Lessons · Industrial control · Quick reference
Industrial control quick reference
109 topics, one line each, in the order Hone teaches them.
Hone is a place to practise programming. This sheet is the whole Industrial control track at a glance: every idea it covers, in the order they are taught, one line each. It is a map rather than a lesson. Read opens the full explanation of an idea; Practise gives you a question on it. Both are free, and reading needs no account at all.
Ladder logic from zero · What a PLC is
what a PLC doesA PLC reads its inputs, runs the whole program top to bottom, writes its outputs, and immediately does it all again, many times a second, forever. Read: What a PLC does · Practise what a PLC does
an input: what the PLC can seeA button, a limit switch, a sensor wires to an input. Each scan the PLC reads every input into memory, and the program reads that copy. Read: Inputs: how the PLC senses · Practise an input: what the PLC can see
an output: what the PLC drivesA motor starter, a valve, a lamp is driven by an output. The program sets output bits; at the end of the scan the PLC writes them to the terminals all at once. Read: Outputs: how the PLC acts · Practise an output: what the PLC drives
rungsA rung is conditions on the left and an output on the right: if the contacts pass power, the coil energises. Read: One line of a ladder · Practise rungs
the two rails a rung sits betweenThe vertical lines on each side of a ladder are the rails: power on the left, the return on the right. A rung is true when contacts form an unbroken path from left to right. Read: The two rails · Practise the two rails a rung sits between
reading a ladder, rung by rungEach rung is read left (conditions) to right (action). The PLC solves the rungs from the top down, one after another, every scan. Read: Left to right, top to bottom · Practise reading a ladder, rung by rung
Ladder logic from zero · Contacts and coils
normally open contact[Name] passes power only while its input is true. Contacts in series are AND. Read: Normally open: passes power when TRUE · Practise normally open contact
normally closed contact[/Name] passes power while its input is false; it opens when the input goes true. Read: Normally closed: passes power when FALSE · Practise normally closed contact
the coil at the end of a rungA coil sits at the right end of a rung and turns on when the contacts to its left form a complete path. Break the path and it drops out on the next scan. Read: The coil is the action · Practise the coil at the end of a rung
contacts in series are ANDContacts in series on one rung must all be closed for power to reach the coil: series is AND. Read: Series is AND · Practise contacts in series are AND
contacts in parallel are ORContacts in parallel branches give power two routes to the coil: either branch closed is enough. Parallel is OR. Read: Parallel is OR · Practise contacts in parallel are OR
the scan cycleRungs solve in order; a coil written on rung 1 is seen by rung 2 in the same scan, but rung 1 sees rung 2's coil from the previous scan. Read: Top to bottom, every scan · Practise the scan cycle
Ladder logic from zero · Hold and time
seal-in circuitsA branch with the output's own coil in parallel with Start holds the rung true after Start drops; a normally closed Stop breaks it. Read: Keep running after the button is released · Practise seal-in circuits
set and reset bits(L Name) turns a bit on and leaves it; (U Name) turns it off. Nothing else changes it. Read: Set and reset · Practise set and reset bits
on-delay timer (TON)[TON T1 2s] accumulates while its input is true and sets T1.DN when it reaches the preset; it resets when the input drops. Read: On-delay · Practise on-delay timer (TON)
the off-delay timerAn off-delay (TOF) keeps its output on for a set time after the input turns off: a fan that runs two minutes after the machine stops. Read: The off-delay · Practise the off-delay timer
counting eventsA count-up counter (CTU) adds one each time its input goes from off to on. Holding the input on does not keep counting: it counts transitions, not time. Read: Counting edges · Practise counting events
acting on the change, not the stateRising-edge logic is true for one scan, the scan a bit goes from off to on. It turns a held button into a single event. Read: Exactly once · Practise acting on the change, not the state
Ladder logic from zero · Do no harm
fail-safe wiringWire stop and safety inputs normally closed so the input is TRUE when healthy; a cut wire looks like a press. Read: A broken wire must stop the machine · Practise fail-safe wiring
wiring an emergency stopAn emergency stop is wired normally closed, so the input is on while everything is fine. A pressed button and a broken wire both read as off, and both stop the machine. Read: Wired to fail safe · Practise wiring an emergency stop
the double coil trapIf two rungs write the same coil, the last one each scan wins; the first is overwritten before outputs update. Read: One coil, two rungs, one winner · Practise the double coil trap
two things that can never both be onAn interlock uses each output's own contact to block the other, so forward and reverse can never be on together even if the logic or a stuck bit tries. Read: Never both at once · Practise two things that can never both be on
the watchdogStep sequences are a chain of latches and timers; a watchdog timer trips if a step takes too long. Read: Sequences and the timer that watches · Practise the watchdog
safety is not the PLCSafety-rated functions (light curtains, e-stops) use dedicated safety relays or safety PLCs; the standard PLC only sees their status. Read: The PLC is not the safety system · Practise safety is not the PLC
