There are multiple things to consider when selecting an I/O device to use:
Quantity of Inputs and Outputs
The first thing to consider when selecting an I/O device are how many inputs and outputs are needed for the application in question. If few inputs and outputs are needed, then a large-scale device capable of many inputs would be a waste of time and would drain budget more than necessary. On the other hand, If many inputs and outputs are needed, then a larger device would be crucial. Depending on the application, even multiple modular devices could be better suited to handle all of the inputs and outputs.
Type of Inputs and Outputs
Once the quantity of inputs and outputs are required of the I/O device, it is time to determine what type of inputs and outputs are needed. With two primary forms of input, it is important to determine whether you will be needing an I/O device that is designed for binary inputs or variable analog inputs. These two inputs are very different and require different forms of I/O input mechanisms to function properly. Then, it is necessary to decide whether the output of the device needs to be modulated binary, unmodulated binary, or variably analog.
Type of Data Transmission
If data needs to be transferred, it needs to be determined if the device should utilize serial transmission at a higher frequency, or parallel transmission at a lower frequency with higher information density.
Once the type of input and output are determined, then the more minor details of the necessary device should be established, such as necessary frequencies, environmental conditions to which the device would be exposed, etc. Once these factors are all addressed, a small selection of I/O devices should be presented for a final choice, at which the final factor to differentiate between the choices would be the budget set aside for said I/O device.
For more detailed information about individual I/O device configurations, please refer to the Purchasing Guide for I/O Devices.