Introduction to PLC hardware- part 2


Earlier we discussed about the basics of PLC hardware .This is the continuation of that .In this session we are focussing on inputs and ouput modules of PLC hardware

You can check the part 1 here


In smaller PLCs, the inputs are usually integrated and specified when purchasing the PLC. For larger PLCs, the inputs are purchased as modules, or cards, with 8 or 16 inputs of the same type on each card. For discussion purposes we will discuss all entries as if they had been purchased as letters. The following list shows the typical ranges for the input voltages, and is more or less in order of popularity.

12-24 Vdc              100-120 Vac              10-60 Vdc           12-24 Vac/dc

5 Vdc (TTL)            200-240 Vac                   48 Vdc               24 Vac

PLC input cards rarely supply power, this means that an external power supply is needed to supply power for the inputs and sensors. The example in Figure shows how to connect an AC input card.

In the example there are two inputs, one is a normally open push button, and the second is a temperature switch, or thermal relay. (NOTE: These symbols are standard)

Connection from supply to input

Both switches are powered by the positive / warm output of the 24 VAC power supply; this is like the positive terminal in a DC source.

The power is supplied to the left side of both switches. When the switches are open, no voltage passes to the input card. If any of the switches is closed, power will be supplied to the input card. In this case, inputs 1 and 3 are used: note that the inputs start at 0. The input card compares these voltages with the common ones. If the input voltage is within a given tolerance range, the inputs will turn on. The ladder logic is shown in the figure for the inputs. Here he uses the Allen Bradley notation for ControlLogix. At the top is the label (name of the variable) for the shelf. The input card (‘I’) is in slot 3, so the address of the card is bob: 3.I.Data.x, where ‘x’ is the input bit number. These addresses can also receive alias labels to make ladder logic less confusing.


It must be a closed loop

Many beginners get confused about where connections are needed in the circuit above. The key word to remember is circuit, which means that there is a complete cycle that the voltage must be able to follow. In the Figure we can begin to follow the circuit (loop) in the power supply. The path goes through the switches, through the input card, and back to the power source, where it flows back to the start. In a complete PLC implementation, there will be many circuits that must be completed.

The importance of common point

A second important concept in PLC hardware is the common one. Here the neutral of the power supply is the common or reference voltage. In effect, we have chosen this to be our 0V reference, and all other voltages are measured in relation to it. If we had a second power source, we would also need to connect the neutral so that both neutrals were connected to the same common. Often, the soil and soil will be confused. The common is a reference, or reference voltage that is used for 0V, but the ground is used to avoid shocks and equipment damage. The floor is connected under a building to a metal pipe or a grid in the ground.

A final concept that tends to catch beginners is that each entry card is isolated. This means that if you have connected a common card to a single card, then the other cards are not connected. When this happens, the other cards will not work correctly. You must connect a common for each of the exit cards.

When deciding which type of input cards to use…..

  • The DC voltages are usually lower and, therefore, safer (ie, 12-24 V).
  • The DC inputs are very fast, the AC inputs require a longer activation time. For example, a 60Hz wave may require up to 1 / 60sec for a reasonable recognition.
  • DC voltages can be connected to a greater variety of electrical systems.
  • AC signals are more immune to noise than DC signals, making them suitable for long distances and noisy (magnetic) environments.
  • AC power is easier and less expensive to supply equipment.
  • AC signals are very common in many existing automation devices.

Output Modules

As with input modules, output modules rarely supply any power, but instead act as switches. External power supplies are connected to the output card and the card will switch the power on or off for each output. Typical output voltages are listed below, and roughly ordered by popularity.

120 Vac            24 Vdc             12-48 Vac            12-48 Vdc           5Vdc (TTL)             230 Vac


These cards usually have 8 to 16 outputs of the same type and can be purchased with different current ratings. A common option when buying exit cards is relays, transistors or triacs. Relays are the most flexible output devices. They are able to change the AC and DC outputs. But they are slower (a switching of approximately 10 ms is typical), they are more bulky, they cost more and they will wear out after millions of cycles. Relay outputs are often called dry contacts. The transistors are limited to the DC outputs, and the Triacs are limited to the AC outputs. The transistor and triac outputs are called switched outputs.

  • Dry contacts: a separate relay is dedicated to each output. This allows mixed voltages (AC or DC and voltage levels up to the maximum), as well as isolated outputs to protect other outputs and the PLC. Response times are usually greater than 10 ms. This method is the least sensitive to variations in voltage and peaks.


  • Switched outputs: a voltage is supplied to the PLC card, and the card switches it to different outputs using a solid-state circuit (transistors, triacs, etc.) The triacs are very suitable for AC devices that require less than 1 A The transistor outputs use NPN or PNP transistors up to 1 A typically. Its response time is much less than 1 ms.

In this example, the outputs are connected to a low intensity light bulb (lamp) and a Relay coil Consider the circuit through the lamp, starting with the 24Vdc supply. When the the output 07 is activated, the current can flow in 07 to the COM, thus completing the circuit, and allowing the light to turn on If the output is off, the current can not flow, and the light does not turn on.

The output 03 for the relay is connected in a similar way. When the output 03 is On, the current will flow through the relay coil to close the contacts and supply 120Vac to the motor. The ladder logic for the outputs is shown in the lower right part of the figure. The notation is for an Allen Bradley ControlLogix. The exit card (‘O’) is on a shelf labeled ‘sue’ in 2. As indicated for the entry card, it is good practice to define and use an alias label for an exit (for example, Motor) instead of using the complete description (for example, demand: 2.O.Data.3). This card could have many different voltages applied from different sources, but all the power supplies would need a single shared common.


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