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.