Why the machine stopped · Before you touch anything
what changedWorking logic does not rot. If a machine ran on Friday and not on Monday, something moved. Read: What changed · Practise what changed
what one scan really doesRead every input into a photograph, solve every rung against the photograph, write every output. Then again. Read: What one scan really does · Practise what one scan really does
finding the rung that stoppedStart at the thing that is not happening, find its coil, and look for the contact that is not made. Read: Finding the rung that stopped · Practise finding the rung that stopped
watching it run versus reading itReading tells you what the program would do. Watching tells you what it is doing. The gap between them is the fault. Read: Watching it run versus reading it · Practise watching it run versus reading it
Why the machine stopped · The coil written twice
the coil written on two rungsAn ordinary coil is written every scan its rung is solved, true or false. Two rungs writing one coil means the lower one decides, always. Read: The coil written on two rungs · Practise the coil written on two rungs
finding a second writerCross-reference the tag: every rung that touches it, and which of those write it. Two writes is your answer. Read: Finding a second writer · Practise finding a second writer
one coil, one rungIf either condition should drive the output, that is a branch. If both must, that is series. Either way it is one rung. Read: One coil, one rung · Practise one coil, one rung
Why the machine stopped · Rung order
rung 2 sees what rung 1 wroteRungs are solved in order, and each one uses the values as they stand at that moment -- including anything an earlier rung has just written. Read: Rung 2 sees what rung 1 wrote · Practise rung 2 sees what rung 1 wrote
logic that is one scan behindPut the same two rungs the other way round and the answer still arrives -- one scan later, every time. Read: Logic that is one scan behind · Practise logic that is one scan behind
moving a rung is a changeRung order decides who wins a double coil and how many scans a chain costs. Reordering is never cosmetic. Read: Moving a rung is a change · Practise moving a rung is a change
Why the machine stopped · The seal-in
how a seal-in holdsThe coil's own contact, in parallel with the start button, keeps the rung true after the button is let go. Read: How a seal-in holds · Practise how a seal-in holds
where the stop belongsIn series with the whole rung, outside the branch -- so it breaks every path, not just one of them. Read: Where the stop belongs · Practise where the stop belongs
an output that will not drop outTwo causes, and they are the first two things to check: a path around the stop, or a second rung writing the coil. Read: An output that will not drop out · Practise an output that will not drop out
hold-to-run, on purposeNo branch means no hold: the output follows the button exactly and stops the moment it is released. Read: Hold-to-run, on purpose · Practise hold-to-run, on purpose
Why the machine stopped · Latches that stick
a latch remembers, a coil does notAn ordinary coil is rewritten every scan and cannot remember anything. A latch writes once and waits to be unlatched. Read: A latch remembers, a coil does not · Practise a latch remembers, a coil does not
the reset that can never runA latch with no unlatch is set for good. A latch whose unlatch cannot be reached is exactly as stuck, and looks better. Read: The reset that can never run · Practise the reset that can never run
set and reset in the same scanWhen a latch and its unlatch both fire in one scan, both run, in order, and the lower one wins. Read: Set and reset in the same scan · Practise set and reset in the same scan
latch, seal-in, or plain coilA seal-in is a rung that stays true. A latch is a bit that stays set. The difference is where the release lives. Read: Latch, seal-in, or plain coil · Practise latch, seal-in, or plain coil
Why the machine stopped · Timers that never finish
a timer adds up timeAn on-delay timer accumulates while its input is true and sets its done bit when it reaches the preset. Read: A timer adds up time · Practise a timer adds up time
one false scan throws it awayA TON is not retentive. Any break in its input, however brief, sets the accumulated time back to zero. Read: One false scan throws it away · Practise one false scan throws it away
done is not rememberedThe done bit is not a memory of having finished. It says the input is true and has been long enough, right now. Read: Done is not remembered · Practise done is not remembered
an input that will not stay stillA signal at the edge of its range flickers, and anything that needs a condition to stay true breaks. Read: An input that will not stay still · Practise an input that will not stay still
Why the machine stopped · The contact that must be normally closed
a healthy stop button reads 1Wired normally closed, the circuit is made at rest. Current flows, and the PLC input is on until somebody presses the button. Read: A healthy stop button reads 1 · Practise a healthy stop button reads 1
closed in the field, open in the rungThe input is already 1 when all is well, so the rung wants an ordinary contact: [Stop], not [/Stop]. Read: Closed in the field, open in the rung · Practise closed in the field, open in the rung
what a broken wire doesWired and written correctly, a break stops the machine. Wired normally open and written [/Stop], the same break leaves it running with a dead stop button. Read: What a broken wire does · Practise what a broken wire does
silence is a faultRaise the alarm on the ABSENCE of the healthy signal, so losing the sensor raises it rather than silencing it. Read: Silence is a fault · Practise silence is a fault
