Ladder logic

Industrial PLC programs in IEC 61131-3 ladder logic. Allen-Bradley tag+address+description labels, Set/Reset coil pairs, parallel branches, seal-in circuits. For manufacturing and automation.

ladder·§ IEC 61131-3

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IEC 61131-3:2013 Ladder Diagram language specification + NEMA ICS 1-2009 North American standard.

Primary References:

IEC 61131-3:2013 — Programmable controllers, Part 3: Programming languages (Ladder Diagram is one of 5)

NEMA ICS 1-2009 — General Standards for Industrial Control and Systems (NA standard)

Allen-Bradley (Rockwell) RSLogix / Studio 5000 ladder conventions

Siemens TIA Portal ladder conventions

1. Structure & Layout

1.1 Overall Diagram Structure

│ Power Rail L │                                  │ Power Rail R │
│              │                                  │              │
│              ├──[element]──[element]──...────────┤              │ ← Rung 0
│              │                                  │              │
│              ├──[element]──[branch]─────────────┤              │ ← Rung 1
│              │               │                  │              │
│              │            [element]             │              │
│              │               │                  │              │
│              ├───────────────┘─────────────────┤              │ ← Rung 2 (continues)
│              │                                  │              │

1.2 Layout Dimensions

Parameter	Default	Meaning
`power_rail_width`	4px	Power rail stroke width
`rung_height`	70px	Height of each rung
`rung_spacing`	15px	Gap between rungs
`element_width`	32px	Standard element width
`element_height`	24px	Standard element height (contacts/coils)
`rung_wire_y_offset`	`rung_height / 2`	Rung horizontal wire center
`element_spacing`	12px	Gap between adjacent elements
`left_rail_x`	20px	Left power rail x coordinate
`right_rail_x`	`canvas_width - 20`	Right power rail x coordinate
`label_offset`	14px	Vertical distance from label to element center

2. Element Symbols

2.1 Contacts (Inputs — Read conditions)

Contacts are located in the left region of the rung and represent input conditions.

Normally Open Contact (XIC / NO)

Symbol: Two parallel vertical lines, 8px apart, not connected
Meaning: When associated bit = 1, the contact closes (conducts)

SVG:

<!-- Left blade -->
<line x1="0" y1="-12" x2="0" y2="12" stroke="#333" stroke-width="2"/>
<!-- Right blade -->
<line x1="8" y1="-12" x2="8" y2="12" stroke="#333" stroke-width="2"/>
<!-- Lead wires (connecting to rung line) -->
<line x1="-12" y1="0" x2="0" y2="0" stroke="#333" stroke-width="2"/>
<line x1="8" y1="0" x2="20" y2="0" stroke="#333" stroke-width="2"/>
<!-- Total width: 32px (−12 to 20) -->

Normally Closed Contact (XIO / NC)

Symbol: Two parallel vertical lines + diagonal slash through the gap

SVG: Same as NO + extra diagonal line:

<!-- Diagonal slash across gap -->
<line x1="0" y1="6" x2="8" y2="-6" stroke="#333" stroke-width="2"/>

Positive Transition Contact (ONS / Rising Edge)

Symbol: Two vertical lines + rising-edge arrow (↑)
SVG: NO + <polygon points="4,-6 4,0 8,-3"/> inside gap

Negative Transition Contact (ONF / Falling Edge)

Symbol: Two vertical lines + falling-edge arrow (↓)
SVG: NO + <polygon points="4,6 4,0 8,3"/> inside gap

Comparison Contact (EQU, GRT, LES, etc.)

Symbol: Rectangle box (24×18px) containing comparison operator (=, >, <, ≥, ≤, ≠)

<rect x="0" y="-9" width="24" height="18" fill="white" stroke="#333" stroke-width="1.5"/>
<text x="12" y="5" font-size="11" text-anchor="middle">≥</text>
<!-- Input/output label references outside the box -->

2.2 Coils (Outputs — Set outputs)

Coils are located on the right side of the rung (adjacent to the right power rail) and represent output drives.

Output Coil (OTE / Standard)

Symbol: Parentheses ( ) — two opposing arcs

SVG:

<!-- Left arc -->
<path d="M 0,0 A 8,12 0 0 0 0,0" fill="none"/>
<!-- Cleaner: two arcs forming parentheses shape -->
<path d="M 8,-12 Q 0,-12 0,0 Q 0,12 8,12" fill="none" stroke="#333" stroke-width="2"/>
<path d="M 16,-12 Q 24,-12 24,0 Q 24,12 16,12" fill="none" stroke="#333" stroke-width="2"/>
<!-- Lead wires -->
<line x1="-12" y1="0" x2="8" y2="0" stroke="#333" stroke-width="2"/>
<line x1="16" y1="0" x2="28" y2="0" stroke="#333" stroke-width="2"/>
<!-- Total width: 40px -->

