📄 ladder logic for pic and avr.mht
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sequencer)=20
<LI>analog inputs, analog (PWM) outputs=20
<LI>integer variables and arithmetic instructions=20
<LI>easy-to-use serial communications, to a PC, LCD, or =
other=20
device=20
<LI>shift registers, look-up tables=20
<LI>EEPROM variables, whose values are not forgotten when =
you lose=20
power=20
<LI>simulator, to test your program before you generate =
PIC/AVR=20
code </LI></UL>
<P>You can <A href=3D"http://cq.cx/ladder.pl#dl">download it =
for=20
free.</A> Or, with more background:</P>
<H3>Introduction</H3>
<P>PLCs are often programmed in ladder logic. This is =
because PLCs=20
originally replaced relay control systems, and forty years =
later, we=20
still haven't quite let go. A PLC, like any microprocessor, =
executes=20
a list of instructions in sequence. Ladder logic tools =
abstract=20
this; you can program the PLC by wiring up relay contacts =
and coils=20
on-screen, and the PLC runtime will simulate the circuit =
that you've=20
drawn. Some of the relay contacts can be tied to input =
signals from=20
the real world; some of the coils can be tied to outputs. =
That way=20
you can make your simulated circuit interact with other =
devices, and=20
actually control things. That is the point.</P>
<P>Actually it's more general than that, because you can =
incorporate=20
timers and counters and arithmetic operations that you =
couldn't=20
(easily) perform with just relays. The circuit concept is =
still=20
useful though, partly just because it's intuitive, but also =
because=20
it abstracts the concurrency issues. It looks like =
this:</P><PRE style=3D"PADDING-RIGHT: 0px; PADDING-LEFT: 0px; FONT-SIZE: =
13px; PADDING-BOTTOM: 10px; PADDING-TOP: 3px; font-face: Lucida =
Console"> || Xa Xb Yout =
||
1 ||-------] [------+-------] [------+-------( )-------||
|| | | ||
|| | Xc | ||
|| +-------]/[------+ ||
</PRE>
<P>This is a simple piece of combinational logic. There are =
three=20
input terms, Xa, Xb, and Xc. There is one output term, Yout. =
The=20
expression is Yout :=3D Xa and (Xb or (not Xc)). This makes =
sense if=20
you think of Xa and Xb as normally open relay contacts, Xc =
as=20
normally closed relay contacts, and Yout as a relay coil. Of =
course=20
it gets more complicated than that:</P><PRE =
style=3D"PADDING-RIGHT: 0px; PADDING-LEFT: 0px; FONT-SIZE: 13px; =
PADDING-BOTTOM: 10px; PADDING-TOP: 3px; font-face: Lucida Console"> =
|| ||
|| Asetpoint ||
1 ||-------------------------------------{READ ADC}----||
|| ||
|| Atemperature ||
||-------------------------------------{READ ADC}----||
|| ||
|| ||
|| ||
|| ||
|| {SUB min_temp :=3D} ||
2 ||------------------------{ Asetpoint - 20 }--------||
|| ||
|| {ADD max_temp :=3D} ||
||------------------------{ Asetpoint + 20 }--------||
|| ||
|| ||
|| ||
|| ||
||[Atemperature >] Yheater ||
3 ||[ max_temp ]+------------------------(R)-------||
|| | ||
|| Xenable | ||
||-------]/[------+ ||
|| ||
||[Atemperature <] Xenable Yheater ||
||[ min_temp ]--------] [--------------(S)-------||
|| ||
|| ||
|| ||
|| ||
|| {SUB check_temp :=3D} ||
4 ||-----------------------{ Asetpoint - 30 }-------||
|| ||
|| ||
|| ||
|| ||
||[Atemperature >] Yis_hot ||
5 ||[ check_temp ]-------------------------( )-------||
|| ||
|| ||
|| ||
||------[END]----------------------------------------||
|| ||
|| ||
</PRE>
<P>This is for a simple thermostat. There are two analog =
inputs; one=20
of them is for the setpoint, so that it might, for example, =
be=20
connected to a pot that the user turns to select the desired =
temperature. The other provides the temperature measurement; =
it=20
might be a semiconductor temperature sensor, or a platinum =
RTD with=20
suitable interfacing circuitry. There is a digital output, =
Yheater.=20
That might control a heating element, through a suitable =
switch (a=20
TRIAC, or a relay, or a solid-state relay, or whatever). =
</P>
<P>We close the loop with a simple hysteretic (bang-bang)=20
