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There are a number of addressing schemes used by PLC manufacturers. Let’s take a quick look at how memory locations (including hardware I/O) are accessed with RSLogix 500. Along the way, let’s define some terms.

File Data Address Format. This File Data Address Format is used with MicroLogix, SLC and PLC-5 range of PLC Devices. The File Data Address Format is a string that defines the location of data in a file based Allen Bradley PLC. This bus enables the communication between CPU and I/O modules to send or receive the data. This communication is established by addressing the I/O modules according to the location from CPU module along the bus. Suppose, if the input module is located in the second slot, then the address must be I2:1.0 (second slot first channel only as an. Allen Nelson - (187 posts): Jan-22-02, 01:49 PM (EST) 1. 'RE: Block transfer Write/read, PLC-5 (Ab)' Because analog I/O has data than can just fit in a single 16-bit word, you need to have a special instruction to get the data into/out of the modules, instead of just being able to look at the data table (You probably knew that, but I'm writing for lurkers too - forgive me if I get too simplistic). The 5 means the input module is in slot 5 of the rack (the sixth slot from the left). The 0 means it is channel 0 (the first output channel on that module). NOTE: PLC addresses always start from 0, not 1. This is common among all programming languages. Addressing always starts at zero unless otherwise defined. Keep that in mind as we go along.

INSTRUCTION – RSLogix’s command language is comprised of “instructions”. An XIC (it looks like a normally open contact –] [– ) is an instruction. A timer is an instruction. A few of the most common instructions are described below.

BIT – an address within the PLC. It can be an input, output or internal coil, among others.

In RSLogix, there are a couple of ways to show the address of a bit. The default is:

[type]:[word]/[bit]

For example, an address that references an output of a MicroLogix 1100 is O:0/0. That is:

O:0/5 means that it is a physical output.
O:0/5 means that it uses Slot 0 (in the case of the 1100, this output is onboard)
O:0/5 means that it is the fifth output on the PLC.

By the way, don’t get the capital “O” confused with a zero.

RUNG – A section of the PLC ladder program that terminates in an output function of some type. Just like in an electrical ladder diagram, a rung has some type of output that is turned on or turned off by the preceding entities in the rung. The first rung in a ladder program is always 0000.

HARDWIRED INPUT – a physical connection to the PLC from an input device (switch or sensor, etc.).

Allen-Bradley uses the capital letter “I” to designate a hardwired input. An address that describes an input on an SLC 500 is I:4/0.

Similar to the output structure,

I:4/0 means that it is a physical input.
I:4/0 means that it uses Slot 4 (the 5th slot in the rack).
I:4/0 means that it is the first input on the card.

Don’t get the capital “I’s” confused with ones.

HARDWIRED OUTPUT – a physical connection from the PLC to an output device (relay or pilot light, etc.) As was said above, an address that references an output of an SLC 500 is O:5/0.

INTERNAL COIL
This is a programmable bit used to simulate a relay within the PLC. The internal coil has no connection to the outside world. It does not connect to an output card. Internal coils are used to store information. The “contacts” of this “relay” can then be used multiple times in other parts of the program.

In RSLogix, the “B3” (binary) file is commonly used for all the internal coils. There are many other words in other files that have bits you can use as internal coils, but we are going to stick with the B3 file for our application.

B3:0/0 means that it references an internal Binary file
B3:0/0 means that it uses the first word in the table
B3:0/0 means that it is the first bit in the word.

Note that, unlike the Output and Input files, you have to use the file number in the address. In this case, the default file number is 3.

TIMER
A timer is a programmable instruction that lets you turn on or turn off bits after a preset time.

The two primary types of timers are TON for “timer on delay” and TOF for “timer off delay”.

Timers in A-B SLC and MicroLogix processors use file 4 for their timers.

T4:0 means that it references an internal Timer file
T4:0 means that it uses the first timer in the table

The address T4:0 simply refers to the timer. Each timer has bits that turn on after the timing function is complete. You can address this bit by simply putting a “/DN” after the timer address. DN stands for “done”.

For example, if timer T4:0 is a TON (timer on delay), then the bit T4:0/DN will turn on after the timer has reached its preset value.

COUNTER
A counter is a programmable instruction that lets you turn on or turn off bits after a preset count has been reached.

