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If you have basic knowledge in relays and what PLC is made of and how it works, it is much easier to understand the programming language that is used for it. But even without this information, these programming languages are easy to learn, it just takes practice to adapt to as many situations as possible.

Because PLCs are basically computers, the programming languages work the same as for the computer. The IT technicians are most likely to learn this, but like every language it is manageable. Online PLC Support is there always to help you out if you are stuck because there will always be situations where there is a better way to do it then you know.

Number systems

There are several number systems made use of in the human interface to programmable logic controllers. At the fundamental level, at the actual computer electronic level, everything in any computer is binary. It is a base 2 number system, 2 states 0 and 1. Therefore, decimal, hexadecimal, octal and binary coded decimal are used to interpret the binary values present in the memory of the programmable logic controller and display them on the screen in the programming software whereas they are always binary in the PLC on the graphical user interface like rslogix 500, or any programming software, you may see a decimal value, hexadecimal, but they are representing binary numbers.

The binary numbers system is second nature to us, so we will use it as a springboard into binary. To have more information, look at the decimal system where you have indexing, exponential, and positional notation. Indexing would be like an odometer. Numbers at indexing are placed from 0 to 9 in various vertical positions, so if you take the least significant digit, which would be the zero on the right, for every ten clicks of that position, the next position to the left is going to click once. So, the second position from the second least significant digit is a ten counter. The third position is going to click once for every ten clicks of the second position which clicks once for every ten clicks of the first position.

Binary digits

Binary digits or BITs, a bit is one position in a binary value. If we convert an array of relays into an array of individual microcircuits, each of which can individually be controlled into two states. In binary, the closest that we can come to 1000 as positional notation is 1024. For years now, the makers of computer memory have quantified their product in layers of memory or multiples of 1024. In recent years the industry has strayed from this convention because the extra 24 elements really began to add up when you get into mega and giga elements.

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Each bit is an actual microcircuit that is connected to the entire array of bits in such a way that it can be turned on or off and in such a way that it can be read or to say that it can be examined to determine which of the two states that it is in on or off. So, with relays, you use the normally open normally closed contacts to query the coil as to its state. In computer memory, you read or write to the bit to set at 1 or 0 and you read from it to see what its state is.

Address or location

Before there where street names and house numbers the word address was strictly a verb never a noun. When we get into programming, we use the instructions that address memory locations, much as you address a person with a question or a statement. When you make a statement addressing an individual you are setting their state. If you ask a question you are reading or determining their state.

In a relay control system, the contacts are used in other circuits to indicate whether or not the mother coil is energized. So, you could say that the normally closed contact has continuity if the relay coil is de-energized. True and false are an alternative to on and off. The normally closed contact is true if the relay is de-energized or off. The normally open contact is true if the relay coil is energized.

If we look at the one row of our relay system from 0 to 15, the layout or the actual location of a coil is defined by row and coil. Row 1 coil 0 through 15, but that is not necessarily the name of the coil. This is the physical location, not the logical location.

Relay panel

For example, take that row separately from the system and name the 15th coil 1CR. This portrays a connection between an actual input device, the relay coil, and the relay contacts. Always remember that there is no electrical connection between the input device and the relay contacts that represent their state. So, one CR represents the state of that pressure switch. The actual input devices turn relay coils on and off, and the contacts read the state of those coils. You could say that the relay coil controlled by the input device represents the state of the input device. When the pressure reaches a pressure that was set, it is said to have reached its set point. So, that pressure switch has a dial on it that you can’t see and you set that dial for a particular pressure. When the pressure is reached that particular sensor is going to close.

The pressure switch is not wired directly into the control panel logic. The state of this input passed through a device that isolates the filled voltage from the electrical circuits of the control panel.

The relay coils are data that represent the state of the field devices such as proximity switches, photoelectric switches, limit switches, conditional switches and many more. However, the contacts mechanically connected to these relay coils do not exist in the scheme of things unless you use them.