Digital I/O basic knowledge - CONTEC

This section explains aspects of digital input and output, from applications and types to interface circuit characteristics, in an easy-to-understand way.

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Contents

1. What is a Digital I/O Board?

A digital I/O board is an interface board that adds the ability to input and output digital signals in parallel to a computer. Using a digital I/O device makes it possible to monitor (read) the statuses of measuring devices as well as the relays and operation switches of various types of control circuits. In addition to controlling output for lamps, LEDs, 7-segment displays, and relays, such products can also be used as an interface for digital communication with controllers such as a PLC (sequencer).

2. Digital I/O Board Types And Applications

2-1. Photocoupler-isolated I/O type

This type provides isolation of the I/O circuit and the logic circuit inside the board using a photocoupler. Transmitting a signal (information) using light makes it possible to prevent the effects of electrical disturbances generated in the operation circuit. However, an external DC power source is required to drive the photocoupler. This is used for connecting the operation circuit to weak electrical devices using 5 to 48 VDC, such as digital switches and indicators.

2-1-1. High-speed photocoupler-isolated I/O type

This type enables faster signal transmission speeds by using a high-speed photocoupler as the isolating element. This type is used when high-speed input and output is required.

2-1-2. High-speed photocoupler-isolated I/O type with built-in power source

This internal logic circuit is equipped with an isolated DC power source. Because power is supplied to the photocoupler's drive and operation circuits, this type is useful when power cannot be provided.

2-2. Contact output type

2-3. Non-isolated I/O type

2-3-1. Bidirectional I/O type

This type features a direct connection without isolation between the logic circuit inside the board and an i8255 PPI or equivalent bidirectional I/O circuit. Using a program allows the number of inputs and outputs to be changed in 8-point units. Use this type of product when connecting to equipment that requires TTL (5 VDC) or LVTTL (3.3 VDC) bidirectional I/O.

3. Output Circuit

Output circuits for digital input and output are categorized into the following types according to application and electrical differences.

3-1. Transistor output (contact-less output)

Using a transistor that functions as a semiconductor device, this output circuit is capable of driving and opening/closing DC loads. This type of output is called contact-less output because there is no real contact involved, unlike contact output.

3-1-1. Photocoupler-isolated open-collector output (current sink type)

This output circuit is where the output transistor collector becomes the output terminal and the status becomes open. The internal logic is "ON (short): 1, OFF (open): 0." In this sink type, when the output transistor becomes "ON" (a load is operated), the current flows from the load to the output terminal. This output is used to send signals to typical weak electrical devices using 12 to 48 VDC.

• Opto-Isolated Open Collector output type (Current sink)

3-1-2. Photocoupler-isolated output (current source type and negative common type)

In this output circuit, the output transistor emitter becomes the output terminal. The internal logic is "ON (short): 1, OFF (open): 0." In this source type, when the output transistor becomes "ON" (a load is operated), the current flows from the output terminal to the load. This output is used to send signals to weak electrical devices using 12 to 48 VDC and is the preferred source type in European countries because it offers high safety concerning ground faults.

• Opto-Isolated Output type (Source output)

3-1-3. Non-isolated TTL open-collector output (negative logic)

With this output circuit, the output transistor collector becomes the output terminal, the status is open, and the input circuit side is pulled up. The internal logic is negative logic of "Low (short): 1, High (open): 0." This output is used to send signals to devices with TTL input circuits and 5 VDC devices.

• Non-Isolated TTL level Open Collector Output (negative logic)

3-1-4. Photocoupler-isolated TTL level output (negative logic)

In this output circuit, the output transistor collector becomes the output terminal. The internal logic is negative logic of "Low: 1, High: 0." This output is used when the operation circuit wiring of devices with TTL input circuits and 5 VDC devices becomes too long or to isolate such devices.

• Opto-Isolated TTL level Output type (negative logic)

3-2. Contact output (reed output)

4. Input Circuit

Input circuits for digital input and output are categorized into the following types.

4-1. DC input

This input circuit can be connected to such contact output circuits as DC transistor outputs or DC operation circuits.

4-1-1. Photocoupler-isolated input (type compatible with current sink output)

This is the input circuit with the cathode side of the photocoupler as the input terminal. This input is used to receive output signals from such sources as sink-type transistor outputs and relay switches. The internal logic is "ON (short): 1, OFF (open): 0." The input terminal is a source-type terminal that feeds an electrical current. This type is used to input signals from typical weak electrical devices using 12 to 48 VDC.

• Opto-Isolated Input type (Current sink output)
• Opto-Isolated Input type (Current sink and source output)

4-1-2. Photocoupler-isolated input (current source output-compatible type and negative common type)

This is the input circuit with the anode side of the photocoupler as the input terminal. This input is used to receive output signals from such sources as source-type transistor outputs and relay switches. The internal logic is "ON (short): 1, OFF (open): 0." The input terminal is a sink-type terminal that accepts an electrical current. This is the preferred type in European countries because it offers high safety concerning ground faults. This type is used to input signals from weak electrical devices using 12 to 48 VDC.

• Opto-Isolated Input type (Source output)
• Opto-Isolated Input type (Current sink and source output)

4-1-3. Non-isolated TTL level input (Negative logic)

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4-1-4. Photocoupler-isolated TTL level input (negative logic)

This is the input circuit with a photocoupler cathode as the input terminal. The internal logic is negative logic of "Low: 1, High: 0." This input is used when the operation circuit wiring of devices with TTL output circuits and 5 VDC devices becomes too long or to isolate such devices.

