A microcomputer assembled on a printed circuit board with certain functions is called a single-board computer. It is equipped with a microprocessor (MPU), a fixed-program read-only memory (ROM or EPROM), a read-write memory (RAM), a programmable input-output interface adapter (PIA and ACIA), a real-time clock, a timer, a bus buffer, a baud rate generator, and other supporting chips. Some single-board computers also have simple input-output devices, such as a small keyboard, a liquid crystal display, and a micro printer. Single-board computers can also be connected to external devices such as floppy disk drives, cassette tape drives, or dot matrix printers. The components of a single-board computer are concentrated on a single printed circuit board, allowing for the achievement of high performance with the least hardware, maximizing the characteristics of the microprocessor, and ensuring high reliability, flexibility, and convenience. It can be inserted as a component in industrial equipment or instruments. Single-board computers are generally expandable and can be assembled into larger microcomputer systems. Since their successful manufacture in 1976, they have been widely used in various industrial control fields to achieve automation and programming of equipment or instruments. Initially, they were mainly 8-bit single-board computers, and after the 1980s, 16-bit single-board computers accounted for more than half. There are many types of single-board computers with various performances, and they support up to 60 chips, with a storage capacity of 4000 to 64000 bytes, and can access up to 8 input-output devices. They have 300 instructions. 8-bit single-board computers use chips such as 8080, M6800, and Z80. 16-bit single-board computers use chips such as 8086 and Z8000.
The hardware of a single-board computer mainly consists of the following five parts: ① Microprocessor and its peripheral circuits. Usually, a simple structure, easy-to-interface, and single-power supply microprocessor is selected to reduce the number of chips. Since the number of components that a general microprocessor can directly drive is limited, additional driver circuit chips can be added. ② Memory and address decoding circuits. Single-board computers are usually used as plug-ins. Once they are finalized, the software is fixed, and only a small amount of storage units are needed. Therefore, the ratio of ROM to RAM in a single-board computer is generally 8:1 to 4:1. Usually, ROM is 4000 to 16000 bytes, and RAM is 256 to 4000 bytes. EPROM can be used first, and then mask ROM production can be carried out after the software is finalized. The RAM capacity is small, and static devices are usually used. The address decoding circuits of the single-board computer should be simplified as much as possible and have an expandable address space. A partial decoding scheme can be adopted. ③ Input-output interface adapters and their accessory circuits. Single-board computers usually have parallel external interfaces and serial communication interfaces. The parallel interface adapter is programmable, and each line can be defined as input or output by the program, which is flexible in configuration and convenient to use, and can be directly connected to external devices without the need to configure logic circuit chips. The communication interface adapter is also programmable, equipped with a baud rate generator and programmable counter/timer chips. ④ Bus and bus buffer. The single-board computer has two types of buses; one is the bus for inter-board communication, called the internal bus; the other is the bus for communication between the single-board computer and external devices or control objects, called the external bus. Currently, the popular internal buses include four standards: S-100 bus (IEEE 696 standard), Multibus bus (IEEE 696.2 standard), EXORciser bus, and Std bus. The external bus of the single-board computer can be directly output or buffered by a universal interface, or it can be made into a standard external bus. The parallel bus standard is IEEE 488, also known as HPIB interface bus, and has been widely used as an instrument standard. This bus consists of 8 bidirectional data lines, 3 byte transmission control lines, and 5 general control lines. In order to directly connect the single-board computer to the IEEE488 bus, special interface devices are required. The interface circuit uses a level conversion chip. When connecting to the telegraph machine, a current loop chip (usually in 60mA and 20mA versions) is used, and it is connected to the RS-232C standard interface through a LOOP-to-EIA conversion circuit chip. Both the internal and external buses of the single-board computer need to consider the driving capacity, and configurable buffer drivers can be used. The single-board computer generally adopts a dual-terminal wiring method; one side is the internal bus, which is compatible with a certain standard bus; the other side is the external bus, which is connected to other plug-ins or control objects. This not only makes the wiring convenient but also facilitates the implementation of isolation measures. ⑤ Monitoring program and peripheral control circuit. The single-board computer must have a monitoring program and corresponding external devices. The monitoring program is generally fixed in ROM and requires the configuration of the necessary external devices and related control circuits according to the monitoring program. The software of the single-board computer includes monitoring programs, debugging programs, diagnostic programs, assembly programs, and compilation programs, etc. In 1979, Intel Corporation launched a direct-insert expansion board, called Multi-Template, and adopted a new bus standard, InterSBX bus. After the single-board computer adopts the Multi-Template structure, various new boards can be used to expand the functions of the single-board computer, such as expanding the number of programmable I/O interfaces and increasing high-speed floating-point computing capabilities, etc. Thus, the single-board computer can be expanded into a multi-processor system.