2. Input/Output Lecture #9 David Andrews dandrews@eecs.ukans.edu

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

4. 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

5. 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

6. 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 ss ss ss ss ss  cntlr s HOLdHOLd s memory request

7. 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…..

8. 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 sensor filter D/dt dt

9. 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 s1 s2 s3

10. 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

11. Reliable Actuators Uncritical actuators: –Simplex single drive Critical actuators: –Fully redundant –Consensus performed by physical environment MM data Actuator control

12. 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