Rockwell Automation T80004 Application Note Field Loop Configuration - Manual (Page 21)

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AN-T80004 Field Loop Configuration
Issue 10 Jun 13 AN-T80004 21
Non volt-free digital inputs
The information above is for isolated volt-free contacts, which are the simplest kind of digital input. The
system is able to apply its own voltage and measure the effect without any external influences apart
from the switch state. However, the input device or system must isolate the contact, e.g. using a relay
or solid state switch.
This section describes approaches for interfacing to digital inputs which are not isolated or not volt-free
simple contacts.
Current controlled inputs
This covers all inputs where the loop current (as opposed to the voltage) is used as an indication of
input state. One example is proximity sensors to Namur EN60947-5-6. These devices pass about 0.5
to 1mA for ‘off’ (proximity detected) and about 2 to 6mA for ‘on’ (proximity absent). They also need a
low voltage supply, e.g. 8V. This limits the available detection range for a digital input module.
The answer is to use an interposing barrier, e.g. MTL 4113P or MTL5018ac, which supplies the
appropriate voltage and detects the loop current. These will also provide contacts for open/short circuit
detection, allowing monitoring of the field loop. They inherently provide intrinsic safety isolation.
Where hazardous area certification is not required, it may be possible to treat the device as an
analogue input, wired to an analogue input module. Since the system’s 250R resistor will drop some
voltage, the supply to the device should be set near the top of its range.
The input may now be treated as a 0 to 20mA loop, and thresholds can be set as described earlier in
this document according to the device specification. For Namur proximity sensors, a common
specification is:
Open circuit <0.28mA <0.07V (with 250R resistor)
Proximity detected 0.35 to 1.2mA 0.09 to 0.3V
Proximity not detected 2.1 to 6mA 0.53 to 1.5V
Short circuit >6.5mA >1.63V
These ranges can be converted to input states using a threshold template, with thresholds set between
the current values shown above.
9V
Channel
0V
System circuit