controller. We have selected plus or minus 20 ADC units of=20
hysteresis. That means that when the temperature falls below =
(setpoint - 20), we turn on the heater, and when it climbs =
above=20
(setpoint + 20), we turn the heater off.</P>
<P>I chose to add a few small frills. First, there is an =
enable=20
input: the heater is forced off when Xenable is low. I also =
added an=20
indicator light, Yis_hot, to indicate that the temperature =
is within=20
regulation. This compares against a threshold slightly =
colder than=20
(setpoint - 20), so that the light does not flicker with the =
normal=20
cycling of the thermostat.</P>
<P>This is a trivial example, but it should be clear that =
the=20
language is quite expressive. Ladder logic is not a =
general-purpose=20
programming language, but it is Turing-complete, accepted in =
industry, and, for a limited class of (mostly =
control-oriented)=20
problems, surprisingly convenient.</P>
<H3>A Ladder Logic Compiler for PIC16 and AVR</H3>
<P>Modern sub-3.00 USD microcontrollers probably have about =
the=20
computing power of a PLC circa 1975. They therefore provide =
more=20
than enough MIPS to run reasonably complex ladder logic with =
a cycle=20
time of a few milliseconds. I think PLCs usually have some =
sort of=20
runtime that's kind of like an interpreter or a virtual =
machine, but=20
if we're doing simple logic on a processor without much =
memory then=20
a compiler might be a better idea.</P>
<P>So I wrote a compiler. You start with an empty rung. You =
can add=20
contacts (inputs) and coils (outputs) and more complicated=20
structures to build up your program. Timers (TON, TOF, RTO) =
are=20
supported. The max/min durations depend on the cycle time of =
the=20
=91PLC,=92 which is configurable; timers can count from =
milliseconds to=20
tens of minutes. There are counters and arithmetic =
operations (plus,=20
minus, times, div).</P>
<P>Circuit elements may be added in series or in parallel =
with=20
existing elements. An I/O list is built from the ladder =
logic drawn.=20
You can have internal relays (Rfoo), for which memory is=20
automatically allocated, or inputs (Xfoo) and outputs =
(Yfoo), to=20
which you must assign a pin on the microcontroller. The =
selection of=20
pins available depends on the microcontroller. I have tried =
to=20
support the most popular PICs and AVRs (see <A=20
href=3D"http://cq.cx/ladder.pl#dl">below</A>).</P>
<P>You can edit the program in graphical form:</P>
<P><IMG src=3D"http://cq.cx/pics/ladder-sample.png"></P>
<P>Then you can test the program by simulating it in real =
time. The=20
program appears on screen with the energized (true) branches =
highlighted, which makes it easy to debug. The state of all=20
variables is shown at the bottom of the screen in the I/O =
list.</P>
<P><IMG src=3D"http://cq.cx/pics/ladder-sim.png"></P>
<P>Once the program works in simulation you can assign pins =
to the=20
inputs and outputs and generate PIC or AVR code. The code =
generator=20
isn't all that difficult. If you realize that a parallel =
circuit is=20
an OR and a series circuit is an AND, it's a second-year CS=20
assignment, and not a very long one. The editor is actually =
much=20
more challenging. It would take some work to make a smart =
compiler,=20
though. For the AVR a good register allocator would provide =
a major=20
speedup. If you wanted to get very fancy then you could =
apply=20
standard logic reduction algorithms, and maybe state =
reduction too.=20
That would be much more difficult. The timers screw things =
up=20
anyways.</P>
<P>Even ignoring all that, my code generator for the AVR is =
very=20
poor. The AVR back end still generates PIC code...for =
example, it=20
does not really take advantage of the fact that the AVR has =
more=20
than one register. Some of the code that it generates is =
just=20
embarrassingly bad. The PIC back end is better, but not =
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