PCC-IV Instruction Manual (rev. 0.2)
Page 2-22
Section 2 - Installation
Grounds, Neutrals, and DC Commons Schematic
Pull-Up/Down Resistor for Voltage Inputs
This sub-section does not apply to 4-20 mA inputs; that is, inputs with the 250-ohm resistor connected.
The 250-ohm resistor is never connected for: 1-5 V, 0-5 V, 0-10 V, and Potentiometer Inputs.
Each analog input has a low pass noise filter with high quality, low leakage capacitors for 60 Hz noise rejection. If
the field wiring breaks, a pot wiper gets dirty, or is otherwise disconnected, the A/D input voltage will remain at the
last value for several hours due to the capacitors in the filter. In many applications, it is desirable for the input to fail
to either 0V or 5V after the input becomes disconnected.
A Pull-Up Resistor (R1) will force the Input to 5V after a wire break.
A Pull-Down Resistor (R2) will force the Input to 0V after a wire break.
If Required, install only one (1) Resistor, either R1 or R2. Do NOT install BOTH resistors.
The value of R1 or R2 depends on the impedance of the field device, the desired response time, and the acceptable
level of error introduced by the resistor. Each application should be analyzed separately.
Example: An input for a 1k position feedback p
ot on an electric actuator with an “R1” pull-up resistor equal to 330k.
This combination adds less than 0.05% error and has a 3-sec. response time. If the pot is wired such that approx.
+5 V is 100% open, then TOUT will drive the actuator fully closed if the feedback pot wiper circuit becomes
disconnected. If the desired effect is to make the actuator fail open, then reverse the +5 and Common wires to the
pot and re-calibrate TOUT.
The CPU, “A”, “B”, and “F” boards do not include pull-up or pull-down resistors. If desired, the user must mount
them externally.
The “G” option board includes an integral 330k Pull-Up Resistor on each Input channel.
SC
(Signal Common)
250.0
0-5 Vdc
to A/D
150k
150k
.68 uF
typ. 2
-
xx
xx
+
JP1-x
Jumper
Field
Terminals
Typical Internal Circuitry for Analog Inputs
0.6 Hz Filter
(-63 db @ 60 Hz)
15
+
+ 5 Vdc
R1
R2
SC
(Signal Common)
250.0
0-5 Vdc
to A/D
150k
150k
.68 uF
typ. 2
-
xx
xx
+
JP1-x
Jumper
Field
Terminals
Typical Internal Circuitry for Analog Inputs
0.6 Hz Filter
(-63 db @ 60 Hz)
15
+
+ 5 Vdc
R1
R2
