Rockwell Automation T80004 Application Note Field Loop Configuration - Manual (Page 24)

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AN-T80004 Field Loop Configuration
Issue 10 Jun 13 AN-T80004 24
Zone Interface and Valve Monitor Inputs
The T8448 and T8449 modules have the same hardware as the T8451 digital output module, but with
different firmware fitted to change their operation. The T8448 Zone Interface Module may have its
channels individually configured as inputs or outputs and is intended to provide all the I/O needed for a
single fire zone. The T8449 Valve Monitor Module has a fixed alternating pattern of inputs and outputs
and is intended for driving safety valves with added testing and monitoring through the inputs.
The inputs on these modules use the monitoring circuits normally used to report the actual state of
each output. These have a limited accuracy of 1/8
th of a volt and so are not suitable for any analogue
inputs except from fire or gas detectors. This accuracy is quite adequate for 24V dc digital inputs.
Termination Options
Both of these modules need to be used with the Versatile Field Termination Assembly (VFTA) T8842.
They therefore need a 40-channel output module VFTA cable TC-209, 210, 509 or 510. The T8842 is
configured using plug-in links and components to allow the use of different field devices. Product
description PD-T8842 describes how to configure the VFTA for:
• Fire detector inputs, needing significant power
• Digital inputs from line monitored volt free contacts
• Powered digital outputs
•
Current loop analogue inputs
• Inputs from detectors which are reset by removing power
•
Valve position switches to convert to a stepped analogue input
The T8842 can connect to three wire devices and supply up to 315mA per channel. If more power is
required (e.g. for beam gas detectors) or separate power is required (e.g. for the beam transmitter), fit
a separate fused terminal for each device.
Diagnostic Test Termination
These two modules perform a test on input channels to determine if the input will sense a closed
contact. To do this, the channel’s output circuits are energised briefly and the input is monitored for a
voltage rise. If the input is wired to a closed non-line monitored volt-free contact (thus shorted to zero
volts) or to a zener diode circuit (thus clamped to a voltage), the voltage will not be able to rise far
enough to satisfy the test and a slice fault will be reported.
To allow the test to run with shorted or clamped inputs, an impedance is needed on the line to allow the
voltage monitored on the module to rise above a threshold. Therefore all digital inputs to these
modules with no resistive line monitoring components require a 1K 0.6W resistor in series with the line,
to create a voltage drop. Product Description PD-T8842 describes where the resistor should be fitted
for each circuit type. In most cases the resistor can be fitted on the VFTA.