Negated Output Coil (OTN)

Same as OTE, with a / slash or NOT annotation inside
SVG: <line x1="8" y1="8" x2="16" y2="-8" stroke="#333" stroke-width="1.5"/>

Set Coil / Latch (OTL)

Same as OTE, with S text inside
SVG: <text x="12" y="5" font-size="12" font-weight="bold" text-anchor="middle">S</text>

Reset / Unlatch Coil (OTU)

Same as OTE, with R text inside
SVG: Same as above, text R

Transition Coils (ONS/ONR)

Rising-edge/falling-edge triggered, with arrow marker inside

2.3 Function Blocks (Complex functional elements)

Function blocks appear in the middle of a rung; a rectangle box contains input/output pins.

Standard size: 60px wide × 50px tall (minimum), expands based on pin count.

<!-- Function block box -->
<rect x="0" y="0" width="60" height="50"
      fill="white" stroke="#333" stroke-width="2"
      class="lt-function-block"/>
<!-- Block type label (top center) -->
<text x="30" y="14" font-size="11" font-weight="bold"
      text-anchor="middle" class="lt-fb-type">TON</text>
<!-- Divider line under type label -->
<line x1="0" y1="18" x2="60" y2="18" stroke="#333" stroke-width="1"/>
<!-- Input pin labels (left side) -->
<text x="4" y="28" font-size="9">IN</text>
<text x="4" y="40" font-size="9">PT</text>
<!-- Input pin wires (left edge) -->
<line x1="-10" y1="25" x2="0" y2="25" stroke="#333" stroke-width="1.5"/>
<line x1="-10" y1="37" x2="0" y2="37" stroke="#333" stroke-width="1.5"/>
<!-- Output pin labels (right side) -->
<text x="36" y="28" font-size="9">Q</text>
<text x="36" y="40" font-size="9">ET</text>
<!-- Output pin wires (right edge) -->
<line x1="60" y1="25" x2="70" y2="25" stroke="#333" stroke-width="1.5"/>
<line x1="60" y1="37" x2="70" y2="37" stroke="#333" stroke-width="1.5"/>

Timer Blocks

Block	Type	Pins (Input)	Pins (Output)
`TON`	Timer On-Delay	`IN`, `PT` (preset time)	`Q` (done), `ET` (elapsed)
`TOFF`	Timer Off-Delay	`IN`, `PT`	`Q`, `ET`
`TP`	Timer Pulse	`IN`, `PT`	`Q`, `ET`
`RTO`	Retentive Timer On	`IN`, `PT`, `R` (reset)	`Q`, `ET`

Counter Blocks

Block	Type	Pins (Input)	Pins (Output)
`CTU`	Count Up	`CU` (count up), `R` (reset), `PV` (preset)	`Q` (done), `CV` (current)
`CTD`	Count Down	`CD`, `LD` (load), `PV`	`Q`, `CV`
`CTUD`	Up/Down	`CU`, `CD`, `R`, `LD`, `PV`	`QU`, `QD`, `CV`

Math Blocks

Rectangle box, width=60, height=40, labeled: ADD, SUB, MUL, DIV. Pins: IN1, IN2 (inputs), OUT (output), EN (enable), ENO (enable output).

Comparison Blocks

Labeled: EQU, NEQ, GRT, GEQ, LES, LEQ. Pins: IN1, IN2, OUT (boolean result).

3. Variable Labels & Addressing

3.1 Label Positioning

Each contact/coil displays the variable address/tag name above or below:

<!-- Tag name above element -->
<text x="x_center" y="y_rung - element_height/2 - 4"
      font-size="9" font-family="monospace" text-anchor="middle"
      class="lt-tag-label">StartBtn</text>
<!-- Address below element (optional) -->
<text x="x_center" y="y_rung + element_height/2 + 12"
      font-size="8" text-anchor="middle" class="lt-address-label">I:0/0</text>

3.2 Rung Numbers

Rung numbers are displayed outside the left power rail:

<text x="left_rail_x - 8" y="y_rung" font-size="10"
      text-anchor="end" class="lt-rung-number">0001</text>

3.3 Rung Comments

Optional comment line above the rung:

<text x="left_rail_x + 10" y="y_rung - rung_height/2 + 8"
      font-size="10" font-style="italic" fill="#555"
      class="lt-rung-comment">Motor start/stop latch logic</text>

4. Parallel Branches

Parallel branches in a rung represent OR logic:

4.1 Branch Start/End Notation

<!-- Branch Start: vertical line down from rung wire -->
<line x1="x_branch_start" y1="y_rung" x2="x_branch_start" y2="y_branch"
      stroke="#333" stroke-width="2"/>

<!-- Branch parallel wire -->
<line x1="x_branch_start" y1="y_branch" x2="x_branch_end" y2="y_branch"
      stroke="#333" stroke-width="2"/>

<!-- Elements on branch wire (at y_branch level) -->

<!-- Branch End: vertical line back up to rung wire -->
<line x1="x_branch_end" y1="y_branch" x2="x_branch_end" y2="y_rung"
      stroke="#333" stroke-width="2"/>

4.2 Multiple Parallel Branches

Each additional branch is placed at y_branch + branch_spacing, with default branch_spacing = rung_height + 10.