• Opto-Isolated TTL level Input type (negative logic)

5. Bidirectional I/O Circuit

5-1. Non-isolated TTL/LVTTL level input/output (positive logic)

6. Selection Hints

CASE 1 Performing relay control at the digital output board

CASE 2 Monitoring switches at the digital input board

When monitoring the ON/OFF status of switches, the number of devices usually becomes the necessary number of inputs. To monitor 20 switches, select a board with 20 or more inputs.

• ※

  The device you are using may offer such functions as alarms, resetting, and handshaking. In such instances, it is necessary to supply the appropriate number of additional inputs/outputs.

CASE 3 Controlling a 7-segment display with BCD/binary data input from the digital output board

CASE 4 Inputting BCD/binary data from the digital switches at the digital input board

CASE 5 Absolute-type (binary data output) rotary encoder

In general, the number of inputs is related to the resolution for one revolution. For example, for a resolution of 256 in one revolution, 8-bit (256=28) binary data will be output. Select a board with 8 or more inputs.

• ※

  The device you are using may offer such functions as alarms, resetting, and handshaking. In such instances, it is necessary to supply the appropriate number of additional inputs/outputs.

• 8bit (8ch) type
• 16bit (16ch) type
• 32bit (32ch) type
• 64bit (64ch) type

• ※

  For incremental-type rotary encoders, use a "counter board" equipped with a pulse-counting function.

• Incremental type Encoder (Counter board)

The term I/O means “input” and “output”. An I/O card facilitates communication with the computer by sending and receiving data. I/O is one of the three main components of computers. Basically, for every input, there’s an output. You pull up a document on your computer and use the keyboard to print it. The keyboard is the input and the printer is the output. This is a simple explanation of I/O, but it’s a good start to help explain what I/O cards are and how they work.

The ports on your laptop or desktop computer are also known as I/O ports for the same reason – you plug in a USB cord, controller, or other peripheral device, which is the input. What the device controls is the output.

 Other examples of input and output devices include:

• Disk drives
• Screens
• Alarms
• Proximity and level switches
• Solenoids
• LEDs

 So what are I/O Cards?

When you look at an I/O card, you’ll probably think it resembles a circuit board – that’s because it is! This board plugs into a slot on the motherboard of a computer so it can control a peripheral device like a controller or mouse. 

I/O cards are also called interface cards, adapters, or controllers. In addition they might be referred to as expansion cards, because they expand the capability of a computer or laptop. Of course, these cards are also used in an industrial capacity and are much larger and more complex than those used for personal computers.

How Do They Work?

I/O cards function as a bridge between a computer and an outside device – this can range from a monitor to a keyboard. The outside device transmits a signal to the I/O card which results in an action being performed.

The part of the I/O card that faces out can accept a variety of different devices and cables. While some cable inputs might be as simple as a USB plug, industrial size I/O cards may be customized to accept specially designed plugs and devices.

When an I/O card is damaged, there is a disconnect with the device, which can cause errors and downtime. Damage can occur in a variety of ways, and much like circuit boards, I/O cards can be repaired or replaced, depending upon the amount of damage they have sustained.

Types of I/O Operations

There are three groups of I/O operations, namely:

• Sensory operations
• Control operations
• Data transfer

Sensory I/O devices perform sensory operations. These devices can measure, detect, process, or simulate human senses. Depending on their intended purpose, both sensory input and output devices may allow vestibular, auditory, kinesthetic, visual, and tactile inputs and outputs.

Control I/O devices, on the other hand, receive input and output requests from the central processor and then send device-specific control signals to the device they control. They are sometimes called device controllers and require a device driver to operate. A device driver is software that directly interacts with the I/O device.

Data transfer devices facilitate the transfer of information between internal and external devices. This process is known as the I/O interface. There are three main methods through which data is transferred: programmed I/O, interrupt-initiated I/O, and direct memory access.

How a PLC Controls I/O Devices

PLC is an acronym for Programmable Logic Controller. It is a control system that monitors the state of input devices and interprets the input to control the state of output devices. PLCs are modular, meaning that you can use several types of input and output devices to suit your needs.

PLCs monitor and control I/O Devices in four steps. The first are called input scan. Here, the PLC detects the state of all input devices connected to it. It then moves to the second step, program scan, where it executes the program logic created by the user. After that, it energizes or de-energizes the output devices connected to it, controlling them in the process. Finally, it runs a series of housekeeping tasks such as internal diagnostics.

What Happens If Your I/O Card is Damaged?

Should your I/O card or module be damaged, it’s not necessary to purchase a new one, ACS Industrial services has a staff of experts who can perform a free evaluation of the card and a repair quote that is dependable and accurate. In many cases repairing a card can save you up to 80% of the cost to purchase a new one.

With more than two decades of service and repair, ACS works with most of the major brand of I/O cards and equipment, including:

• Emerson PACSystems RX3i
• Siemens
• Rockwell Automation / Allen Bradley
• Mitsubishi Electric
• Schneider Electric
• ABB
• Honeywell Process

ACS usually has repaired I/O cards back to you within 7-15 days, and if it’s an emergency that requires immediate attention, we also offer a Rush Repair Service with shipping back to you between 2-5 days.

You can find out more about our I/O repair services on our website page or call us today at 800-605-6419.

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