SERVICE AND TROUBLESHOOTING
TABLE 28 - OUTDOOR AIR SENSOR
TEMPERATURE/VOLTAGE/
CORRELATION
VOLTAGE
TEMP °F
(Signal Input
to Return)
0
0.7
5
0.8
10
0.9
15
1.0
20
1.1
25
1.2
30
1.4
35
1.5
40
1.7
45
1.8
50
2.0
55
2.2
60
2.3
65
2.5
70
2.6
75
2.8
80
2.9
85
3.1
90
3.2
95
3.4
100
3.5
105
3.6
110
3.7
115
3.8
120
3.9
125
4.0
130
4.1
TEMP °C
-18
-15
-12
-9
-7
-4
-1
2
4
7
10
13
16
18
21
24
27
29
32
35
38
9
41
43
46
49
52
54
139
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