1.1 SPECIAL I/O MODULES

 

• Counters,

• each card will have 1 to 16 counters generally.

• typical sample speeds 200KHz

• often allow count up/down

• 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.

 

• 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.

 

• 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,

• touch screens

• screen and buttons

• LCD/LED and buttons

• keypad to talk to PLC

 

• 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.

 

 

• Specialty cards for IBM PC interface.

- 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

• Siemens CP580 the Simatic AT

- 1 com port (RS-232C)

- 1 serial port (?)

- 1 RS-422 serial port

- RGB monitor driver (VGA)

- keyboard

- 3.5” disk

- TTY interface

- 9 pin RS-232C mouse

 

• Diagnostic Modules

• Plug in and all they do is watch for trouble.

 

• Short distance RF transmitters,

• e.g., Omron V600/V620 ID system

• a basic method for transmission of a text based message

• tags on parts carry message

• transceivers receive and transmit changes.

• 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.

 

• BCD input/output module - allows numbers to be output/input in 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.

 

• 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.

 

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