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INTRODUCTION
LDmicro generates native code for certain Microchip PIC16 and Atmel AVR microcontrollers. Usually software for these microcontrollers is written in a programming language like assembler, C, or BASIC. A program in one of these languages comprises a list of statements. These languages are powerful and well-suited to the architecture of the processor, which internally executes a list of instructions.
PLCs, on the other hand, are often programmed in `ladder logic.' A simple program might look like this:
|| Xbutton1 Tdon Rchatter Yred || 1 ||-------]/[---------[TON 1.000 s]-+-------]/[--------------( )-------|| || | || || Xbutton2 Tdof | || ||-------]/[---------[TOF 2.000 s]-+ || || || || || || || || Rchatter Ton Tnew Rchatter || 2 ||-------]/[---------[TON 1.000 s]----[TOF 1.000 s]---------( )-------|| || || || || || || ||------[END]---------------------------------------------------------|| || || || ||
(TON is a turn-on delay; TOF is a turn-off delay. The --] [-- statements are inputs, which behave sort of like the contacts on a relay. The --( )-- statements are outputs, which behave sort of like the coil of a relay. Many good references for ladder logic are available on the Internet and elsewhere; details specific to this implementation are given below.)
A number of differences are apparent:
The program is presented in graphical format, not as a textual list of statements. Many people will initially find this easier to understand.
At the most basic level, programs look like circuit diagrams, with relay contacts (inputs) and coils (outputs). This is intuitive to programmers with knowledge of electric circuit theory.
The ladder logic compiler takes care of what gets calculated where. You do not have to write code to determine when the outputs have to get recalculated based on a change in the inputs or a timer event, and you do not have to specify the order in which
these calculations must take place; the PLC tools do that for you.
LDmicro compiles ladder logic to PIC16 or AVR code. The following processors are supported:
- PIC16F
- PIC16F
- PIC16F876 (untested)
- PIC16F88 (untested)
- PIC16F819 (untested)
- PIC16F887 (untested)
- PIC16F886 (untested)
- ATmega
- ATmega
- ATmega162 (untested)
- ATmega32 (untested)
- ATmega16 (untested)
- ATmega8 (untested)
It would be easy to support more AVR or PIC16 chips, but I do not have any way to test them. If you need one in particular then contact me and I will see what I can do.
Using LDmicro, you can draw a ladder diagram for your program. You can simulate the logic in real time on your PC. Then when you are convinced that it is correct you can assign pins on the microcontroller to the program inputs and outputs. Once you have assigned the pins, you can compile PIC or AVR code for your program. The compiler output is a .hex file that you can program into your microcontroller using any PIC/AVR programmer.
LDmicro is designed to be somewhat similar to most commercial PLC programming systems. There are some exceptions, and a lot of things aren't standard in industry anyways. Carefully read the description of each instruction, even if it looks familiar. This document assumes basic knowledge of ladder logic and of the structure of PLC software (the execution cycle: read inputs, compute, write outputs).
ADDITIONAL TARGETS
It is also possible to generate ANSI C code. You could use this with any processor for which you have a C compiler, but you are responsible for supplying the runtime. That means that LDmicro just generates source for a function PlcCycle(). You are responsible for calling PlcCycle every cycle time, and you are responsible for implementing all the I/O (read/write digital input, etc.) functions that the PlcCycle() calls. See the comments in the generated source for more details.
Finally, LDmicro can generate processor-independent bytecode for a virtual machine designed to run ladder logic code. I have provided a sample implementation of the interpreter/VM, written in fairly portable
Choose the rest of the name so that it describes what the object does, and so that it is unique within the program. The same name always refers to the same object within the program. For example, it would be an error to have a turn-on delay (TON) called Tdelay' and a turn-off delay (TOF) calledTdelay' in the same program, since each counter needs its own memory. On the other hand, it would be correct to have a retentive timer (RTO) called Tdelay' and a reset instruction (RES) associated withTdelay', since it that case you want both instructions to work with the same timer.
Variable names can consist of letters, numbers, and underscores (_). A variable name must not start with a number. Variable names are case-sensitive.
The general variable instructions (MOV, ADD, EQU, etc.) can work on variables with any name. This means that they can access timer and counter accumulators. This may sometimes be useful; for example, you could check if the count of a timer is in a particular range.
Variables are always 16 bit integers. This means that they can go from -32768 to 32767. Variables are always treated as signed. You can specify literals as normal decimal numbers (0, 1234, -56). You can also specify ASCII character values ('A', 'z') by putting the character in single-quotes. You can use an ASCII character code in most places that you could use a decimal number.
