There are several ways to access data from a Siemens PLC. These different ways or methods vary depending on the model or range of the PLC in question. The following sections describe the communication interfaces that you can find on a PLC. programmable Siemens.
Serial communications
Except for one or two models, serial communications are available through an expansion module for all Siemens controllers. The serial port connects the controller to barcode readers, printers, operator interfaces and other SIMATIC controllers. RS232, RS422 and EIA-485, formerly RS-485, communication interfaces can be used to access data from Siemens PLCs via serial communications. Devices can be accessed using a variety of protocols, including Modbus ASCII and Modbus RTU in master / slave configuration.
MPI communications
MPI is a multi-node network used for programming or to communicate with SIMATIC controllers. An MPI interface is integrated in the processors of modular SIMATIC controllers. However, there are no native MPI interfaces on the S7-1200 and S7-1500, it is replaced by a profinet port.
MPI is a proprietary Siemens communication mode, used for inter-PLC data exchange. MPI type communications use S7 Basic Communication, S7 Communication and Global Data.
S7 protocol is a Siemens proprietary protocol which facilitates the transfer of data from one Siemens controller to another. The S7 protocol is a subset of Profibus DP and uses many of the same commands as Profibus DP. An adapter is however recommended in applications where you want to connect the MPI port to a Profibus network.
Communications via Profibus DP
Profibus was born out of a combined push by the German government, German companies, and other industry leaders in the late 1980s. Profibus defines a serial physical layer via RS485 with special drivers to achieve speeds around 12Mbits. A standard twisted pair cabling system is typical, but Profibus can also be implemented with fiber optics and other physical media.
Profibus allows several smart sensors to be interconnected on the same data bus. A master device allows slave peripherals to be interrogated in order to collect data or to check the status of the devices.
Profibus has two huge advantages over other sensor bus technologies. The first is the speed and the second is the size of the data. 12 Mbits is really very fast. In some applications we won’t even need to reach this degree of speed. Indeed, most electronic sensors do not consume astronomical amounts of data.However, even if we can achieve high communication speed with the profibus, the infrastructure to implement a profibus network can easily become very expensive, if we take into account the transmitters / receivers and the wiring costs .
Otherwise, the advantage of profibus is enormous. For example, if we take the data size of Profibus compared to DeviceNet, there is a nice gap. Also, Profibus has an image size of 244 bytes. compare that with the CAN bus, it’s monstrous knowing that for the CAN it’s 8 bytes.
One of the things some people hate about Profibus is the representation of data. All Profibus devices look like controller I / O racks. This means that a device is a series of slots and each slot hosts a module. This is easy to set up for simple devices like I / O devices. You can simply choose to have the first slot host a module. module that is a 16-bit digital input device and the second one houses an 8-bit digital output module and so on. If you have analog inputs / outputs as well, you can also configure the device using four slots and four modules. It’s for other non-IO devices that this is a bit inconvenient.
Communications via Profinet IO
Profinet IO is very similar to Profibus, but not really Profibus over Ethernet. Although Profibus uses cyclic communications to exchange data with PLCs at a maximum speed of 12Mbits, Profinet IO uses cyclic data transfer to exchange data with Simatic PLCs over Ethernet. As with Profibus, both the PLC and the device with which it must communicate must have a prior understanding of the data structure; in both cases the data systems are organized as slots containing modules.
Profinet IO uses three different communication channels to exchange data with programmable logic controllers and other devices. The standard TCP / IP channel is used for parameterization, configuration and acyclic read / write operations. The RT or real-time channel is used for standard cyclic data transfer and alarms. RT communication bypasses the standard TCP / IP interface to speed up data exchange with programmable logic controllers. The third channel, isochronous real time (IRT) is a very high speed channel used for motion control applications.
