Programmable logic control and synchronous link control are two terms used in computer programming for different types of electronics systems. The two types of control systems are both designed to make automation easier in the industries where they are typically utilized. PLC and SLC systems have central processing units and an input-output interface system. The CPU controls the processes in each but does so through the input and output system that connects to the device it controls. Aside from the general purpose of increased automation control, PLCs and SLCs have some significant differences.


Programmable logic controllers are used in applications such as computer networking, motion and process control systems, data storage and handling, and other complex control systems, such as sequential relay control and the use of distributed control systems. PLCs are often used in manufacturing to control machinery responsible for production. Synchronous link controllers can also be used in process-control applications as well as telecommunications-control systems, real-time financial systems, and even in the defense and airline industries. For instance, in the airline industry, SLCs are used in wide area networks, or WANs, to allow for simultaneous transmission and reception of data for critical airline operations. In the financial industry, this kind of technology is needed for the submission of real-time trades on stock markets where getting the most current price on an investment is essential, as is getting the trade submitted before the price changes.


The main difference between PLCs and SLCs is in terms of the type of programming used for each. PLCs are programmed using ladder logic control systems. These controllers are programmed by using external control terminals or software programs that are transferred to the controller using a network connection. In some cases, the programming logic is added to the controller with a removable microchip processor. Synchronous logic control systems tend to be somewhat more versatile in terms of the programming and editing options available. SLCs can be operated independently through multiple communications links. Whereas PLCs typically require dedicated facilities for ongoing control, SLCs use this multiple communications strategy as a way to limit the number of dedicated facilities required to maintain control of the system.


PLCs have evolved over the years to become highly functional devices used across a wide variety of industries. Because of the need for wide-ranging use of these types of controllers, programmers working on PLCs have developed portable microcontroller systems that are used to navigate and change the programming logic used in various types of electronics devices. SLCs, while versatile in terms of the number of access points, do not have the same portability. Instead, SLCs are primarily limited to mainframe systems.


The functionality and programming of PLCs and SLCs highlight one of the other major differences between the two types of controllers. Communications through SLCs can be wide-ranging in terms of the number of points of access because these controllers can be accessed through multiple ports across a network. With PLCs, access to the control and programming of the controller is limited to the number of available physical access ports. This limited access does not mean that the controllers cannot be accessed across a network, but it does limit the number of individuals who may operate those controls at any given time.