At the bottom of the screen you will see a list of all the objects in the program. This list is automatically generated from the program; there is no need to keep it up to date by hand. Most objects do not need any configuration. Xname',Yname', and `Aname' objects must be assigned to a pin on the microcontroller, however. First choose which microcontroller you are using (Settings -> Microcontroller). Then assign your I/O pins by double-clicking them on the list.
You can modify the program by inserting or deleting instructions. The cursor in the program display blinks to indicate the currently selected instruction and the current insertion point. If it is not blinking then press or click on an instruction. Now you can delete the current instruction, or you can insert a new instruction to the right or left (in series with) or above or below (in parallel with) the selected instruction. Some operations are not allowed. For example, no instructions are allowed to the right of a coil.
The program starts with just one rung. You can add more rungs by selecting Insert Rung Before/After in the Logic menu. You could get the same effect by placing many complicated subcircuits in parallel within one rung, but it is more clear to use multiple rungs.
Once you have written a program, you can test it in simulation, and then you can compile it to a HEX file for the target microcontroller.
SIMULATION
To enter simulation mode, choose Simulate -> Simulation Mode or press . The program is shown differently in simulation mode. There is no longer a cursor. The instructions that are energized show up bright red; the instructions that are not appear greyed. Press the space bar to run the PLC one cycle. To cycle continuously in real time, choose Simulate -> Start Real-Time Simulation, or press . The display of the program will be updated in real time as the program state changes.
You can set the state of the inputs to the program by double-clicking them in the list at the bottom of the screen, or by double-clicking an `Xname' contacts instruction in the program. If you change the state of an input pin then that change will not be reflected in how the program is displayed until the PLC cycles; this will happen automatically if you are running a real time simulation, or when you press the space bar.
COMPILING TO NATIVE CODE
Ultimately the point is to generate a .hex file that you can program into your microcontroller. First you must select the part number of the microcontroller, under the Settings -> Microcontroller menu. Then you must assign an I/O pin to each Xname' orYname' object. Do this by double-clicking the object name in the list at the bottom of the screen. A dialog will pop up where you can choose an unallocated pin from a list.
Then you must choose the cycle time that you will run with, and you must tell the compiler what clock speed the micro will be running at. These are set under the Settings -> MCU Parameters... menu. In general you should not need to change the cycle time; 10 ms is a good value for most applications. Type in the frequency of the crystal that you will use with the microcontroller (or the ceramic resonator, etc.) and click okay.
Now you can generate code from your program. Choose Compile -> Compile, or Compile -> Compile As... if you have previously compiled this program and you want to specify a different output file name. If there are no errors then LDmicro will generate an Intel IHEX file ready for programming into your chip.
Use whatever programming software and hardware you have to load the hex file into the microcontroller. Remember to set the configuration bits (fuses)! For PIC16 processors, the configuration bits are included in the hex file, and most programming software will look there automatically. For AVR processors you must set the configuration bits by hand.
INSTRUCTIONS REFERENCE
be the rightmost instruction in its rung.
> COIL, RESET-ONLY Rname Yname ----(R)---- ----(R)----
If the signal going into the instruction is true, then the given internal relay or output pin is cleared false. Otherwise the internal relay or output pin state is not changed. This instruction instruction can only change the state of a coil from true to false, so it is typically used in combination with a set-only coil. This instruction must be the rightmost instruction in its rung.
> TURN-ON DELAY Tdon -[TON 1.000 s]-
When the signal going into the instruction goes from false to true, the output signal stays false for 1.000 s before going true. When the signal going into the instruction goes from true to false, the output signal goes false immediately. The timer is reset every time the input goes false; the input must stay true for 1000 consecutive milliseconds before the output will go true. The delay is configurable.
The `Tname' variable counts up from zero in units of scan times. The TON instruction outputs true when the counter variable is greater than or equal to the given delay. It is possible to manipulate the counter variable elsewhere, for example with a MOV instruction.
> TURN-OFF DELAY Tdoff -[TOF 1.000 s]-
When the signal going into the instruction goes from true to false, the output signal stays true for 1.000 s before going false. When the signal going into the instruction goes from false to true, the output signal goes true immediately. The timer is reset every time the input goes false; the input must stay false for 1000 consecutive milliseconds before the output will go false. The delay is configurable.
The `Tname' variable counts up from zero in units of scan times. The TON instruction outputs true when the counter variable is greater than or equal to the given delay. It is possible to manipulate the counter variable elsewhere, for example with a MOV instruction.
