Rockwell Automation T80004 Application Note Field Loop Configuration - Manual (Page 23)

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AN-T80004 Field Loop Configuration
Issue 10 Jun 13 AN-T80004 23
Hall Effect and Open Collector/Open Drain
These devices are all essentially semiconductor switches in diferent configurations.
A Hall Effect sensor is a simple proximity detector, using a semiconductor. When magnetic material is
nearby, the semiconductor will conduct like a diode (one way). When the device is away from magnetic
material, it stops conducting. It may be imagined as a transistor without a base connection. This device
has a semiconductor voltage drop when conducting (about 0.6V). It may be treated as a volt-free
contact and monitored with two 8 volt (7V5, 8V2 etc.) zener diodes as shown below.
Open collector outputs also act as volt-free contacts but are wired on the collector side. Since these by
definition are not isolated or protected, they should only be used when the input module and open
collector output may be referenced together on the same local cabinet supply. Open collector outputs
are often present on older electronic logic devices. Open drain outputs are the equivalent for FET
devices. These outputs will draw significant current when they are switched low/’off’, but are essentially
open circuit when switched high. They therefore need an external pull-up resistor.
A suggested circuit is shown below.
When the output is high, it is open circuit. The current path is therefore through the 1K resistor, 4V
zener and the module’s internal impedance (shown here for an 8403). The voltage at the digital input
will be about 15 to 17 volts, depending on whether the module has a partner fitted.
When the output is low, it will conduct. The voltage at the input will drop to 8.2 volts plus the
semiconductor diode voltage drop.
The circuit requires the zero volt returns to be joined between systems, and so this should not be used
to connect distant systems. If the zener diodes are fitted near the output end, all line faults can be
detected except an open circuit on the zero volt link which will appear as a high output.
Field Supply
Module
Field Supply 24V
Module
5K (2K5 with
partner)
1K
4V
8V2