6.Other memory devices
Generally microcomputers used to control fluid power equipment are “dedicated” systems and will only require ROM （usually EPROM）and RAM. Other means of storage are:
Discs: these are used when a lot of memory is required. The engineer may come across them when producing programs on a “development system” with discs. A development system is essentially a general purpose microcomputer with extra facilities. Disc drives are not very robust and are unsuitable for use in harsh environment.
7.Input/Output
The input/output (I/O) of a microcomputer is the part which is accessible for connecting to external devices. It will usually require an interface. There are various methods of obtaining input/output: parallel output, direct connection to the data bus, isolated input/output put, and serial input/output.
(1)Parallel port device
This is a component which provides ports or connections into or out of the microprocessor. Each port is usually of eight bits each in parallel; this is due to the configuration of components which make up the I/O devices.
A port can be treated as a whole word or a series of single bits. In some cases each bit can be used as either an input or an output as required. An example of input would be a signal from a limit switch, and for an output the signal to control the energizing of a solenoid on a valve.
Such devices are very versatile, easy to apply and wide use in engineering applications.
a.PIO, parallel input/output;
b.VIA, versatile interface adaptor;
c.PIA, peripheral interface adaptor;
d.RAM I/O, RAM input/output
(2)Direct connecting to the bus
The microcomputer can be expanded by connecting component components to the bus. Tailor-made plug-in units, say a board to switch eight solenoids, can be obtained.
(3)Isolated Input/output
Some microprocessors have locations reserved for I/O with a special set of instructions for fast access and simplified programming. The locations are not part of the normal memory.
(4)Serial port
Data are transmitted by a series of timed pulses rather like Morse code, but the receiving unit has a way of understanding what the pulses mean. It saves having a mass of wires as one wire will suffice. The rate at which these pulses are transmitted is called the “baud” rate. It would only be used where the fluid power system was some distance away from the microcomputer, because of the need to have devices to interpret the signals.
8.DC power supplies
A complete power system with microcomputer control will require various power supplied at different voltage levels. The required stability of the voltage will differ for the various elements, for example, a typical microcomputer needs a stable supply at 5V whilst a solenoid will have a moderately stable 24V DC supply or even an AC voltage supply.
Power provided by the mains needs to be converted to produce DC voltage. There are two main types of power supply which do this linear (low frequency) and switch mode (light frequency).
(1)Linear power supplies
The circuit has four main elements to convert alternating current into smooth DC supply:
a.transfer, which is used to convert AC AT MAINS VOLTAGE, SAY, 240V, 50Hz to the required voltage at 50 Hz.
b.Rectifier, which is used to convert the transformed voltage into a fluctuating DC voltage.
c.Capacity, which is used to remove the fluctuations in the DC voltage.
d.Zener diode, which may be added to provide a standard voltage.
A linear power supply will be:
a. inexpensive
b. easy and quick to construct using the simple components, it may be purpose-built to suit any particular requirements.
The drawbacks for such a system will be that:
a.transformer is large and heavy.
b.A large capacitor is needed.
c.The output can be affected by mains interference.
d.The low efficiency of about 30% means that a lot of energy is wasted as heat.
An example of the use for such a power supply is the powering of solenoid valves.
(2)Switch mode supplies
The total system is sophisticated. In simple terms, the method of operation is that the input power is first rectified and then switched on and off at about 20kHz. In effect, a high frequency source is produced, which can then be harnessed in much the same way as the linear power supply.
The characteristics of this type of power supply are that:
a.it is small and light;
b.it gives very little power loss and hence low heat generation;
c.it operates from a widely fluctuating input voltage.
The switch mode power supply is frequently used for powering microcomputers.
9.Buffers
Microcomputer input/output are normally buffered. A buffer is a small amplifier which is considered as being expendable. When excessive power surges are experienced by the microcomputer, the buffer is destroyed, thus isolating and protecting the microcomputer from damage.
The term “buffer” is also used to describe an intermediated storage file for data in memory. This is not something with the engineer will normally be involved.
Vocabulary
1.harsh a. 恶劣的
2.robust a. 稳定的
3.configuration n. 结构，组态
4.Morse Code 莫尔斯电码
5.suffice v. 足够
6.interference n. 干扰
7.fluctuation n. 脉动，波动
8.expendable a. 可消耗的
9.intermediate a. 中间的
10.adaptor n. 转换器
11.transformer n 变压器
12.rectifier n. 整流器
13.capacitor n. 电容器
14.Zener diode 齐纳（稳压）二极管
15.buffer n. 缓冲寄存器
16.come across 碰到
17.Baud rate 波特率