> RETENTIVE TIMER Trto -[RTO 1.000 s]-
This instruction keeps track of how long its input has been true. If its input has been true for at least 1.000 s, then the output is
true. Otherwise the output is false. The input need not have been true for 1000 consecutive milliseconds; if the input goes true for 0.6 s, then false for 2.0 s, and then true for 0.4 s, then the output will go true. After the output goes true it will stay true even after the input goes false, as long as the input has been true for longer than 1.000 s. This timer must therefore be reset manually, using the reset instruction.
The `Tname' variable counts up from zero in units of scan times. The TON instruction outputs true when the counter variable is greater than or equal to the given delay. It is possible to manipulate the counter variable elsewhere, for example with a MOV instruction.
> RESET Trto Citems ----{RES}---- ----{RES}----
This instruction resets a timer or a counter. TON and TOF timers are automatically reset when their input goes false or true, so RES is not required for these timers. RTO timers and CTU/CTD counters are not reset automatically, so they must be reset by hand using a RES instruction. When the input is true, the counter or timer is reset; when the input is false, no action is taken. This instruction must be the rightmost instruction in its rung.
> ONE-SHOT RISING _
--[OSR_/ ]--
This instruction normally outputs false. If the instruction's input is true during this scan and it was false during the previous scan then the output is true. It therefore generates a pulse one scan wide on each rising edge of its input signal. This instruction is useful if you want to trigger events off the rising edge of a signal.
> ONE-SHOT FALLING _
--[OSF _]--
This instruction normally outputs false. If the instruction's input is false during this scan and it was true during the previous scan then the output is true. It therefore generates a pulse one scan wide on each falling edge of its input signal. This instruction is useful if you want to trigger events off the falling edge of a signal.
> SHORT CIRCUIT, OPEN CIRCUIT
The output condition of a short-circuit is always equal to its input condition. The output condition of an open-circuit is always false. These are mostly useful for debugging.
-[ var2 ]- -[ vartwo]- -[ Cup]-
If the input to this instruction is false then the output is false. If the input is true then the output is true if and only if the given condition is true. This instruction can be used to compare (equals, is greater than, is greater than or equal to, does not equal, is less than, is less than or equal to) a variable to a variable, or to compare a variable to a 16-bit signed constant.
> COUNTER Cname Cname --[CTU >=5]-- --[CTD >=5]--
A counter increments (CTU, count up) or decrements (CTD, count down) the associated count on every rising edge of the rung input condition (i.e. what the rung input condition goes from false to true). The output condition from the counter is true if the counter variable is greater than or equal to 5, and false otherwise. The rung output condition may be true even if the input condition is false; it only depends on the counter variable. You can have CTU and CTD instructions with the same name, in order to increment and decrement the same counter. The RES instruction can reset a counter, or you can perform general variable operations on the count variable.
> CIRCULAR COUNTER Cname --{CTC 0:7}--
A circular counter works like a normal CTU counter, except that after reaching its upper limit, it resets its counter variable back to 0. For example, the counter shown above would count 0, 1, 2, 4, 5, 6, 7, 0, 1, 2, 3, 4, 5, 6, 7, 0, 2,.... This is useful in combination with conditional statements on the variable `Cname'; you can use this like a sequencer. CTC counters clock on the rising edge of the rung input condition condition. This instruction must be the rightmost instruction in its rung.
> SHIFT REGISTER {SHIFT REG }
-{ reg0..3 }-
A shift register is associated with a set of variables. For example, this shift register is associated with the variables reg0',reg1', reg2', andreg3'. The input to the shift register is reg0'. On every rising edge of the rung-in condition, the shift register will shift right. That means that it assignsreg3 := reg2', reg2 := reg1'. andreg1 := reg0'. `reg0' is left unchanged. A large shift register can easily consume a lot of memory. This instruction must be the rightmost instruction in its rung.
> LOOK-UP TABLE {dest := }
-{ LUT[i] }-
A look-up table is an ordered set of n values. When the rung-in condition is true, the integer variable dest' is set equal to the entry in the lookup table corresponding to the integer variablei'. The index starts from zero, so `i' must be between 0 and (n-1). The behaviour of this instruction is not defined if the index is outside this range. This instruction must be the rightmost instruction in its rung.
> PIECEWISE LINEAR TABLE {yvar := } -{ PWL[xvar] }-
This is a good way to approximate a complicated function or curve. It might, for example, be useful if you are trying to apply a calibration curve to convert a raw output voltage from a sensor into more convenient units.
