Issue
140 March 2002
Replacing
Relays with Ladder Logic
Part
1: Getting Ready for the Climb
by
Fred Eady
Fred’s
reaching for the sky with his latest project so it’s
no surprise that ladder logic came into play. With a
well-balanced programmable logic controller board, Fred
observed the “Do not disturb” labels on the board and
tried figuring out the hardware via the software.
Start
•
What's Fred Up To? • Inside
The Hardware • Climbing
The Ladder • Top Of
The Ladder • Sources
& PDF
Ladder logic.
That sounds like something painters and window washers
would use and be good at. On the other hand, my plumber
friend and his father both fell off the same ladder performing
the same plumbing task at the same house with the ladder
in the same position for each of their falls. If you were
to poll me on who’s who of ladder logic, I’d say the couple
of plumbers I know are definitely not experts at it.
Ladder logic
came along in the fabulous 1960s and matured with the
great music my teenagers think was written by contemporary
rap artists. Unlike the rappers, ladder logic is an original
and didn’t have much except relay coils to take anything
from. In fact, I’d wager that some of our most popular
micros took some pointers from the beginnings of automated
industrial control in which ladder logic was a key player.
Ladder logic’s
climb to popularity originated in Motown as auto manufacturers
were trying to find a suitable replacement for the relay-based
automation found on their assembly lines. Out went the
relays and in came the Programmable Logic Controller (PLC),
and right there with it was a newfangled programming lingo
called ladder logic.
The new programming
language adopted some of the old relay logic lingo. Ladder
logic programs are full of relays, coils, and contacts.
Some of these relay components are real and many more
of them are logical, or, for a better word, virtual, as
they only exist in the mind of the PLC.
As I studied
example ladder logic programs and experimented with my
own ladder logic creations, I realized that the relay
coils and contacts in my programs were the bits that control
the behavior of my embedded programs. Taking the analogy
a bit further, the ladder logic inputs and outputs perform
the same functions as I/O pins on microcontrollers. With
the inclusion of timer coils, counter coils, and sequencers,
ladder logic is to PLCs what C is to microcontrollers
with one major difference. The microcontroller behind
a ladder logic program is on steroids and a daily regimen
of BowFlex strength training.
