PLC terminology


  • 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.