Estimate how many discrete devices your system will have. You may also want to consider which types (AC, DC, voltage levels, and current requirements etc.) are needed.

PLCs use discrete inputs and outputs to monitor and control binary devices. For example, if a limit switch detects the presence of an object, it provides an "ON" input signal to the PLC; if no object is detected, it provides an "OFF" signal (or the lack of any signal). An example of a discrete output is a light, the PLC outputs an "ON" signal to turn on the light, and outputs an "OFF" signal (more accurately, the lack of a signal) to turn off the light.

NOTE: The values used by this selector are for natively addressable I/O. Thousands more I/O points can often be accessed by various networking operations (MODBUS 'reads' and 'writes' for example).

Why this is important: The number and type of devices your system will include is directly linked to the amount of I/O that will be necessary for your system. You will need to choose a controller that supports your I/O count requirements and has modules that support your signal types.

Estimate how many analog devices your system will have. You may also want to consider which types (voltage, current, temperature, etc.) are needed.

PLCs use analog inputs and outputs to monitor and control continuous signals which vary over time. An example of an analog input is a thermocouple (an industrial thermometer); which sends a continuously varying temperature signal in to the PLC. An example of an analog output might be a motor speed command, where a 0 volt signal commands a zero speed (stopped), ranging up to a 10 volt signal which indicates that the motor should spin at full speed.

NOTE: The values used by this selector are for natively addressable I/O. Thousands more I/O channels can often be accessed by various networking operations (Modbus 'reads' and 'writes' for example).

Why this is important: The number and type of devices your system will include is directly linked to the amount of I/O that will be necessary for your system. You will need to choose a controller that supports your I/O count requirements and has modules that support your signal types.

Estimate how many high-speed input devices your system will require.

High speed inputs are needed when input signals may fluctuate faster (or nearly so) than the scan time of the PLC. The signals can't be processed via the "normal scan" of the PLC, therefore special input hardware is needed to capture the frequency of the signals. Most PLCs provide a method for handling such signals, and the programmer can access info about the frequency in the ladder logic, without having to process each and every transition itself.

Estimate how many high-speed output devices your system will require.

High speed outputs are similarly to the high speed inputs, and for the same reason: it is not practical to program the high-speed switching in the ladder code. The programmer can stipulate a frequency, and the PLC hardware handles the actual switching, independent of the ladder logic scan. High speed outputs are often used for motion control (positioning) applications, where the frequency of the PLC output is used by a servo or stepper system to determine a desired motor shaft position, or the length of a particular "move". These signals are often referred to as "step and direction signals. Take a look at our SureStep™ stepper systems and our SureServo™ servo systems. And search on the phrase "step and direction" for more info about using PLCs as motion controllers.

AutomationDirect now offers motion controllers (PLCs) that use the EtherCAT protocol to send highly-precise motion commands over an Ethernet network (see below for additional motion control related selection criteria). With network control, the wiring complexity is greatly reduced and you may not need any high-speed outputs for motion control applications.

Choose from these options if your application requires Expansion I/O.

Serial and Ethernet-based Expansion I/O hardware are two popular options. This I/O may also be referred to as distributed I/O, and may require a particular protocol, such as Modbus.

Local Expansion I/O refers to an additional rack of I/O modules located near the main rack. Local I/O is typically required when the necessary I/O modules simply will not fit in the main rack. Response times are excellent, as Local I/O is typically updated "every scan" along with the I/O in the main rack.

Remote I/O refers to I/O modules that are housed in a separate rack, typically in a remote location (although sometimes remote I/O is installed near the main PLC rack). Remote I/O can be very advantageous, especially with large, far flung machinery, and can reduce wiring costs significantly. Response times are good, but the communications distance typically means that I/O will be updated asynchronously from the scan.

Some PLCs outputs are capable of producing a high-speed AND variable-width pulse train - also known as Pulse Width Modulation (PWM).

PWM is a method of dynamically reducing the effective power delivered by an electrical signal by dividing it into discrete pulses, which vary in width and frequency. Often used for controlling DC motors, PWM is also used to control valves, pumps, hydraulics, and other mechanical parts.

