ROC364 Instruction Manual
3-6 Input and Output Modules Rev Jun/05
3.4.1 Analog Input Loop Module
The Analog Input Loop module monitors either loop current or output voltage from field devices.
The module provides source power at terminal A for the loop. The AI Loop module operates by
measuring the voltage at terminals B and C. For current loop monitoring, scaling resistor R1 generates a
voltage across terminals B and C that is proportional to the loop current (I). A 250-ohms scaling resistor
(R1) is supplied by the factory (0.1%, 1/8 watts) to accommodate either 0 to 20 milliamps or 4 to 20
milliamps current loop transmitters. This translates to a maximum operating input voltage of 5 volts dc,
which is the upper limit of the module.
When using a transmitter with a maximum current requirement different from 20 milliamps, R1 should
be scaled to achieve full-scale deflection at 5 volts dc. The formula for determining a new value of R1 is
given in Figure 3-2, where “I Maximum” is the upper end of the operating current range, such as 0.025
amps for a 0 to 25 milliamps device.
I
ROC-POWERED
CURRENT LOOP
DEVICE
I MAXIMUM
R1 =
TO SELECT PROPER VALUE OF R1:
V
S
= SOURCE VOLTAGE FROM MODULE = 11 TO 30 VDC, 25 mA MAX
5 VOLTS
DOC0153J
+T
B
C
R1=250
I LIMIT
I LOOP
V
S
+
-
+
-
Figure 3-2. AI Loop Module Field Wiring for Current Loop Devices
Figure 3-3 shows a typical voltage signal input. Terminal B is the “+” signal input and terminal C is the
“–” signal input. These terminals accept a voltage signal in the 0 to 5 volts range. Since terminal C
connects to a signal ground (non-isolated logic ground), the Analog Input must be a single-ended.
Ensure that no scaling resistor (R1) is installed when the module is used to sense a voltage signal.
Figure 3-3. AI Loop Module Field Wiring for Voltage Devices
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