Plug-in data acquisition boards: Digital I/O boards

Digital I/O interfaces are commonly used in a PC based DAQ systems to provide monitoring and control for industrial processes, generate patterns for testing in the laboratory and communicate with peripheral equipment such as data loggers and printers which have parallel digital I/O capabilities.

The digital I/O interface of any DAQ board is that component of the board which consists of ICs capable of input or output of TTL-compatible signals. A signal is defined to be TTL-compatible if its logic low level is between 0 V and 0.8 V and its logic high level is between 2.2 V and F.5 V. Typically, the digital interface is a number of digital I/O lines grouped into ports, each port usually consisting of four or eight lines, although this is specific to the particular board used. it's most common for all the digital I/O lines of a particular port to be configured for either input or output, although there are circumstances where the direction of individual lines of a port can be configured independently. By reading from or writing to a port, the logical states of multiple digital lines can be simultaneously retrieved or set. The important parameters of digital I/O interfaces include the number of digital lines available and how many are configurable for input and output (or both), the rate at which data can be transferred on the digital lines, and the device drive capability of the digital output lines.

Many multi-purpose plug-in DAQ boards currently on the market, including A/D boards, D/A boards, and counter/timer boards, provide digital I/O interfaces with a varying number of digital I/O lines. Where the digital I/O capabilities of these DAQ boards does not meet the requirements of a specific application, or, a single digital I/O board is all that is required, then specialized and dedicated plug-in digital I/O boards are used. A typical digital I/O board is shown in ill. F.29.

ill. F.29 Typical digital I/O board block diagram

ill. F.29 shows a typical digital input/output circuit. A data acquisition card or device will often combine the inputs and outputs on the same channel. The I/O channels will do either inputs or outputs but not both at the same time. The software configures the I/O channel on the card or device as either an input or output. The voltage applied to the channel, defined as an input, is usually in the form of a ground or common. This ground biases the LED of the opto-coupler on. The LED in the opto-coupler shines on the base of the transistor and turns it on. This in turn tells the input circuitry in the card that the input is 'on'.

When the channel is configured as a digital output the outgoing ground turns on the transistor. This supplies a ground to the output channel. This ground is used by the device in the field, a relay or sold state relay, to turn something on. The chips that are used to do digital output often have either diode or capacitor snubber networks on their outputs. These built in snubber networks are not intended to replace external networks that are required on long lines.

Non-latched digital I/O:

Non-latched digital I/O is the mode of operation in which the state of a digital output line is updated immediately a digital value is written to the digital I/O port. In addition, for digital I/O lines configured as input lines, the current digital value present on the line when the port is read is the value that is returned. Non-latched digital I/O is the most common and simplest implementation used in digital I/O interfaces and is supported by all boards with digital I/O lines. The direction of the digital lines of a digital I/O port is conveniently set by software and can be changed as many times as required.

Latched digital I/O:

For applications that require handshaking of digital data, latched digital I/O is used. In this mode of operation, an external signal determines when the data is either input to or output from the digital I/O port. The signals that are used to control the transfer of data are sometimes known as handshake lines. They are used to ensure that the digital interface is ready to input digital information appearing at the input lines, sent from a remote device or instrument, or a remote device or instrument is ready to receive data available to be sent on the output lines of a board's digital I/O interface. They could also provide digital control, in particular, to switch AC or DC power relays or alarm relays, or provide the PC tremendous power for a variety of industrial control applications.

Where digital I/O lines are used to drive panel LED displays or switch AC and DC power through relays, a high data transfer rate is not required. What is more important, however, is that the number of output lines should match the number of processes that are controlled, and that the amount of drive current required to turn the devices on/off are less than the available drive current from the output lines of the digital I/O interface.

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