• discrete or Digital • analog Discrete devices are inputs and outputs that have only two states: on and off. As a result, they send/receive simple signals to/from a PLC. These signals consist of only 1s and 0s. A 1 means that the device is on and a 0 means that the device is off. Analog devices are inputs and outputs that can have an infinite number of states. These devices can not only be on and off, but they can also be barely on, almost totally on, not quite off, etc. These devices send/receive complex signals to/from a PLC. Their communications consist of a variety of signals, not just 1s and 0s. The overhead light and switch are both examples of discrete devices. The switch can only be either totally on or totally off at any given time. The same is true for the light. A thermometer and a control valve are examples of the other type of I/O devices, analog. A thermometer is an analog input device because it provides data that can have an infinite number of states. Temperature isn't just hot or cold. It can have a variety of states, including warm, cool, moderate, etc. A control valve is an analog output for the same reason. It can be totally on or totally off, but it can also have an infinite number of settings between these two states.
signals, they sometimes have a hard time communicating with the PLC. While PLCs are powerful devices, they can’t always speak the “language” of every device connected to them. That’s where the I/O modules we talked about earlier come in. The modules act as “translators” between the field devices and the PLC. They ensure that the PLC and the field devices all get the information they need in a language that they can understand.
program is a software program in the PLC’s memory. It’s what puts the control in a programmable controller. The user or the system designer is usually the one who develops the control program. The control program is made up of things called instructions. Instructions are, in essence, little computer codes that make the inputs and outputs do what you want in order to get the result you need. There are all different kinds of instructions and they can make a PLC do just about anything (add and subtract data, time and count events, compare information, etc.). All you have to do is program the instructions in the proper order and make sure that they are telling the right devices what to do and …you have a PLC-controlled system. And remember, changing the system is a snap. If you want the system to act differently, just change the instructions in the control program. Different PLCs offer different kinds of instructions. That’s part of what makes each type of PLC unique. However, all PLCs use two basic types of instructions: • contacts • coils Contacts are instructions that refer to the input conditions to the control program—that is, to the information supplied by the input field devices. Each contact in the control program monitors a certain field device. The contact waits for the input to do something in particular (e.g., turn on, turn off, etc.—this all depends on what type of contact it is). Then, the contact tells the PLC’s control program, “The input device just did what it’s supposed to do. You’d better check to see if this is supposed to affect any of the output devices.”
that is, to what each particular output device is supposed to do in the system. Like a contact, each coil also monitors a certain field device. However, unlike a contact, which monitors the field device and then tells the PLC what to do, a coil monitors the PLC control program and then tells the field device what to do. It tells the output device, “Hey, the PLC just told me that the switch turned on. That means that you’re supposed to turn on now. So let’s go!” Let's talk again about that souped-up switching circuit, in which a wall switch and an overhead light are connected to a PLC. Let's say that turning on the switch is supposed to turn on the light. In this situation, the PLCs control program would contain a contact that examines the input device the wall switch for an on condition and a coil that references the light. When the switch turns on, the contact will energize, meaning that it will tell the PLC that the condition itÕs been looking for has happened. The PLC will relay this information to the coil instruction by energizing it. This will let the coil know that it needs to tell its referenced output the light : to turn on.
the PLC control program, and changing the status of the outputs accordingly is called the scan.
• INPUT RELAYS-(contacts)These are connected to the outside world. They physically exist and receive signals from switches, sensors, etc. Typically they are not relays but rather they are transistors. • INTERNAL UTILITY RELAYS-(contacts) These do not receive signals from the outside world nor do they physically exist. They are simulated relays and are what enables a PLC to eliminate external relays. There are also some special relays that are dedicated to performing only one task. Some are always on while some are always off. Some are on only once during power-on and are typically used for initializing data that was stored. • COUNTERS-These again do not physically exist. They are simulated counters and they can be programmed to count pulses. Typically these counters can count up, down or both up and down. Since they are simulated they are limited in their counting speed. Some manufacturers also include high-speed counters that are hardware based. We can think of these as physically existing. Most times these counters can count up, down or up and down. • TIMERS-These also do not physically exist. They come in many varieties and increments. The most common type is an on-delay type. Others include off- delay and both retentive and non-retentive types. Increments vary from 1ms through 1s. • OUTPUT RELAYS-(coils)These are connected to the outside world. They physically exist and send on/off signals to solenoids, lights, etc. They can be transistors, relays, or triacs depending upon the model chosen. • DATA STORAGE-Typically there are registers assigned to simply store data. They are usually used as temporary storage for math or data manipulation. They can also typically be used to store data when power is removed from the PLC. Upon power-up they will still have the same contents as before power was removed. Very convenient and necessary!! |
| PLC Main Important Parts. |
| Important Parts Of PLC. |
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