What you learn today
- Commands via radio
- Feedback
- Safety on connection loss
The pump station at the ice field is two kilometres from the base. No data cable reaches that far. A pair of logic transmitters bridges the distance – and brings a new danger: what does the pump do when the connection breaks?
What the logic transmitter returns
A logic transmitter (StructureLogicTransmitter) in active mode has a Setting of its own. A transmitter in passive mode is linked to another one and passes reading and writing on to it. How to pair the two is covered in day 116.
One detail from the game code matters for safety: if the passive transmitter has no link or no power, it reports itself as not readable. An l or lbn on it halts with IncorrectLogicType – and while the chip stands still, the pump keeps its last command. Without a link it cannot be written either, without power it silently drops what you write. That is why both receivers hang on pins, and before every access you ask first:
move lost 1 # to begin with: no connection
move radio 0
bdns cmdradio check # device gone?
bdnvl cmdradio Setting check # power, link?
l radio cmdradio Setting
move lost 0
check:If the chip at the base stops, the transmitter stands still too: it simply keeps its last value. To the receiver that looks like a valid command that does not change. A radio command therefore needs a heartbeat.
Commands with a number
As with Monday's protocol, every command gets a running number. The base counts it up regularly, even if the command does not change:
| Bits | Content |
|---|---|
| 0–7 | number |
| 8 | pump on (1) or off (0) |
The receiver remembers the last number. If it does not change for too long, the connection is gone – exactly as with the watchdog:
seq tmp seq old
add still still tmp
mul still still tmp
move old seq
sgt tmp still TIMEOUT # quiet for too long
or lost lost tmpSafety on connection loss
What “safe” means depends on the plant. For a pump station it usually means: pump off, valves closed, light on. The receiver enforces this state itself without waiting for a command:
seqz tmp lost
and cmd cmd tmp # no connection: off
sbn PUMP HASH("Pumpe") On cmdFeedback
The base does not only want to send but to know what is happening out there. A second transmitter pair carries the reply back: the number seen last and the state the pump really has. If the number matches, the command has arrived. If the state does not match the command, the safety outside is kicking in.
The reply needs its own transmitter pair. If the receiver writes back the same way, it overwrites the base's command.
Diagram
Exercise
Task
Write the pump station's receiver. The logic transmitters hang on pins: “Befehl” on d0, “Antwort” on d1. “Befehl” delivers bits 0–7 number and bit 8 pump:
- pump “Pumpe” follows bit 8
- if the number does not change for more than TIMEOUT = 20 ticks, or
d0is empty or not readable: pump off, light “Funk” on - feedback into “Antwort”: number in bits 0–7, actual pump command in bit 8 – only if
d1is there and writable
Hints
Hint 1
Reading: first move lost 1 and move radio 0, then bdns and bdnvl on d0 before the l.
Hint 2
Unpack: ext seq radio 0 8 and ext cmd radio 8 1, loss as with the watchdog.
Hint 3
Reply: bdns and bdnvs on d1, then move reply seq, ins reply cmd 8 1 and s.
Quiz
3 questions. You pass from 60 % – the server does the grading.
Complete the day
- Passed the simulator exercise
- Passed the quiz with at least 60 %
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