GE D20 Technical Overview page 54

D20/D200 Technical Overview
2.4.6.1 D20S Digital Input Processing
The following digital input processing is carried out by the D20S:
• Software Debouncing: Used to verify that a digital input point has actually changed state.
Two levels of software debouncing are provided:
− First-level debouncing, which can be provided on a per-point basis, is used to filter
glitches and contact bounce. The first-level debounce period can be configured, on a per-
point basis, to any value between 0 and 255 ms, in 1 ms increments.
− Second-level debouncing is used to qualify data that has been first-level debounced. The
input state must remain stable through the second-level debounce period, configurable on
an individual point basis, before a valid change will be reported.
• Chatter Filtering: A digital input is disabled if the number of state changes encountered
during a defined time interval is excessively high. While the chatter filter is ON, all state
transitions are ignored. While it is OFF, state transitions are gated through. Reports are
issued whenever the chatter filter state changes from ON to OFF, or from OFF to ON. The
filter period is programmable, on a per-system basis, from 6 to 65535 ms, in increments of 1
ms. The lock-out period, which defines the minimum number of filter periods during which
the chatter filter is ON, is programmable on a per-point basis, from 0 to 65535, in increments
of 1 ms. The maximum expected number of state transitions is programmable, on a per-point
basis, from 0 to 255, in increments of 1 ms.
• Report Limiting: You can specify the minimum number of state transitions that must occur
before a report is sent. The minimum number of state transitions can be programmed, on a
per-point basis, to any value between 0 and 255 ms, in increments of 1 ms. If it is set to 0, a
report will be issued every time a state transition occurs.
2.4.6.2 D20S Base Application
The base application on the D20S controls four main functions:
• Field Input Scanning collects data about the status of various field points, records debounce
changes, controls filtering, time-tags various changes, and queues them in a change-of-state
queue for processing. The input scan period is 1 millisecond, and the state of all 64 points is
recorded during each scan.
• Field Input Processing processes the changes of state (recorded during the scan process)
that are queued. The results of processing are either buffered COS, SOE or transition counts
for accumulator inputs.
• Communication Interface, with the D20 main processor, consists of a set of interface-
function address tables that are downloaded with the base-application code. The addressed
functions are application controls and data transfer functions.
• Maintenance Interface consists of a set of interface-function address tables containing
addresses for maintenance and diagnostic process functions.
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PRPI-019-3.00-10
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