5. DSL Grammar (Ladder Logic)

document       = header statement*
header         = "ladder" quoted_string? NEWLINE

statement      = comment | variable_decl | rung_def

comment        = "#" [^\n]* NEWLINE

variable_decl  = "var" id ":" data_type ("=" init_value)? NEWLINE
data_type      = "bool" | "int" | "float" | "timer" | "counter" | "string"
init_value     = "true" | "false" | INT | FLOAT | quoted_string

rung_def       = "rung" INT? rung_comment? ":" NEWLINE INDENT
                   rung_element+
                 DEDENT

rung_comment   = quoted_string

rung_element   = series_element
               | parallel_block

series_element = contact | coil | function_block | jump

parallel_block = "parallel:" NEWLINE INDENT
                   (series_element+)
                   ("branch:" NEWLINE INDENT series_element+ DEDENT)+
                 DEDENT

contact        = contact_type "(" IDENTIFIER ")" NEWLINE
contact_type   = "XIC" | "XIO" | "ONS" | "ONF"    # NO, NC, rising, falling edge
               | "EQU" | "NEQ" | "GRT" | "GEQ" | "LES" | "LEQ"  # compare

coil           = coil_type "(" IDENTIFIER ")" NEWLINE
coil_type      = "OTE" | "OTN" | "OTL" | "OTU"   # output, negated, set, reset

function_block = fb_type "(" fb_params ")" NEWLINE
fb_type        = "TON" | "TOFF" | "TP" | "RTO"
               | "CTU" | "CTD" | "CTUD"
               | "ADD" | "SUB" | "MUL" | "DIV" | "MOV"
fb_params      = IDENTIFIER ("," IDENTIFIER)*

jump           = "JMP" "(" LABEL_ID ")" NEWLINE
               | "LBL" "(" LABEL_ID ")" NEWLINE

IDENTIFIER     = /[a-zA-Z][a-zA-Z0-9_]*/
LABEL_ID       = /[a-zA-Z][a-zA-Z0-9_]*/
INT            = /[0-9]+/
FLOAT          = /[0-9]+\.[0-9]+/
quoted_string  = '"' /[^"]*/ '"'
INDENT         = increase in whitespace
DEDENT         = decrease in whitespace
NEWLINE        = /\n/

DSL Example (Motor Start/Stop):

ladder "Motor Control"

var StartBtn: bool
var StopBtn: bool
var EmergencyStop: bool = false
var MotorLatch: bool = false
var MotorRun: bool = false
var RunIndicator: bool = false

rung 0 "Start latch with stop condition":
  XIC(StartBtn)
  XIO(StopBtn)
  XIO(EmergencyStop)
  OTL(MotorLatch)

rung 1 "Unlatch on E-stop":
  XIC(EmergencyStop)
  OTU(MotorLatch)

rung 2 "Motor run output":
  XIC(MotorLatch)
  OTE(MotorRun)

rung 3 "Run indicator":
  XIC(MotorRun)
  OTE(RunIndicator)

DSL Example (Timer + Counter):

ladder "Timer Counter Example"

var RunSignal: bool
var RunTimer: timer
var CycleCounter: counter
var AlarmOut: bool

rung 0 "TON timer when running":
  XIC(RunSignal)
  TON(RunTimer, RunSignal, T#5s)

rung 1 "Count on timer done":
  XIC(RunTimer.Q)
  CTU(CycleCounter, RunTimer.Q, 100)

rung 2 "Alarm when max cycles":
  XIC(CycleCounter.Q)
  OTE(AlarmOut)

DSL Example (Parallel Branch — OR logic):

ladder "Parallel Branch"

rung 0 "Start from push button OR remote":
  parallel:
    XIC(LocalStart)
    branch:
      XIC(RemoteStart)
  XIC(MotorLatch)
  XIO(StopBtn)
  OTE(MotorRun)

