Rockwell Automation 1715-OF8I Redundant I/O System User Manual - Manual (Page 132)

132 Rockwell Automation Publication 1715-UM001C-EN-P - March 2014
Chapter 3 Digital I/O Operation
Enable Change of State
The Point column (on the left side of the Configuration tab) lets you determine
whether a change of state occurs when a field device becomes Off to On or On to
Off.
1. On the Module Properties dialog box, click the Configuration tab.
2. Do one of the following:
• Check a checkbox to enable COS for a specific point for either Off to
On or On to Off.
• Clear the checkbox to disable the COS for a specific point.
3. Click OK.
See Threshold Values for Digital Inputs on page 261 for setting custom threshold
configurations.
Field-side Diagnostics
If using the 1715-IB16D thresholds, the user is responsible for designing a circuit
that provides unique voltage levels for Wire OFF, OFF, ON and shorted to 24V
DC. This typically involves an inline and shunt resistor. Typical values for the
inline resistor are 3.9 k. A typical value for the shunt resistor is 15 k. This
generates an OFF voltage of 5V DC and an ON voltage of 14V DC if the supply
voltage is 24V DC.
The digital input module has the ability to detect the voltage level, much like an
analog input module. There are no circuit tests on field wiring. The user defines
their diagnostic granularity by using custom thresholds built in to the channel
configuration. For full diagnostics, the user designs a circuit for which the OFF
state is, for example, 5V DC, and the ON state, which is, for example, 15V DC.
Wire OFF creates 0V DC, which falls below the OFF threshold, and OpenWire
fault status is set HI. A short to 24V DC is above the ON threshold, and Short
Circuit fault status is sent HI.