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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.

ROC-POWERED

CURRENT LOOP

DEVICE

I MAXIMUM

R1 =

TO SELECT PROPER VALUE OF R1:

= SOURCE VOLTAGE FROM MODULE = 11 TO 30 VDC, 25 mA MAX

5 VOLTS

DOC0153J

R1=250

I LIMIT

I LOOP

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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Emerson ROC364 Manual de instrucciones Pagina 40