LDmicro

LDmicro: Logic Ladder Diagram for PIC and AVR

Quick summary: There is a compiler that starts with a ladder diagram and generates native PIC16 or AVR code. Features include:

digital inputs and outputs
timers (TON, TOF, RTO)
counters (CTU, CTD, `circular counters' for use like a sequencer)
analog inputs, analog (PWM) outputs
integer variables and arithmetic instructions
easy-to-use serial communications, to a PC, LCD, or other device
shift registers, look-up tables
EEPROM variables, whose values are not forgotten when you lose power
simulator, to test your program before you generate PIC/AVR code

This program is free software; source code and executables are available for download.

Introduction

PLCs are often programmed in ladder logic. This is because PLCs originally replaced relay control systems, and forty years later, we still haven't quite let go. A PLC, like any microprocessor, executes a list of instructions in sequence. Ladder logic tools abstract this; you can program the PLC by wiring up relay contacts and coils on-screen, and the PLC runtime will simulate the circuit that you've drawn. Some of the relay contacts can be tied to input signals from the real world; some of the coils can be tied to outputs. That way you can make your simulated circuit interact with other devices, and actually control things. That is the point.

Actually it's more general than that, because you can incorporate timers and counters and arithmetic operations that you couldn't (easily) perform with just relays. The circuit concept is still useful though, partly just because it's intuitive, but also because it abstracts the concurrency issues.

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