6. SVG Structure

<svg class="lt-ladder" data-diagram-type="ladder">
  <defs>
    <style>
      .lt-power-rail   { stroke: #333; stroke-width: 4; fill: none; }
      .lt-rung-wire    { stroke: #333; stroke-width: 2; fill: none; }
      .lt-contact      { stroke: #333; stroke-width: 2; fill: none; }
      .lt-coil         { stroke: #333; stroke-width: 2; fill: none; }
      .lt-fb-box       { stroke: #333; stroke-width: 2; fill: white; }
      .lt-fb-type      { font: bold 11px monospace; fill: #333; }
      .lt-tag-label    { font: 9px monospace; fill: #333; }
      .lt-address      { font: 8px monospace; fill: #666; }
      .lt-rung-number  { font: 10px monospace; fill: #999; }
      .lt-rung-comment { font: italic 10px sans-serif; fill: #555; }
      .lt-branch-wire  { stroke: #333; stroke-width: 2; fill: none; }
    </style>
  </defs>
  <title>Ladder Logic Diagram — [name]</title>
  <desc>[description]</desc>

  <!-- Left power rail -->
  <line class="lt-power-rail" x1="20" y1="0" x2="20" y2="{total_h}"/>

  <!-- Right power rail -->
  <line class="lt-power-rail" x1="{w-20}" y1="0" x2="{w-20}" y2="{total_h}"/>

  <!-- Rungs -->
  <g id="lt-rungs">
    <g id="rung-0" data-rung="0">
      <!-- Rung number -->
      <text class="lt-rung-number" ...>0000</text>
      <!-- Rung comment -->
      <text class="lt-rung-comment" ...>Motor start/stop</text>
      <!-- Rung horizontal wire -->
      <line class="lt-rung-wire" x1="20" y1="y" x2="{w-20}" y2="y"/>
      <!-- Elements in rung -->
      <g id="rung-0-elem-0" data-type="XIC" data-tag="StartBtn">
        <!-- contact SVG -->
        <text class="lt-tag-label" ...>StartBtn</text>
      </g>
      <g id="rung-0-elem-1" data-type="OTL" data-tag="MotorLatch">
        <!-- coil SVG -->
        <text class="lt-tag-label" ...>MotorLatch</text>
      </g>
    </g>
  </g>
</svg>

7. Test Cases

Case 1: Single Contact → Single Coil

ladder
rung 0:
  XIC(MotorStart)
  OTE(MotorOut)

Verify: one rung, NO contact on the left, output coil on the right, both ends connected to power rails.

Case 2: Series Contacts (AND logic)

ladder
rung 0:
  XIC(A)
  XIC(B)
  XIO(C)
  OTE(Y)

Verify: A/B/C three contacts in series, C is NC (has diagonal slash).

Case 3: Parallel Branch (OR logic)

ladder
rung 0:
  parallel:
    XIC(Btn1)
    branch:
      XIC(Btn2)
  OTE(Lamp)

Verify: Btn1 and Btn2 are on parallel branches that merge before the Lamp coil.

Case 4: TON Timer

ladder
var RunTimer: timer
rung 0:
  XIC(StartSignal)
  TON(RunTimer, StartSignal, T#10s)
rung 1:
  XIC(RunTimer.Q)
  OTE(DoneOutput)

Verify: TON block shows IN and PT input pins, Q output pin, T#10s parameter annotated.

Case 5: Set/Reset Latch

ladder
rung 0:
  XIC(SetBtn)
  OTL(MotorLatch)
rung 1:
  XIC(ResetBtn)
  OTU(MotorLatch)

Verify: OTL coil shows S inside, OTU coil shows R inside.

Case 6: Counter

ladder
var PulseCnt: counter
rung 0:
  XIC(PulseInput)
  CTU(PulseCnt, PulseInput, 50)
rung 1:
  XIC(PulseCnt.Q)
  OTE(BatchDone)

Verify: CTU block has CU/PV inputs, Q output, rung 1 reads the .Q bit.

8. Implementation Priority

Priority	Feature	Complexity	User Value
P0 (MVP)	XIC, XIO contacts + OTE coil	Low	Core — 90% of rungs use these three
P0	Power rails + rung horizontal wire	Low	Core
P0	Series elements layout	Low	Core
P0	Tag name labels above elements	Low	Core
P1	OTL (Set) / OTU (Reset) coils	Low	High
P1	Parallel branch (OR logic)	Medium	High — required for industrial logic
P1	TON / TOFF timer blocks	Medium	High
P1	CTU counter block	Medium	High
P1	ONS / ONF transition contacts	Low	Medium
P2	Comparison contacts (EQU, GRT, etc.)	Medium	Medium
P2	Math blocks (ADD, SUB, MOV)	Medium	Medium
P2	Rung comments + rung numbers	Low	Medium
P2	CTUD up/down counter	Medium	Low
P3	JMP / LBL jump instructions	Medium	Low
P3	IEC symbol style (vs NEMA)	Medium	Low

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