There are different types of counters available in the RSLogix, but the CTU (counter up) instruction covers everything we will talk about here.

Counters in A-B SLC and MicroLogix processors use file 5.

C5:0 means that it references an internal Counter file
C5:0 means that it uses the first counter in the table

The address C5:0 simply refers to the counter. Each counter has bits that turn on after the counting function is complete. You can address this bit by simply putting a “/DN” after the counter address. DN stands for “done”.

For example, if counter C5:0 is a CTU (counter up), then the bit C5:0/DN will turn on after the counter has reached its preset value.

–] [– Normally Open Contact
When used with a hardwired input, this instruction is off until there is a voltage applied to the input. The bit address then goes high, or on, and the instruction becomes “true.” It works the same way when it has the same address as an internal coil, except that the coil must be turned on by logic in the program.

Allen-Bradley calls these normally open contacts “XIC”, or “eXamine If Closed” instruction.

An XIC instruction can reference a hardwired input, a hardwired output, an internal coil or a timer done bit, among others.

–]/[– Normally Closed Contact
This is an inverted normally open contact.

When used with a hardwired input, this instruction is “true” until there is a voltage applied to the input. It then goes low, or off, and becomes “false.”

It also can be used with an internal coil, becoming true when the coil is off and becoming false when the coil is on.

Allen-Bradley calls these normally closed contacts “XIO”, or “eXamine If Open” instructions.

-( )- Output Coil
When used with a hardwired output, this function is off until the logic in the program allows it to turn on. It then becomes “true”, and will energize the device that is wired to the respective output.

If it is used as an internal coil, it will toggle the instructions associated with it. That is, it will close a normally open instruction and open a normally closed instruction.

Allen-Bradley calls these outputs “OTE”, or “OutpuT Energize”.

An OTE may be used with a hardwired output or an internal coil.

TRUE – A state that indicates an instruction is allowing logic to “flow” through it.

Also, if the logic in a rung turns on the output of the rung, then the rung is said to be true.

FALSE – Without stating the obvious, this is the opposite of true.

Excerpted from PLC Programming with RSLogix 500

Signals and slots are used for communication between objects. The signals and slots mechanism is a central feature of Qt and probably the part that differs most from the features provided by other frameworks. Signals and slots are made possible by Qt's meta-object system.

Introduction

In GUI programming, when we change one widget, we often want another widget to be notified. More generally, we want objects of any kind to be able to communicate with one another. For example, if a user clicks a Close button, we probably want the window's close() function to be called.

Other toolkits achieve this kind of communication using callbacks. A callback is a pointer to a function, so if you want a processing function to notify you about some event you pass a pointer to another function (the callback) to the processing function. The processing function then calls the callback when appropriate. While successful frameworks using this method do exist, callbacks can be unintuitive and may suffer from problems in ensuring the type-correctness of callback arguments.

Signals and Slots

In Qt, we have an alternative to the callback technique: We use signals and slots. A signal is emitted when a particular event occurs. Qt's widgets have many predefined signals, but we can always subclass widgets to add our own signals to them. A slot is a function that is called in response to a particular signal. Qt's widgets have many pre-defined slots, but it is common practice to subclass widgets and add your own slots so that you can handle the signals that you are interested in.

The signals and slots mechanism is type safe: The signature of a signal must match the signature of the receiving slot. (In fact a slot may have a shorter signature than the signal it receives because it can ignore extra arguments.) Since the signatures are compatible, the compiler can help us detect type mismatches when using the function pointer-based syntax. The string-based SIGNAL and SLOT syntax will detect type mismatches at runtime. Signals and slots are loosely coupled: A class which emits a signal neither knows nor cares which slots receive the signal. Qt's signals and slots mechanism ensures that if you connect a signal to a slot, the slot will be called with the signal's parameters at the right time. Signals and slots can take any number of arguments of any type. They are completely type safe.

All classes that inherit from QObject or one of its subclasses (e.g., QWidget) can contain signals and slots. Signals are emitted by objects when they change their state in a way that may be interesting to other objects. This is all the object does to communicate. It does not know or care whether anything is receiving the signals it emits. This is true information encapsulation, and ensures that the object can be used as a software component.

Slots can be used for receiving signals, but they are also normal member functions. Just as an object does not know if anything receives its signals, a slot does not know if it has any signals connected to it. This ensures that truly independent components can be created with Qt.

