Running program.
Can send messages to it.
It can poll files.
Have file nanes called start,stop,config.txt
You have a number of interconnected boxes. They need to signal to each other.
Inside the box logic is kept alive by a clock tick.
The logic can have inputs and outputs.
Outputs can be a voltage or current, or light or tones some other media.
Inputs can convert a voltage or current to a logic or analogue value.
The software needs a logic or analogue value.
Inputs can detect voltage or current or tone.
A telephone circuit is managed.
phone - circuit - phone
It has a call on it, but what condition is on the circuit outside the call?
How do you signal a start of the call and end of the call.
The circuit is bothway, in that it supports speech in both directions.
Outside the call, a tone is applied.
An old fashioned phone needed a current to power the microphone.
It had a hook switch to hang the receiver on.
A bell was connected when the receiver was on hook.
STATE | CONDITION |
disconnected | noTone |
IDLE | on hook |
Alerting | ringing bell |
CALL | receiver off hook and current flowing |
End call | receiver on hook and current stops |
For a managed telephone circuit.
STATE | CONDITION |
disconnected | noTone |
IDLE | TONE |
CALL | noTone |
CLEAR | TONE |