when both directions are called at onceEach rung requires the other direction to be absent, so calling both makes each one block the other and neither runs. Read: When both directions are called at once · Practise when both directions are called at once
More
what an I/O address namesQuestions on Hone; no lesson yet. Practise what an I/O address names
an alarm somebody will still readQuestions on Hone; no lesson yet. Practise an alarm somebody will still read
a reading that variesQuestions on Hone; no lesson yet. Practise a reading that varies
scaling raw counts to real unitsQuestions on Hone; no lesson yet. Practise scaling raw counts to real units
a bit, and sixteen of themQuestions on Hone; no lesson yet. Practise a bit, and sixteen of them
protecting safety data on a networkQuestions on Hone; no lesson yet. Practise protecting safety data on a network
clearing a counterQuestions on Hone; no lesson yet. Practise clearing a counter
when a counter is doneQuestions on Hone; no lesson yet. Practise when a counter is done
a switch that chattersQuestions on Hone; no lesson yet. Practise a switch that chatters
on or off, nothing betweenQuestions on Hone; no lesson yet. Practise on or off, nothing between
what a fieldbus replacesQuestions on Hone; no lesson yet. Practise what a fieldbus replaces
the first scan bitQuestions on Hone; no lesson yet. Practise the first scan bit
forcing a point on a running PLCQuestions on Hone; no lesson yet. Practise forcing a point on a running PLC
a reusable blockQuestions on Hone; no lesson yet. Practise a reusable block
HMI tagsAn HMI reads bits from the PLC. Indicator tags are coils that mirror state, including 'commanded but not running'. Read: What the screen shows · Practise HMI tags
an index into an arrayQuestions on Hone; no lesson yet. Practise an index into an array
what 32 points meansQuestions on Hone; no lesson yet. Practise what 32 points means
a bit set later in the same scanQuestions on Hone; no lesson yet. Practise a bit set later in the same scan
a bit wired to nothingQuestions on Hone; no lesson yet. Practise a bit wired to nothing
a button that springs backQuestions on Hone; no lesson yet. Practise a button that springs back
the contact with a slashQuestions on Hone; no lesson yet. Practise the contact with a slash
one-shotsRising edge: input true now AND a memory of last scan false. Falling edge: the reverse. Write the memory after. Read: True for exactly one scan · Practise one-shots
when outputs updateThe PLC reads all inputs, solves all rungs, then writes all outputs. Nothing physical changes mid-scan. Read: Outputs update at the end of the scan · Practise when outputs update
what a PID loop doesQuestions on Hone; no lesson yet. Practise what a PID loop does
what PLC stands forQuestions on Hone; no lesson yet. Practise what PLC stands for
an address or tagQuestions on Hone; no lesson yet. Practise an address or tag
an analog inputQuestions on Hone; no lesson yet. Practise an analog input
when an output coil energizesQuestions on Hone; no lesson yet. Practise when an output coil energizes
a counter's presetQuestions on Hone; no lesson yet. Practise a counter's preset
what a CTU countsQuestions on Hone; no lesson yet. Practise what a CTU counts
a discrete inputQuestions on Hone; no lesson yet. Practise a discrete input
what an HMI isQuestions on Hone; no lesson yet. Practise what an HMI is
why ladder looks like a wiring diagramQuestions on Hone; no lesson yet. Practise why ladder looks like a wiring diagram
a latch that stays setQuestions on Hone; no lesson yet. Practise a latch that stays set
a normally-closed contactQuestions on Hone; no lesson yet. Practise a normally-closed contact
a normally-open contactQuestions on Hone; no lesson yet. Practise a normally-open contact
true for one scan onlyQuestions on Hone; no lesson yet. Practise true for one scan only
a branch is ORQuestions on Hone; no lesson yet. Practise a branch is OR
a timer that remembersQuestions on Hone; no lesson yet. Practise a timer that remembers
the order rungs run inQuestions on Hone; no lesson yet. Practise the order rungs run in
what SCADA stands forQuestions on Hone; no lesson yet. Practise what SCADA stands for
the scan cycle, in orderQuestions on Hone; no lesson yet. Practise the scan cycle, in order
a seal-in that holds it onQuestions on Hone; no lesson yet. Practise a seal-in that holds it on
in series is ANDQuestions on Hone; no lesson yet. Practise in series is AND
a timer-on-delayQuestions on Hone; no lesson yet. Practise a timer-on-delay
an unlatch that clears itQuestions on Hone; no lesson yet. Practise an unlatch that clears it
the watchdog timerQuestions on Hone; no lesson yet. Practise the watchdog timer
why not an ordinary computerQuestions on Hone; no lesson yet. Practise why not an ordinary computer
reading every input at onceQuestions on Hone; no lesson yet. Practise reading every input at once
it runs with nothing plugged inQuestions on Hone; no lesson yet. Practise it runs with nothing plugged in
the second controllerQuestions on Hone; no lesson yet. Practise the second controller
what PLCs replacedQuestions on Hone; no lesson yet. Practise what PLCs replaced
retentive timersQuestions on Hone; no lesson yet. Practise retentive timers
why scan time mattersQuestions on Hone; no lesson yet. Practise why scan time matters
a sequential function chartQuestions on Hone; no lesson yet. Practise a sequential function chart
what a safety integrity level saysQuestions on Hone; no lesson yet. Practise what a safety integrity level says
when text beats a ladderQuestions on Hone; no lesson yet. Practise when text beats a ladder
what a timer is forQuestions on Hone; no lesson yet. Practise what a timer is for