INPUT/OUTPUT DEVICES FOR DISCRETE DATA
Discrete data can be processed by a digital computer without needing the kinds of conversion procedures required for continuous analog signals. As indicated earlier, discrete data divide into three categories: (a) binary data, (b) discrete data other than binary. and (c) pulse data. The first two categories are communicated between the process and the computer by means of contact input and contact output interfaces, while pulse data are entered into and sent from the computer using pulse counters and pulse generators.
Contact Input/Output Interfaces
Contact interfaces are of two typesinput and output. These interfaces read binary data from the process into the computer and send binary signals from the computer to the process, respectively. The terms input and output refer to the computer.
A contact input interface is a device by which binary data are read into the computer from some external source (e.g., the process). It consists of a series of simple contacts that can be either closed or open (on or off) to indicate the status of binary devices connected to the process such as limit switches (contact or no contact), valves [open ur closed), or motor push buttons (on or off). The computer periodically scans the actual status of the contacts to update the values stored in memory.
Ihc contact input interface can also he used to enter discrete data other than binary, This type of data i~generated by devices such as a photo~lectr.ic sens~H array and can be stored in a binary register consisting of multiple bits. The individual bit values (Oor 1) can he entered through the contact input int~rfacc.ln effect, a certain number of cont~cts in the mput interface are assigned to the binary register, the number of contacts being equal 10 the number of bits in the register. The binary number can be converted to a conventional base 10 number as needed in the application
The wntaCf output interface is the device that communicates on/off signals from the computer to the process. The contact positions are set in either of fWO states: ON or apr.
These positions are maintained until changed hy the computer. perhaps in response 10 events in the process. In computer process control applications, hardware controlled by the contact output interface include alarms, indicator lights (on control panelsj.solencids. and constant speed motors. The computer controls the sequence of ON/OFF activities in a work cycle through this contact output interface.
The contact output interface can be used to transmit a discrete data value other than binary by assigning an array of contacts in the interface for that purpose. The 0 and I values of the contacts in the array an: evaluated as a group to determine the corresponding discrete numbcr.Jn erreet, this procedure is the reverse of that used by the contact input interface for discrete data other than binary.
Pulse Counters and Generators
Discrete data can also exist in the form of a series of pulses. Such data is generated by digital transducers such as optical encoders. Pulse data are also used to control certain devices such as stepper motors.
A pulse counter is a device used to convert a series of pulses (call it a pulse train, as shown in Figure 4.1) into a digital value. The value is then entered into the computer through its .nput channel. The most common type of pulse counter is one that counts electrical pulses. II is constructed using sequential logic gates, called flipflops. which are electronic devices that possess memory capability and hence can be used to store the results of the counting procedure
Pulse counters can he used for both counting and measurement applications. A typical counting application might he to add up the number of packages moving past a photoelectric sensor along a conveyor. A typical measurement application is to indicate the rotational speed of a shaft. One possible method to accomplish the measurement is for the shaft to he connected to an optical encoder. which generates a certain number of electrical pulses for each rotation. To determine rotational speed, the pulse counter measures the number of pulses received during a certain time period and divides this by the time period and by the number of pulses in each revolution of the encoder.
A pulse generator is a device that produces a series of electrical pulses whose total number and frequency are specified by the control computer. The total number of pulses might be used to drive the axis of a positioning system. The frequency of the pulse train, or pulse rate. could be used to control the rotational speed of a stepper motor. A pulse generator operates by repeatedly closing and opening an electrical contact, thus produoing a sequence of discrete electrical pulses. The amplitude (voltage level) and frequency are designed tu be compatible with the device being controlled.