Assume that you are trying to approximate a function that converts an integer input variable, x, to an integer output variable, y. You know the function at several points; for example, you might know that
f(0) = 2 f(5) = 10 f(10) = 50 f(100) = 100
This means that the points
(x0, y0) = ( 0, 2) (x1, y1) = ( 5, 10) (x2, y2) = ( 10, 50) (x3, y3) = (100, 100)
lie on that curve. You can enter those 4 points into a table associated with the piecewise linear instruction. The piecewise linear instruction will look at the value of xvar, and set the value of yvar. It will set yvar in such a way that the piecewise linear curve will pass through all of the points that you give it; for example, if you set xvar = 10, then the instruction will set yvar = 50.
If you give the instruction a value of xvar that lies between two of the values of x for which you have given it points, then the instruction will set yvar so that (xvar, yvar) lies on the straight line connecting those two points in the table. For example, xvar = 55 gives an output of yvar = 75. (The two points in the table are (10, 50) and (100, 100). 55 is half-way between 10 and 100, and 75 is half-way between 50 and 100, so (55, 75) lies on the line that connects those two points.)
The points must be specified in ascending order by x coordinate. It
LDmicro can generate code to use the PWM peripheral built in to certain microcontrollers. If the input condition to this instruction is true, then the duty cycle of the PWM peripheral is set to the value of the variable duty_cycle. The duty cycle must be a number between 0 and 100; 0 corresponds to always low, and 100 corresponds to always high. (If you are familiar with how the PWM peripheral works, then notice that that means that LDmicro automatically scales the duty cycle variable from percent to PWM clock periods.)
You can specify the target PWM frequency, in Hz. The frequency that you specify might not be exactly achievable, depending on how it divides into the microcontroller's clock frequency. LDmicro will choose the closest achievable frequency; if the error is large then it will warn you. Faster speeds may sacrifice resolution.
This instruction must be the rightmost instruction in its rung. The ladder logic runtime consumes one timer to measure the cycle time. That means that PWM is only available on microcontrollers with at least two suitable timers. PWM uses pin CCP2 (not CCP1) on PIC16 chips and OC2 (not OC1A) on AVRs.
> MAKE PERSISTENT saved_var --{PERSIST}--
When the rung-in condition of this instruction is true, it causes the specified integer variable to be automatically saved to EEPROM. That means that its value will persist, even when the micro loses power. There is no need to explicitly save the variable to EEPROM; that will happen automatically, whenever the variable changes. The variable is automatically loaded from EEPROM after power-on reset. If a variable that changes frequently is made persistent, then the EEPROM in your micro may wear out very quickly, because it is only good for a limited (~100 000) number of writes. When the rung-in condition is false, nothing happens. This instruction must be the rightmost instruction in its rung.
> UART (SERIAL) RECEIVE var --{UART RECV}--
LDmicro can generate code to use the UART built in to certain microcontrollers. On AVRs with multiple UARTs only UART1 (not UART0) is supported. Configure the baud rate using Settings -> MCU Parameters. Certain baud rates may not be achievable with certain crystal frequencies; LDmicro will warn you if this is the case.
If the input condition to this instruction is false, then nothing happens. If the input condition is true then this instruction tries to receive a single character from the UART. If no character is read then the output condition is false. If a character is read then its ASCII value is stored in `var', and the output condition is true
for a single PLC cycle.
> UART (SERIAL) SEND var --{UART SEND}--
LDmicro can generate code to use the UARTs built in to certain microcontrollers. On AVRS with multiple UARTs only UART1 (not UART0) is supported. Configure the baud rate using Settings -> MCU Parameters. Certain baud rates may not be achievable with certain crystal frequencies; LDmicro will warn you if this is the case.
If the input condition to this instruction is false, then nothing happens. If the input condition is true then this instruction writes a single character to the UART. The ASCII value of the character to send must previously have been stored in `var'. The output condition of the rung is true if the UART is busy (currently transmitting a character), and false otherwise.
Remember that characters take some time to transmit. Check the output condition of this instruction to ensure that the first character has been transmitted before trying to send a second character, or use a timer to insert a delay between characters. You must only bring the input condition true (try to send a character) when the output condition is false (UART is not busy).
Investigate the formatted string instruction (next) before using this instruction. The formatted string instruction is much easier to use, and it is almost certainly capable of doing what you want.
> FORMATTED STRING OVER UART var -{"Pressure: \3\r\n"}-
LDmicro can generate code to use the UARTs built in to certain microcontrollers. On AVRS with multiple UARTs only UART1 (not UART0) is supported. Configure the baud rate using Settings -> MCU Parameters. Certain baud rates may not be achievable with certain crystal frequencies; LDmicro will warn you if this is the case.
