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FORM 150.73-NM1
ISSUE DATE: 09/04/2020
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, 30 VDC 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 VDC to 36 VDC could be
measured for the DC voltage on the digital inputs. This
voltage is in reference to ground. The unit case should
be sufficient as a reference point when measuring digi-
tal 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 = +5 VDC regulated supply to sensor.
J6-9 = VDC input signal to the microboard.
See Table 35 on page 145 for voltage readings
that correspond to specific outdoor tempera-
tures.
J6-3 = drain (shield connection = 0 VDC) Return
JOHNSON CONTROLS
SECTION 9 – SERVICE AND TROUBLESHOOTING
TABLE 35 - 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
41
43
46
49
9
52
54
145
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