PLC Terminology

PLC Terminology

PLC Programmers

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

  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.


For communication with remote computers. This is now an option on many CPUs.

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

Takes inputs from an encoder and tracks position. This allows encoder changes that are much faster than the PLC can scan.

Human Machine Interface (HMI)

A-B/Siemens/Omron/Modicon/etc offer human interface systems. The user can use touch screens, screen and buttons, LCD/LED and a keypad.

ASCII module

Adds an serial port for communicating with standard serial ports RS-232/422.

IBM PC computer cards

An IBM compatible computer card that plugs into a PLC bus, and allows use of common software.
For example, Siemens CP580 the Simatic AT;

– serial ports: RS-232C, RS-422, TTY

– RGB monitor driver (VGA)

– keyboard and mouse interfaces

– 3.5” disk


Each card will have 1 to 16 counters at speeds up to 200KHz.
The counter can be set to zero, or up/down, or gating can occur with an external input.


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.

PID modules

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.

Stepper motor

Allows control of a stepper motor from a PLC rack.

Servo control module

Has an encoder and amplifier pair built in to the card.

Diagnostic Modules

Plug in and they monitor the CPU status.

Specialty cards for IBM PC interface

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


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

Thumb Wheel Module

Numbers can be dialed in on wheels with digits from 0 to 9.

BCD input/output module

Allows numbers to be output/input in BCD.

BASIC module

Allows the user to write programs in the BASIC programming language.

Short distance RF transmitters

e.g., Omron V600/V620 ID system
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. Messages are stored as ASCII text.

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.

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