Input module

Inputs

In smaller PLCs the inputs are normally built in and are 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 inputs as

if they have been purchased as cards. The list below shows typical ranges for input voltages,

and is roughly in order of popularity.

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 3.2 shows how

to connect an AC input card.

Note: inputs are normally high impedance. This means that they will

use very little current.

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 and

will be discussed in chapter 24.) Both of the switches are powered by the hot output of the

24Vac power supply - this is like the positive terminal on a DC supply. Power is supplied

to the left side of both of the switches. When the switches are open there is no voltage

passed to the input card. If either of the switches are closed power will be supplied to the

input card. In this case inputs 1 and 3 are used - notice that the inputs start at 0. The input

card compares these voltages to the common. If the input voltage is within a given tolerance

range the inputs will switch on. Ladder logic is shown in the figure for the inputs.

Here it uses Allen Bradley notation for PLC-5 racks. At the top is the location of the input

card I:013 which indicates that the card is an Input card in rack 01 in slot 3. The input

number on the card is shown below the contact as 01 and 03.

Many beginners become confused about where connections are needed in the circuit

above. The key word to remember is circuit, which means that there is a full loop that

the voltage must be able to follow. In Figure 3.2 we can start following the circuit (loop) at

the power supply. The path goes through the switches, through the input card, and back to

the power supply where it flows back through to the start. In a full PLC implementation

there will be many circuits that must each be complete.

A second important concept is the common. Here the neutral on 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 relative to it. If we had a second power supply, we

would also need to connect the neutral so that both neutrals would be connected to the

same common. Often common and ground will be confused. The common is a reference,

or datum voltage that is used for 0V, but the ground is used to prevent shocks and damage

to equipment. The ground is connected under a building to a metal pipe or grid in the

ground. This is connected to the electrical system of a building, to the power outlets,

where the metal cases of electrical equipment are connected. When power flows through

the ground it is bad. Unfortunately many engineers, and manufacturers mix up ground and

common. It is very common to find a power supply with the ground and common mislabeled

Remember - Don’t mix up the ground and common. Don’t connect them together if the

common of your device is connected to a common on another device.

One final concept that tends to trap beginners is that each input card is isolated.

This means that if you have connected a common to only one card, then the other cards are

not connected. When this happens the other cards will not work properly. You must connect

a common for each of the output cards.

There are many trade-offs when deciding which type of input cards to use.

• DC voltages are usually lower, and therefore safer (i.e., 12-24V).

• DC inputs are very fast, AC inputs require a longer on-time. For example, a 60Hz

wave may require up to 1/60sec for reasonable recognition.

• DC voltages can be connected to larger variety of electrical systems.

• AC signals are more immune to noise than DC, so they are suited to long distances,

and noisy (magnetic) environments.

• AC power is easier and less expensive to supply to equipment.

• AC signals are very common in many existing automation devices.