<H3>Simulating the Program</H3>
            <P>Now we are ready to simulate our circuit. Choose Simulate =
-&gt;=20
            Simulation Mode. The display will change; the ladder diagram =
will=20
            appear mostly greyed, but you won't see anything changing =
with time.=20
            That is because the PLC is not yet cycling. To start it =
cycling,=20
            choose Simulate -&gt; Start Real-Time Simulation. Now you =
will see=20
            things happening: the oscillator is obviously running, but =
the LED=20
            (=91Yled=92) is still off, which is what we want, because no =
one has=20
            pressed the button yet. To simulate pressing the button,=20
            double-click the text =91Xbutton=92 in the list at the =
bottom of the=20
            screen. You have now simulated bringing the pushbutton input =
high;=20
            this is what would happen if someone depressed (but did not =
yet=20
            release) the pushbutton.=20
            <P><IMG=20
            =
src=3D"http://cq.cx/pics/ldtut-simulation-just-clicked.png"></P>
            <P>You can see that the program is working: the =91Cstate=92 =
counter is=20
            now equal to 1, which corresponds to the =91steady on=92 =
state, which is=20
            what we want. The LED output is high; you can see that its =
value is=20
            1 in the list, and the =91Yled=92 coil appears red on the =
diagram.=20
            Double-click the =91Xbutton=92 text in the list to simulate =
releasing=20
            the button, then double-click it again to simulate pressing =
it=20
            again; the =91Yled=92 coil will start blinking, as designed. =
If you=20
            simulate a third button press then the output will go steady =

low.</P>
            <H3>Compiling to an IHEX File</H3>
            <P>So now we are fairly sure that the program works. At this =
point=20
            we are ready to generate actual code, and try it in the =
micro.=20
            First, exit simulation mode by choosing Simulate -&gt; =
Simulation=20
            Mode, or by pressing Escape.</P>
            <P>Next we must choose a microcontroller. We decided earlier =
that we=20
            would be using a PIC16F876, so choose Settings -&gt; =
Microcontroller=20
            -&gt; Microchip PIC16F876 28-PDIP or 28-SOIC. We also have =
to tell=20
            LDmicro what kind of crystal we will be using, and what the =
cycle=20
            time will be. Choose Settings -&gt; MCU Parameters, and fill =
in our=20
            clock speed of 20&nbsp;MHz. Leave the cycle time at =
10&nbsp;ms; that=20
            will usually be a good value.</P>
            <P>Now we can assign pins to our inputs and outputs. =
Double-click=20
            =91Xbutton=92 in the list at the bottom of the screen, and =
choose pin 14=20
            of the PIC, which corresponds to MCU port RC3. (There is =
usually no=20
            reason for you to care which port you are using; just look =
at the=20
            pin number.)</P>
            <P><IMG =
src=3D"http://cq.cx/pics/ldtut-assigning-io-pin.png"></P>
            <P>Click =91OK,=92 and then repeat the process for =
=91Yled=92, which you can=20
            see from the schematic should go to pin 15. The other =
elements in=20
            the list are internal variables and bits in memory, so there =
is no=20
            need to assign pins to them. LDmicro will allocate memory =
for them=20
            when you compile.</P>
            <P>So now you are ready to compile. Choose Compile -&gt; =
Compile,=20
            and specify where you want to put the IHEX file. Then use =
whatever=20
            PIC programming equipment you have available to load that =
into your=20
            device, and you are ready to try it out.</P>
            <P>This completes my tutorial. It is possible to write much =
more=20
            complex programs than that, of course. A program this simple =
uses=20
            only a very small fraction of your processor's memory, so =
there is=20
            room for many more rungs of logic. LDmicro also offers =
specialised=20
            instructions, for things like arithmetic, analog (A/D) =
inputs, PWM,=20
            and even text output to a character-based LCD. Consult the =
manual=20
            for details.</P>
            <P>I don't see why you would need to, but you can <A=20
            href=3D"http://cq.cx/dl/ldtut.ld">download the tutorial =
program</A>=20
            premade.</P>
            <P><I>December 2005, Cambridge MA</I></P>
            <P></P><BR><IMG height=3D10=20
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