Let’s first discuss an assembly process that would place a cap on top of a bottle. The procedure to attach that cap may be by compression, meaning that the lid is pressed onto the bottle, or the lid may be twisted or turned onto the bottle. This automation process would take that bottle into some sort of piece holder while another piece holder would hold the cap. The automated process would attach that cap to the bottle in a single action, either by pressing the parts together or one of the piece holders may spin in order to twist the cap onto the bottle. This automated process produced a product that can be physically counted. Of course, before the cap was attached, the bottle was probably filled with some sort of liquid and the possible final step of this process may be to attach a label to the front of the bottle. It really doesn’t matter how many steps it takes to complete the product, the end result is a countable item. However, just a small process such as this one isn’t the only type of thing that would be considered discrete automation.

Let’s take a look at the process of making a cell phone. There are several parts that are used to create a cell phone. Those parts may include a plastic or metal case, an LCD screen which can be a touch screen or just a display, possibly a keypad if it’s not integrated as a touch screen, main circuit board, battery, etc. Now each of these components may be made on a different production line or in a different facility or even in a different country. The process of creating each of those parts would be considered a discrete automation process. Each process is creating a quantifiable or countable part. When those parts are created in separate processes, they would then be shipped or moved to another production line that would then assemble those products into the whole, complete cell phone. The process to assemble the cell phone may take the cell phone case and first install the screen. After that screen is installed, it may then be followed by the circuit board, the battery, the memory card, etc. When the cell phone is assembled, again, itis assembled using a discrete automation process. There are all of those components that were created by discrete automation processes meaning, they could count a component or part at the end of the process. Then you can take all of those countable part sand assemble them. The final product is a countable product which makes it a discrete automation process.

In discrete automation, the process is not necessarily continuous. The process of creating something via discrete automation could be started and stopped at any time. In other words, let’s say that plant A would create the cell phone case in a discrete automation process. Plant A needs to ful fill an order to produce 50,000 cases in various colors. The order is for 10,000 cases in stainless steel, 10,000 black, 10,000 white, 10,000 gold, and 10,000 rainbow tie-dye. The plant’s process can create 1000 cases an hour of a solid color and 500 an hour of a mixed or multi color. The plant has only 1 shift and that shift works 8 hour days and no weekends. So Monday, plant A starts production on the stainless steel case. The shift was able to make 6,500 cases and plan to resume production the next day. On Tuesday, the plant resumes production of the stainless steel case for the final 1500 count. If it takes an hour to change over the machine to produce a different color, the order for plant A would take at least 8 working days to complete. If however, plant A had a 24-hour shift, this order would be complete in approximately 3 days. Basically, discrete automation production of components may be started and stopped no times or multiple times. The starting and stopping of an automation process can be done in either discrete or analog automation, however, starting and stopping a discrete process is as easy as stopping a machine after the last component is produced and then starting it up again when you want to resume production. This is one the easiest forms of process automation. In a future lesson I will contrast the discrete and analog automation processes and demonstrate that with an analog process, the chore of starting and stopping is a much more difficult task. So to sum it all up, discrete automation is the production of parts that are of a quantifiable nature. That may include cell phones, soda bottles, automobiles, airplanes, toys, etc. As you know, an automobile contains many, many parts. The parts required for an automobile are also quantifiable in nature. A car requires 2 or 4 doors, measurable, 1hood and 1 trunk, measurable, 1 engine and 1 transmission, again measurable. Obviously, each of those required parts is not necessarily a single part in and of itself but an assembly of other quantifiable parts. Once all of those parts are put together, in a discrete automation process, you are left with a countable part, an automobile. This is discrete automation.

Now let’s see how the wiring will change when we utilize a Profibus-DP network. Okay, at the moment our I/O modules are situated right next to the CPU. To use Profibus-DP we can put an enclosure near the sensors in the field area and then move the I/O modules into it; then we can connect the sensors to the I/O modules.

Now, to enable data transfer between the PLC in the control room and the I/O modules in the new enclosure we are going to install an IM or Interface Module here; then we can use Profibus-DP and an RS-485 cable to transfer all data. So, previously the PLC’s I/O modules were arranged centrally but by introducing a network bus between the main controller and its I/O modules we DECENTRALIZED the I/Os, moving them to this enclosure in the field area. That’s why this type of profibus has the added suffix of “DP” or “Decentralized Peripherals”. We usually call these Decentralized Peripherals “Remote I/O.” So using Profibus-DP, instead of wiring each individual sensor, actuator or other facility to the PLC individually, we can install a set of “remote I/Os” next to these facilities in the field area and then transfer the data to the control area using a single RS-485 cable. This can decrease the cost of wiring dramatically and on the top of that, since the data transmission method here is “digital”, industrial environment noise has less of an impact on data, so data communication between the control and field areas will be more robust. But there are a number of disadvantages with Profibus-DP. For instance, because we are transferring data with a single cable, ifit malfunctions in any way we’ll lose all the data from the field facilities.

To prevent this problem, some applications have some of the main signals connected directly to the PLC and transfer the rest of the signals using Profibus-DP. With this wiring configuration, even if the data cable goes down for any reason, the signals from important facilities are not lost. This method is called hybrid. Another solution is to utilize Profibus-DP as a “redundant” network. In this method we use two “RS-485”cables to connect the remote I/Os to the PLC. In this configuration if the main cable goes down, the data transfer can be switched to the reserve cable with no problems. So to recap, “Profibus-DP” is a type of industrial network which decreases the amount of wiring required by “decentralizing” the PLC’s I/O modules.