IDEC WindLDR Ladder Logic Basics

Quick Answer

Ladder logic is the primary programming language for IDEC FC6A PLCs. WindLDR provides a graphical ladder editor with contacts, coils, timers, counters, comparison, math, and data move instructions. Programs use IDEC memory types: I (input), Q (output), M (internal relay), D (data register), T (timer), and C (counter).

IDEC Memory Types in Ladder

• I — Physical input relays mapped to input terminals (least-significant digit is octal)
• Q — Physical output relays mapped to output terminals (least-significant digit is octal)
• M — Internal memory relays for intermediate logic and flags
• D — Data registers (16-bit words) for numeric values; special/system registers also live in the upper D range
• T — Timer contacts / values (WindLDR 6+ also exposes TC current and TP preset device types)
• C — Counter contacts / values (CC current, CP preset in WindLDR 6+)

Contacts and Coils

• Normally Open (A / NO) — Passes continuity when the referenced bit is ON (true)
• Normally Closed (B / NC) — Passes continuity when the referenced bit is OFF (false)
• OUT / OUTN — Energizes (or inverted-energizes) the output while the rung is true
• SET — Latches the bit ON until explicitly reset
• RST — Resets a latched bit (or clears a timer / counter) to OFF / zero

Timer Instructions

FC6A on-delay timers are named by time base (not “TMR”):

• TML — 1 s time base
• TIM — 100 ms time base
• TMH — 10 ms time base
• TMS — 1 ms time base

Off-delay variants are TMLO, TIMO, TMHO, and TMSO. For rising/falling-edge one-shots, use SOTU / SOTD (not a “TMRSS” timer).

Counter Instructions

• CNT — Adding (up) counter
• CDP — Dual-pulse reversible counter
• CUD — Up/down selection reversible counter
• CNTD / CDPD / CUDD — Double-word (32-bit) versions of the above — not the same as “up/down”

Comparison and Math Instructions

Data-register comparisons commonly use DC= / DC>= (and related forms). Counter comparisons use CC= / CC>=. Math instructions (ADD, SUB, MUL, DIV) and MOV operate on D registers (and other word devices per the instruction table). Always confirm operand types in the FC6A Ladder Programming Manual.

Example: Motor Start/Stop with Seal-In

Rung 0: [NO I0] --+-- [OUT Q0]     (Start PB → Motor)
                   |
        [NO Q0] --+                  (Seal-in)
        [NC I1] ----                 (Stop PB, NC)
Rung 1: [NO I2] ---- [RST Q0]       (Fault/reset path clears motor bit)

Example: Timed Fill Cycle

Rung 0: [NO I0] ---- [TIM T0 K300]  (Start → 30 s at 100 ms base)
Rung 1: [NO T0] ---- [OUT Q1]       (Timer done → Open fill valve)

K300 with TIM (100 ms) equals 30 seconds. The same 30-second delay with TML (1 s) would use K30.