When selecting the brand of PLC components (Allen-Bradley, Siemens, Modicon) you must consider factors such as standards, support, cost and capability. Often times your facility, if it applies, will request a certain brand of PLC. If not, your first instinct may be to select the cheapest PLC you can find. But with cheaper PLCs you may find support is limited and the hardware may not be as robust. The downside to the bigger brand PLCs are the programming software and hardware can be more expensive. Often times you get what you pay for but there are also excellent components out there priced reasonably. You should also consider if the system is new or existing. For instance, if you are upgrading a CPU of an existing system it may be beneficial to select a processor that is compatible with existing I/O to save money. Next you must determine how environmental issues may affect your application. You should always research the product to be sure the equipment satisfies the environmental constraints or you must design the installation to meet those requirements (i.e., panel enclosures, purging, etc.).
Before ordering any equipment you should determine how much discrete and analog I/O the system will have and what voltages the field devices will have. You will need to choose a PLC model that supports the amount and type of I/O. On many systems it is a good idea to add about 20% to your I/O count for spares to cover I/O you may have missed during planning or future expandability. Another important factor in determining the type of CPU and I/O is whether the system requires any special capabilities such as high-speed counting or servo control. Planning ahead will help you select the correct processor and I/O for your specific application. It is also important to try to determine how large your program will be to determine the amount of memory is required. If the system you are programming for is small obviously less memory is required. Always consider expandability when planning your control system. A small system may easily turn into a larger system and require more memory. The location of I/O needs to be taken into account to determine if local or remote I/O can be used. Remote I/O can save money by reducing panel cost and cost of electrical installation.
Finally you should know your communication requirements. This is often determined by the type of operator interface you may want to use or components that you need to connect to such as drives and I/O. In many instances you must purchase a communication module separately. Check the availability and costs of communicating to your devices and choose accordingly. Troublshooting and commisioning the communications portion of a control system can be one of the more difficult tasks you will confront. Minimizing the number of different communication methods on a control system will generally speed the development, installation and troubleshooting time in your system. Planning ahead can save you time and money implementing your control system.
Before you can begin programming a PLC you must first select the hardware components for your control system. This hardware will include power supply, CPU, I/O (Input/Output). Many considerations must be taken into account before selecting you components such what PLC manufacturer (brand) you would like to use, memory requirements, CPU requirements, voltage level requirements (24Vdc, 120Vac, 220Vac), environmental requirements, communication requirements, etc. .