This section explains aspects of digital input and output, from applications and types to interface circuit characteristics, in an easy-to-understand way.
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).
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.
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.
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.
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.
Output circuits for digital input and output are categorized into the following types according to application and electrical differences.
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.
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.
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.
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.
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.
Input circuits for digital input and output are categorized into the following types.
This input circuit can be connected to such contact output circuits as DC transistor outputs or DC operation circuits.
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.
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.
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.
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.
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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.
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.
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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.
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For incremental-type rotary encoders, use a "counter board" equipped with a pulse-counting function.
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