By knowing the number of any type of I/O lines we need and the number of lines available on a given module, the final shopping list of modules and the size of the PLC system are determined. In addition, build in at least 20 per cent extra capacity to allow to future modifications or to solve problems identified during commissioning.
Discrete I/O modules
For each input we need to determine the followings:
For each output we need to consider the followings:
* Voltage level.
* The power that PLC outputs need to switch.
* Output resistance and electrical noise can be an issue in cases where low level signals are to be switched.
* The use of a.c. outputs can often be an advantage.
- In most cases the voltage is higher, giving a reduction in current for any particular load, the consequent reduction in the wire size required giving a reduction in wiring costs.
- A second and often more important advantage is the reduction of electromagnetic interference (EMI).
Analog I/O modules
The following terms, common for analog input and output modules, used to describe their performance;
- Resolution defines how accurately the analog to digital (A/D) or digital to analog (D/A) converter within the module can represent an analog voltage as a binary number, or vice versa.
- Isolation refers to the ability of each input or output to work at voltage levels independent of the system ground.