Data Acquisition

Latched digital I/OFor applications that require handshaking of digital data, latched digital I/O is used. In thismode of operation, an external signal determines when the data is either input to or outputfrom the digital I/O port. The signals that are used to control the transfer of data aresometimes known as handshake lines. They are used to ensure that the digital interface isready to input digital information appearing at the input lines, sent from a remote device orinstrument, or a remote device or instrument is ready to receive data available to be sent onthe output lines of a board’s digital I/O interface. They could also provide digital control, inparticular, to switch AC or DC power relays or alarm relays, or provide the PC tremendouspower for a variety of industrial control applications.Where digital I/O lines are used to drive panel LED displays or switch AC and DC powerthrough relays, a high data transfer rate is not required. What is more important, however, isthat the number of output lines should match the number of processes that are controlled, andthat the amount of drive current required to turn the devices on/off are less than the availabledrive current from the output lines of the digital I/O interface.5.10.1 Switch sensingIn many applications, and particularly in industrial monitoring and control, switches form aprimary interface for control actions that must be initiated by an operator. Operator controlledpanel switches can be used to indicate that an action should be performed by the system.Alternatively, where switches have multiple contacts, one contact can actually perform theaction required (i.e. turning on a pump), while another contact can be used to indicate that theaction was actually initiated. The monitoring of abnormal system conditions can also be madeeasier by using limit switches to indicate that an alarm condition has been reached. In each ofthese cases, and in many other applications, the condition of the switch contact must bedetermined, requiring that the switches be interfaced and sensed by DAQ hardware.Since switches are passive devices with no power source, they must be made to emit TTLsignal levels for direct connection to a TTL compatible digital I/O interface. The open/closedposition of the switch is then deduced by the TTL logic level read at the digital input. Thiscan be carried out quite easily, as demonstrated in the two switch-sensing connections shownin Figure 5.30.In the first switch-sensing connection, a pull-up resistor connected to one side of the switchis pulled up to the supply voltage level, which is normally available from the DAC board.The open position of the switch contact is deduced by the high logic level read at the digitalinput. When the switch contact is in the closed position the digital input is connected todigital ground. This configuration has higher noise immunity and has the added advantagethat one terminal is connected straight to ground and can be grounded at a convenient pointnear the location of the switch.In the second switch-sensing configuration, a pull-down resistor is used to present a lowlogic level (digital ground) at the digital input, when the switch contact is open. When theswitch contact is in the closed position, the digital input is connected directly to the 5 Vsupply voltage. The value of the pull-up or pull-down resistor is determined by the supplyvoltage and the digital input current sink capability.