32.2 SPECIAL I/O MODULES
Many different special I/O modules are available. Some module types are listed below for illustration, but the commercial selection is very large. Generally most vendors offer competitive modules. Some modules, such as fuzzy logic and vision, are only offered by a few supplier, such as Omron. This may occasionally drive a decision to purchase a particular type of controller.
A wide variety of CPU's are available, and can often be used interchangeably in the rack systems. the basic formula is price/performance. The table below compares a few CPU units in various criteria.
1. PLC Software for Personal Computers - Similar to the specialized programming units, but the software runs on a multi-use, user supplied computer. This approach is typically preferred.
2. Hand held units (or integrated) - Allow programming of PLC using a calculator type interface. Often done using mnemonics.
3. Specialized programming units - Effectively a portable computer that allows graphical editing of the ladder logic, and fast uploading/downloading/monitoring of the PLC.
When dealing with lower TTL voltages (0-5Vdc) most input cards will not recognize these. These cards allow switching of these voltages.
Takes inputs from an encoder and tracks position. This allows encoder changes that are much faster than the PLC can scan.
A-B/Siemens/Omron/Modicon/etc offer human interface systems. The user can use touch screens, screen and buttons, LCD/LED and a keypad.
Thermocouples can be used to measure temperature, but these low voltage devices require sensitive electronics to get accurate temperature readings.
These cards measure voltages in various ranges, and allow monitoring of continuous processes. These cards can also output analog voltages to help control external processes, etc.
There are 2 types of PID modules. In the first the CPU does the calculation, in the second, a second controller card does the calculation.
This allows communications or networks protocols in addition to what is available on the PLC. This includes DH+, etc.