each card will have 1 to 16 counters generally.

the counter can be set to zero, or up/down, or gating can occur with an external input.

High Speed Counter - When pulses are too fast to be counted during normal PLC ladder scans, a special counter can be used that will keep track of the pulses.

Position controller - A card that will drive a motor (servo motor or stepper motor), and use feedback of the motor position to increase accuracy (feedback is optional with stepper motors).

Voice Recognition/Speech - In some cases verbal I/O can be useful. Speech recognition methods are still very limited, the user must control their speech, and background noise causes problems.

Thermocouple - Thermocouples can be used to measure temperature, but these low voltage devices require sensitive electronics to get accurate temperature readings.

Analog Input/Output - 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.

- when the CPU does the calculation the PID loop is slower.

- when a specialized card controls the PID loop, it is faster, but it costs less.

Typical applications - positioning workpieces.

Programmers - There are a few basic types of programmers in use. These tend to fall into 3 categories,

1. Hand held units (or integrated) - Allow programming of PLC using a calculator type interface. Often done using mnemonics.

2. Specialized programming units - Effectively a portable computer that allows graphical editing of the ladder logic, and fast uploading/downloading/monitoring of the PLC.

3. 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 over 2.

Man Machine Interface - A-B/Siemens/Omron/Modicon/etc offer human interface systems. The user can use,

PLC CPU's - 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.

- Siemens/Allen-Bradley/ etc have cards that fit into IBM buses, and will communicate with PLC's.

IBM PC computer cards - An IBM compatible computer card that plugs into a PLC bus, and allows use of common software

Plug in and all they do is watch for trouble.

a basic method for transmission of a text based message

ID Tags - Special "tags" can be attached to products, and as they pass within range of pickup sensors, they transmit an ID number, or a packet of data. This data can then be used, updated, and rewritten to the tags by the PLC

Ethernet/modem cards for communication with remote computers. This is now an option on many CPUs

Thumb Wheel Module - this allows numbers to be dialed in on wheels with digits from 0 to 9. The values are read as BCD.

TTL input/outputs - when dealing with lower TTL voltages (0-5Vdc) most input cards will not recognize these. These cards allow switching of these voltages.

Encoder counter module - this module will take inputs from an encoder and track position. This allows encoder changes that are much faster than the PLC.

BASIC module - allows the user to write programs in the BASIC programming language.

ASCII module - adds an serial port for communicating with standard serial ports RS-232/422.

Communications - this allows communications or networks protocols in addition to what is available on the PLC. This includes DH+, etc.

36.1 SPECIAL I/O MODULES