What is a PLC?
A PLC (Programmable Logic Controller ) is a digital electronic system specifically designed for industrial applications. It uses a programmable memory to store instructions for performing logical operations, sequential control, timing, counting, and arithmetic operations, controlling various types of machinery or production processes through digital or analog inputs and outputs.
What are the functions of a PLC?
A simple application of a PLC is to convert inputs into device outputs through logic. This is similar to typing letters on a keyboard (input) and then displaying those letters on a screen (output). PLCs in industrial applications generally do the same thing. This can be a very simple task, such as when you press a button, a robot monitors your conveyor belt through sensors and makes it complete a series of complex actions.
How to choose a PLC? There is a wide range of PLCs to choose from in terms of price and complexity. How do you choose the right PLC?
The first thing to do is to determine what your PLC wants to control (output) and how you want to control it (input). Based on this information, you can determine the number of inputs and outputs you need.
In addition, there are other important factors to consider, such as power supply, user interface, communication protocol, and programming.
Common PLC Communication Protocols
The following simple descriptions will give you a quick overview of the main differences between these commonly used PLC communication protocols.
CompoBus
This communication protocol has two modes: high-speed and long-distance. In high-speed mode, the communication rate can reach 750kbps, but the maximum communication distance is only 100 meters. In long-distance mode, the communication distance can reach 500 meters, but the maximum communication rate is only 93.75kbps. This communication mode is Omron's proprietary protocol.
DeviceNet
This is a digital network used to connect simple devices. Because it provides not only communication cables but also power, it can be established more quickly. This communication protocol is used on the CAN bus. DeviceNet and Ethernet/IP both use the Common Industrial Protocol (CIP).
Ethernet/IP
Don't confuse Ethernet/IP with TCP/IP. The former is a protocol that uses Ethernet architecture to connect to various devices, while the latter is the Internet protocol. Compared to other protocols, Ethernet/IP requires a steeper learning curve due to the performance requirements of software programming.
EtherCAT
EtherCAT was originally developed by Beckhoff in Germany. EtherCAT is a real-time industrial Ethernet technology focused on short-cycle (less than 100μs) data. In an EtherCAT network, when a data frame passes through a device node, the node copies the data and transmits it to the next node. At the same time, it identifies the data corresponding to this node and performs the appropriate processing. If a node needs to send data out, it will also insert the data to be sent into the data transmitted to the next node.
RS-232C
Due to limitations in cable length and device compatibility, this protocol was typically used on older computers to connect printers, scanners, keyboards, etc. With the development of RS-422 and RS-485, RS-232C was gradually phased out, though it is still used in pre-installed systems. "C" represents the version of the RS232 protocol; prior to this, there were RS232B and RS232A. The main difference between the versions is the power supply voltage: RS232A uses 25V, RS232B uses 12V, and RS232C uses 15V. In 1962, "C" became the standard version; if the protocol does not specify a version or list it as RS-232, it is assumed to be version C.
RS-422
RS-422 uses differential data transmission, offering higher transmission rates and longer communication distances compared to RS-232C. Up to 10 nodes can be connected via RS-422.
RS-485
RS-485 was developed alongside RS-422. This protocol enables multi-point networks and allows multiple devices and drivers to run simultaneously. Since RS-485 and RS-422 use the same underlying technology, RS-485 can be used to drive some RS-422 devices.
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