Inputs, Processing, Outputs
All modern, microprocessor-controlled systems use various sensors, switches, voltage signal circuits, ground signal circuits, etc. to provide information to a system control module . These sensors, switches, and other components are known as the input devices; the information or signals they provide are known simply as the inputs . Typically, each input has its own, dedicated terminal or pin at the control module.
Some examples of inputs to the climate control system control module are:
- heater core temperatures (from the heater core temperature sensors)
- evaporator temperature (from the evaporator temperature sensor)
- refrigerant pressure (from the compound pressure switch on the receiver/dryer)
- stratified air thumbwheel position (from the thumbwheel potentiometer)
- vehicle speed (from the vehicle speed sensor)
- A/C compressor request (from the A/C compressor "snowflake" button)
- the start signal (from the ignition switch).
The system control module repetitively accepts all the inputs (many times each second) and processes them.
Engineers and researchers thought about how the module should respond to specific inputs, combinations of inputs, or even missing inputs, and then programmed the control module microprocessor to perform in each case.
Each time the control module processes the inputs, it provides power or ground circuits to operate solenoids, stepper motors, relays, lamps, etc., or it may send signals to other control modules. The signals or power/ground circuits provided by the control module are known as its outputs . The actual component operated (relay, solenoid, motor, etc.) is referred to as the output device.
As an example, the IHKA climate module operates both the blower motor and the water valves.
- The input used to request blower speed is the blower speed knob, which goes to the processing area for blower speed control. If the input device is faulty, or the circuit is broken, the blower control processing area does not receive all the information necessary, and will operate the blower speed based on a substitute value.
- The water valve control processing, however, which does not use the blower speed knob input, will continue to function normally.
To perform diagnosis efficiently, it's important to understand which inputs are used to control a specific output. Or perhaps more importantly, which inputs will not affect a specific output .
BMW uses Inputs, Processing, Outputs diagrams (IPOs) to provide a quick view of the important features of microprocessor controlled systems. Typically (as shown in the previous graphics), inputs are shown on the left side of the IPO, the processor (control module) is shown in the middle, and the outputs are shown on the right.