Select this option if your application requires serial communications ports.

Serial ports are used for a wide range of communication functions. From a hardware standpoint there are two popular types — RS-232, used for point-to-point serial connections; and RS-485, used for multi-drop connections to multiple devices. Serial devices may also require a particular software protocol. Common serial protocols include Modbus RTU and ASCII (both are supported by ALL AutomationDirect PLCs with serial ports).

If you have a preference for a particular PLC form factor, select it here.

AutomationDirect offers a choice of PLC form factors:

• Stackable Brick: Our CLICK and BRX PLCs can function as standalone controllers with a handful of inputs and outputs built-in to the CPU — or you can add up to eight modules, "stacked" to the right of the CPU. The P1000 is also a stackable design - but the CPU has no built-in I/O. Stackable PLCs take up only as much space as is needed for the amount of I/O required.
• Rack-based: P2000 and P3000 Productivity Series PLCs require a rack (sometime called a base), and these racks are available in various lengths depending on how many modules may be required for a particular application.

If you have a preference for a particular Memory Type, select it here.

Fixed type refers to the classic PLC memory naming convention, with abbreviations such as "I" for integer, or "S" for String, followed by a number to differentiate the various bits and words. Examples such as I34, or S105, may seem familiar to many PLC programmers.

Tagnames are a more recent method for naming the memory locations, whereby the programmer is allowed to define logical names to the variables 'at will' — even prior to determining what type of memory location is required. There are a few rules governing the selection of names, and there are many conventions that can make life much easier for the programmer. The software does automatic housekeeping of all memory locations. There is no concern about physical addressing and memory conflicts.

Tagnames also lend well to the use of structure data types. Structures (and their associated tagnames) are defined based on function rather than data type.

Structure data types:

Structure data types are specific sets of tags combined together for convenience. Using Structures in instructions can speed up program development because multiple tags are created and assigned automatically. In the Productivity Suite, for example, you simply check the "Use Structure" box and enter the name for your structure. The Productivity Suite software does the rest, creating all the tags required for that instruction.

Select this option if your application requires PID Control.

Paraphrased from Wikipedia, the free encyclopedia: http://en.wikipedia.org/wiki/PID_controller

Proportional-integral-derivative control (PID control) is a control loop feedback technique widely used in industrial control systems. A PID loop calculates an error value as the difference between a measured process variable and a desired setpoint. The controller attempts to minimize the error by adjusting the process through use of a manipulated variable.

All AutomationDirect PLCs allow programming via the USB port on your PC.

USB is a fast and easy way ("Plug and Play") to connect to a PLC and program, troubleshoot, and/or monitor your PLC application.

Select CLICK PLUS CPUs offer a built-in Wi-Fi port. ProductivityOpen may support Wi-Fi via 3rd party shields (not sold by AutomationDirect)

Wi-Fi enables a wireless Ethernet connection for installation, programming and communications to factory devices.

Select this option if you prefer the convenience of Built-in Data Logging.

While data logging is a reasonably simple feature to add to most control systems, the Do-more BRX, Productivity1000, Productivity2000 and Productivity3000 PLCs offer built-in data logging. With the Productivity1000 and 2000, a microSD card can be inserted into the front of the CPU module to store data. With the Productivity3000, a USB memory stick can be plugged into the CPU for similar functionality. For BRX, 1MB of internal RAM is available to use as data storage along with data logging to an external microSD card.

With these PLCs, the data can be retrieved by physically accessing the flash memory device, or the files can be accessed by enabling the optional built-in webserver or by sending the log files via e-mail (if e-mail is enabled for the PLC) on a periodic or on-demand basis.

Select this option if you require an embedded web server. An embedded web server allows access to data files and system tags remotely from a web browser. The secure password protected login prevents unwanted access and keeps data safe.

Select this option if you require a REST API programming interface. The HTTP-based RESTful API (REpresentational State Transfer, Application Programming Interface) will return the contents of PLC memory locations in JSON formatted records.