You can connect as many signals as you want to a single slot, and a signal can be connected to as many slots as you need. It is even possible to connect a signal directly to another signal. (This will emit the second signal immediately whenever the first is emitted.)

Together, signals and slots make up a powerful component programming mechanism.

Signals

Signals are emitted by an object when its internal state has changed in some way that might be interesting to the object's client or owner. Signals are public access functions and can be emitted from anywhere, but we recommend to only emit them from the class that defines the signal and its subclasses.

When a signal is emitted, the slots connected to it are usually executed immediately, just like a normal function call. When this happens, the signals and slots mechanism is totally independent of any GUI event loop. Execution of the code following the emit statement will occur once all slots have returned. The situation is slightly different when using queued connections; in such a case, the code following the emit keyword will continue immediately, and the slots will be executed later.

If several slots are connected to one signal, the slots will be executed one after the other, in the order they have been connected, when the signal is emitted.

Signals are automatically generated by the moc and must not be implemented in the .cpp file. They can never have return types (i.e. use void).

A note about arguments: Our experience shows that signals and slots are more reusable if they do not use special types. If QScrollBar::valueChanged() were to use a special type such as the hypothetical QScrollBar::Range, it could only be connected to slots designed specifically for QScrollBar. Connecting different input widgets together would be impossible.

Slots

A slot is called when a signal connected to it is emitted. Slots are normal C++ functions and can be called normally; their only special feature is that signals can be connected to them.

Since slots are normal member functions, they follow the normal C++ rules when called directly. However, as slots, they can be invoked by any component, regardless of its access level, via a signal-slot connection. This means that a signal emitted from an instance of an arbitrary class can cause a private slot to be invoked in an instance of an unrelated class.

Plc-5 2 Slot Addressing Software

You can also define slots to be virtual, which we have found quite useful in practice.

Compared to callbacks, signals and slots are slightly slower because of the increased flexibility they provide, although the difference for real applications is insignificant. In general, emitting a signal that is connected to some slots, is approximately ten times slower than calling the receivers directly, with non-virtual function calls. This is the overhead required to locate the connection object, to safely iterate over all connections (i.e. checking that subsequent receivers have not been destroyed during the emission), and to marshall any parameters in a generic fashion. While ten non-virtual function calls may sound like a lot, it's much less overhead than any new or delete operation, for example. As soon as you perform a string, vector or list operation that behind the scene requires new or delete, the signals and slots overhead is only responsible for a very small proportion of the complete function call costs. The same is true whenever you do a system call in a slot; or indirectly call more than ten functions. The simplicity and flexibility of the signals and slots mechanism is well worth the overhead, which your users won't even notice.

Note that other libraries that define variables called signals or slots may cause compiler warnings and errors when compiled alongside a Qt-based application. To solve this problem, #undef the offending preprocessor symbol.

A Small Example

A minimal C++ class declaration might read:

A small QObject-based class might read:

The QObject-based version has the same internal state, and provides public methods to access the state, but in addition it has support for component programming using signals and slots. This class can tell the outside world that its state has changed by emitting a signal, valueChanged(), and it has a slot which other objects can send signals to.

All classes that contain signals or slots must mention Q_OBJECT at the top of their declaration. They must also derive (directly or indirectly) from QObject.

Slots are implemented by the application programmer. Here is a possible implementation of the Counter::setValue() slot:

The emit line emits the signal valueChanged() from the object, with the new value as argument.

In the following code snippet, we create two Counter objects and connect the first object's valueChanged() signal to the second object's setValue() slot using QObject::connect():

Calling a.setValue(12) makes a emit a valueChanged(12) signal, which b will receive in its setValue() slot, i.e. b.setValue(12) is called. Then b emits the same valueChanged() signal, but since no slot has been connected to b's valueChanged() signal, the signal is ignored.

Note that the setValue() function sets the value and emits the signal only if value != m_value. This prevents infinite looping in the case of cyclic connections (e.g., if b.valueChanged() were connected to a.setValue()).

By default, for every connection you make, a signal is emitted; two signals are emitted for duplicate connections. You can break all of these connections with a single disconnect() call. If you pass the Qt::UniqueConnectiontype, the connection will only be made if it is not a duplicate. If there is already a duplicate (exact same signal to the exact same slot on the same objects), the connection will fail and connect will return false.

