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A single-board computer system is illustrated in Fig. 9.1, with input and output units suitable for the solution of control problems. The central processing unit is a microprocessor that coordinates the control of all sequences within the system in accordance with the program stored in the program memory. A read-only memory (ROM) does not lose its contents even if the line voltage is removed. In the system in Fig. 9.1, the ROM contains a program that allows data to be entered via the keyboard and displayed on the screen. This program is a kind of mini-operating system; it is called a monitor program or simply a monitor. A RAM loses its contents when the voltage is removed. It is provided to receive programs that are newly developed and intended to solve a specific control task. The input/output devices enable the user to enter data, particularly programs, through the keyboard into the RAM and to monitor the inputs. Binary signals generated by the sensing process can be read in through the input/output devices, generating an actuating process as
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Data address control bus 0 1 2 3 4 5 6 7 Input register (port I) 0 1 2 3 4 5 6 7 Onput register (port O)
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Microprocessor
Actuators
FIGURE 9.1
Single-board computer.
output response. Registers of this type, which are especially suitable for signal input and output, are also known as ports. The system input/output units are connected by a bus system, through which they can communicate with one another. Bus is the designation for a system of lines to which several system components are connected in parallel. However, only two connected devices are able to communicate with one another at any time (Fig. 9.1).
Sensors for Input Control
A microcomputer is used for the dual-level input control of a reservoir (Fig. 9.2). The level of the liquid is kept constant within the specified limits. For this purpose, two sensors are placed at the desired maximum and minimum locations: Maximum sensing level = maximum limit Minimum sensing level = minimum limit Maximum and minimum sensors are installed to recognize the upper and lower limits. The motor is switched ON when the minimum level has been reached: Max = 0 Min = 0
Communications
FIGURE 9.2
Dual-level control.
and OFF when the maximum level has been reached: Max = 1 Min = 1 If the level is between the two levels Max = 0 Min = 1 the motor maintains the current status. The constant level is disturbed by varying the bleed-off valve. These requirements are shown in a fact table, where P represents a current status and PN represents the required pump status (Table 9.1).
Max 0 0 0 0 1 1 TABLE 9.1
Min 0 0 1 1 1 1
P 0 1 0 1 0 1
Port I I I I I I
PN 1 1 0 1 0 0
Fact Table
Nine
FIGURE 9.3
Program owchart.
FIGURE 9.4
Ports to connect sensors and actuators.
The program flowchart in Fig. 9.3 can be used as a programming model. The ports to connect the sensors and actuators are specified in Fig. 9.4. The programming model is converted into computer program statements. For this, it is necessary for the statement commands to be formulated in machine code. In turn, this makes it necessary to understand the type of microprocessor being used. The well-known Z80 type is used in the example in Table 9.2. The program appears in the abbreviated hexadecimal form. The hexadecimal counting system is based on the number 16. Like the decimal system, the hexadecimal system uses the numerics 0 to 9 and, in addition, the letters A to F, which represent the numbers 10 to 15. Addresses and commands are entered on a keyboard that is comprised of the keys for the hexadecimal alphanumerics 0 to F and various special keys. An address that has been set and the data it contains can be read from a six-digit numerical display.
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