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Chapter 9. Computer Interface




9.1.1   A Typical Microcomputer Configuration

A typical microcomputer system for a control application is shown in Figure 9-1. The heart of the computer is the microprocessor (CPU). There are two memory modules, one read-and-write random access memory (RAM) and one read-only memory (ROM).
TheRAM module is the working memory and serves for storage and manipulation of process operating parameters, such as measurement data, set points, and upper and lower control limits. The ROM module contains the operating system or monitor and the application program. During the execution of the application program, individual instructions will be read out of the memory, brought to the control unit, andexecuted. In case the process is optimized with an optimization algorithm, this program will also be located in the ROM. The parallel digital input/output module allows process communication with digital control elements such as relays, alarm devices, and indicator lights. The serial input/output module serves as an interface for data peripherals such as a CRT or a keyboard. It accepts parallelinformation from the microcomputer bus and converts it to serial information so that it can be sent in serial form over a communication line. This device also can be operated in reverse mode.
Figure 9-1 Typical microcomputer configuration. (From Ref. 9.)
The A/D and D/A converters process signals coming from or going to analog peripherals such as measuring instruments, sensors, andactuators. The individual modules of this computer system are interconnected with a universal bus system consisting of the data, address, and control buses. In the following sections different input/output modules for such a computer system will be described. It is very typical with many of these modules that the I/O control circuit and the interface circuit are incorporated into one module. Thisconcept often makes interfacing tasks with microcomputers much simpler than with control computers.

9.1.2   10.2.2 Parallel Input/Output Module

This module is used for parallel input and output of digital process data. The communication in a microcomputer system is done in a word-serial mode which means that for the example of the 8-bit computer, it is possible to send 8 bits ofinformation in parallel to an output device. Likewise it is also possible to read 8 bits of information simultaneously from an input device (Figure 9-2). There are several I/O devices available which are based on this concept. One module is the programmable input/output (PIO) interface from Intel, and an other one is the peripheral interface adaptor (PIA) from Motorola. Figure 9-3 shows a schematic diagramof a PIA device [1].
On the computer side, this module is connected to the data bus via eight input/output lines D0 to D7. With the help of the chip select or register select lines it is possible to address internal registers.
Figure 9-2 Principle of parallel data input.
Figure 9-3 Motorola 6820 PIA. (From Ref. 1.)
The R/W line specifies the read or write mode. TheEnable pulse EN is used to condition the PIA's internal interrupt control circuit and also for timing of the peripherals control signals. A reset line allows the device to be initialized. There are two interrupt line IRQA and IRQB one for each output channel.
On the process side there are two individual data channels. Each of which can exchange 8 bits of input or output information with theperipheral. Each channel has two control lines, for example, channel A has the lines CA1 and CA2. Over these lines status information, such as buffer full, transmission acknowledged, or send the next byte of information, may be exchanged between the interface and the device. Some parallel input/output modules contain three output channels. In order to minimize interfacing efforts for many applications, it...
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