Plc 5 Slot Addressing

This example illustrates that objects can work together without needing to know any information about each other. To enable this, the objects only need to be connected together, and this can be achieved with some simple QObject::connect() function calls, or with uic's automatic connections feature.

A Real Example

The following is an example of the header of a simple widget class without member functions. The purpose is to show how you can utilize signals and slots in your own applications.

LcdNumber inherits QObject, which has most of the signal-slot knowledge, via QFrame and QWidget. It is somewhat similar to the built-in QLCDNumber widget.

The Q_OBJECT macro is expanded by the preprocessor to declare several member functions that are implemented by the moc; if you get compiler errors along the lines of 'undefined reference to vtable for LcdNumber', you have probably forgotten to run the moc or to include the moc output in the link command.

After the class constructor and public members, we declare the class signals. The LcdNumber class emits a signal, overflow(), when it is asked to show an impossible value.

If you don't care about overflow, or you know that overflow cannot occur, you can ignore the overflow() signal, i.e. don't connect it to any slot.

If on the other hand you want to call two different error functions when the number overflows, simply connect the signal to two different slots. Qt will call both (in the order they were connected).

A slot is a receiving function used to get information about state changes in other widgets. LcdNumber uses it, as the code above indicates, to set the displayed number. Since display() is part of the class's interface with the rest of the program, the slot is public.

Several of the example programs connect the valueChanged() signal of a QScrollBar to the display() slot, so the LCD number continuously shows the value of the scroll bar.

Note that display() is overloaded; Qt will select the appropriate version when you connect a signal to the slot. With callbacks, you'd have to find five different names and keep track of the types yourself.

Signals And Slots With Default Arguments

The signatures of signals and slots may contain arguments, and the arguments can have default values. Consider QObject::destroyed():

When a QObject is deleted, it emits this QObject::destroyed() signal. We want to catch this signal, wherever we might have a dangling reference to the deleted QObject, so we can clean it up. A suitable slot signature might be:

To connect the signal to the slot, we use QObject::connect(). There are several ways to connect signal and slots. The first is to use function pointers:

There are several advantages to using QObject::connect() with function pointers. First, it allows the compiler to check that the signal's arguments are compatible with the slot's arguments. Arguments can also be implicitly converted by the compiler, if needed.

Plc-5 2 Slot Addressing Tool

You can also connect to functors or C++11 lambdas:

In both these cases, we provide this as context in the call to connect(). The context object provides information about in which thread the receiver should be executed. This is important, as providing the context ensures that the receiver is executed in the context thread.

The lambda will be disconnected when the sender or context is destroyed. You should take care that any objects used inside the functor are still alive when the signal is emitted.

The other way to connect a signal to a slot is to use QObject::connect() and the SIGNAL and SLOT macros. The rule about whether to include arguments or not in the SIGNAL() and SLOT() macros, if the arguments have default values, is that the signature passed to the SIGNAL() macro must not have fewer arguments than the signature passed to the SLOT() macro.

All of these would work:

But this one won't work:

...because the slot will be expecting a QObject that the signal will not send. This connection will report a runtime error.

Note that signal and slot arguments are not checked by the compiler when using this QObject::connect() overload.

Advanced Signals and Slots Usage

For cases where you may require information on the sender of the signal, Qt provides the QObject::sender() function, which returns a pointer to the object that sent the signal.

Lambda expressions are a convenient way to pass custom arguments to a slot:

Using Qt with 3rd Party Signals and Slots

It is possible to use Qt with a 3rd party signal/slot mechanism. You can even use both mechanisms in the same project. Just add the following line to your qmake project (.pro) file.

It tells Qt not to define the moc keywords signals, slots, and emit, because these names will be used by a 3rd party library, e.g. Boost. Then to continue using Qt signals and slots with the no_keywords flag, simply replace all uses of the Qt moc keywords in your sources with the corresponding Qt macros Q_SIGNALS (or Q_SIGNAL), Q_SLOTS (or Q_SLOT), and Q_EMIT.

See also QLCDNumber, QObject::connect(), Digital Clock Example, Tetrix Example, Meta-Object System, and Qt's Property System.

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