When the rung-in condition for this instruction goes from false to true, it starts to send an entire string over the serial port. If the string contains the special sequence \3', then that sequence will be replaced with the value ofvar', which is automatically converted into a string. The variable will be formatted to take exactly 3 characters; for example, if var' is equal to 35, then the exact string printed will bePressure: 35\r\n' (note the extra space). If instead var' were equal to 1432, then the behaviour would be undefined, because 1432 has more than three digits. In that case it would be necessary to use\4' instead.
If the variable might be negative, then use \-3d' (or-4d'
|| {DIV y :=} || ||-----------{ 1200 / x }-----------||
Mathematically, these two are equivalent; but if you try them, then you will find that the first one gives an incorrect result of y = 0. That is because the variable `temp' underflows. For example, when x = 3, (1 / x) = 0.333, but that is not an integer; the division operation approximates this as temp = 0. Then y = temp * 1200 = 0. In the second case there is no intermediate result to underflow, so everything works.
If you are seeing problems with your math, then check intermediate results for underflow (or overflow, which `wraps around'; for example, 32767 + 1 = -32768). When possible, choose units that put values in a range of -100 to 100.
When you need to scale a variable by some factor, do it using a multiply and a divide. For example, to scale y = 1.8x, calculate y = (9/5)x (which is the same, since 1.8 = 9/5), and code this as y = (9*x)/5, performing the multiplication first:
|| {MUL temp :=} || ||---------{ x * 9 }----------|| || || || {DIV y :=} || ||-----------{ temp / 5 }-----------||
This works for all x < (32767 / 9), or x < 3640. For larger values of x, the variable `temp' would overflow. There is a similar lower limit on x.
CODING STYLE
I allow multiple coils in parallel in a single rung. This means that you can do things like this:
|| Xa Ya || 1 ||-------] [--------------( )-------|| || || || Xb Yb || ||-------] [------+-------( )-------|| || | || || | Yc || || +-------( )-------|| || ||
Instead of this:
|| Xa Ya || 1 ||-------] [--------------( )-------|| || || || ||
|| Xb Yb || 2 ||-------] [--------------( )-------|| || || || || || || || || || Xb Yc || 3 ||-------] [--------------( )-------|| || ||
This means that in theory you could write any program as one giant rung, and there is no need to use multiple rungs at all. In practice that would be a bad idea, because as rungs become more complex they become more difficult to edit without deleting and redrawing a lot of logic.
Still, it is often a good idea to group related logic together as a single rung. This generates nearly identical code to if you made separate rungs, but it shows that they are related when you look at them on the ladder diagram.
In general, it is considered poor form to write code in such a way that its output depends on the order of the rungs. For example, this code isn't very good if both Xa and Xb might ever be true:
|| Xa {v := } || 1 ||-------] [--------{ 12 MOV}--|| || || || Xb {v := } || ||-------] [--------{ 23 MOV}--|| || || || || || || || || || [v >] Yc || 2 ||------[ 15]-------------( )-------|| || ||
I will break this rule if in doing so I can make a piece of code significantly more compact, though. For example, here is how I would convert a 4-bit binary quantity on Xb3:0 into an integer:
|| {v := } || 3 ||-----------------------------------{ 0 MOV}--|| || || || Xb0 {ADD v :=} || ||-------] [------------------{ v + 1 }-----------|| || || || Xb1 {ADD v :=} ||
DO NOT USE CODE GENERATED BY LDMICRO IN APPLICATIONS WHERE
SOFTWARE
FAILURE COULD RESULT IN DANGER TO HUMAN LIFE OR DAMAGE TO
PROPERTY. THE
AUTHOR ASSUMES NO LIABILITY FOR ANY DAMAGES RESULTING FROM THE
OPERATION
OF LDMICRO OR CODE GENERATED BY LDMICRO.
This program is free software: you can redistribute it and/or modify it under the terms of the GNU General Public License as published by the Free Software Foundation, either version 3 of the License, or (at your option) any later version.
This program is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License for more details.
You should have received a copy of the GNU General Public License along with this program. If not, see .
Jonathan Westhues
Rijswijk -- Dec 2004 Waterloo ON -- Jun, Jul 2005 Cambridge MA -- Sep, Dec 2005 Feb, Mar 2006 Feb 2007 Seattle WA -- Feb 2009
Email: user jwesthues, at host cq.cx
