Input/Output Lecture #9

1. Input/OutputLecture #9 David Andrews dandrews@eecs.ukans.edu

2. What We Will Cover Today • Time In I/O • Data • Control • Sampling • Polling • Interrupts • Sensors • Single • Redundant • Actuators • Single • Redunant

3. Dual Role of Time • I/O signals from field bus comprised of • Value • Time • Value: contents of a register • Time: trigger signal, the control signal that determines when value is transferred from register to another subsystem • Can Be Known Apriori • Periodic sample • Can be controlled by computer • Polling • Can be Initiated by environment • Interrupts

4. The Meaning of Input/Output • Everything that crosses controller boundary • Input • Captures state of RT entities • RT entities observed by Sensors • Output • Control of RT entities • Actuator signals • I/O required to be Reliable in real time systems

5. Ds Ds Ds Ds Ds H O L d s mcntlr s memory request Input Mechanisms • Sampling • State of RT entity is periodically measured • Time between samples is the sampling rate • Provides predictable timing • Samples usually buffered at sensor interface • Temporal control within computer system • Periodic samples read in from sensor buffer. • Polling • Similar to sampling • “Demand” Driven by the computer system • Computer system observes RT value at time of request

6. Interrupts • RT entity may influence control flow in computer • What are implications of this ? • When are interrupts useful ? • Demand (Event Driven) processing of random events • Processing response time shorter than sampling period • Gives temporal control to environment • Some say this is bad…..

7. filter sensor D/dt dt Input Preprocessing • Provide cleaner view of entity • Low pass filtering • “Smooths” out perturbations • Glitches • Meaningless harmonics • Plausability test • Can your body temperature be 400 F ? • Composite Variables • Rates: How is this calculated ? • Averages

8. s1 s2 s3 Reliability in Sensors • Non-critical sensors • Can fail and not dramatically affect system • Can be single sensor • Critical Sensors • Must continue to function with faults • Usually requires multiple identical sensors • How many ? • Coordination • Can “vote”by comparing results

9. Actuation • Actuation • Issue change command for RT Entity • Can also post process and optimize • Actuation may be • Immediate • Deferred • Periodic • Fail Silent • One can fail, but the other can manage control requirements

10. data Actuator control Actuator control M M Reliable Actuators • Uncritical actuators: • Simplex single drive • Critical actuators: • Fully redundant • Consensus performed by physical environment

11. Summary – Input/Output • I/O is the connection between RT entities and images For input, may use sampling, polling, or interrupts • Interrupts are convenient – but open control of the controller to the environment. AVOID if at all possible • Standard sensors and actuators are simplex, single path – for critical tasks, employ redundant sensors, connected to redundant RT representatives • Input pre-processing can be a cheap means to catch large error classes