Chem 454

corresponds roughly to lecture of January 2x

• one/Zero
• true / false
• typically 0-0.2V / 3-5 V

• we could, for example allow 10 different voltages to be present
• we'd need circuitry that could differentiate, say, between 1 volt and 2 volts
  • we'd need to avoid intermediate values
  • for example
    • a "1" must lie between 0.90 and 1.10 volts
    • a "2" would lie between 1.90 and 2.10 volts
    • and no component should allow values between 1.10 and 1.90 volts
  • we'd also need some fairly complex circuits if they behave in 10 different ways

• circuits that respond to only two values are much less sensitive to errors
• such binary circuits are simpler and much easier to build
• you need 3-4 such devices to do the work of one decimal device
  • but the four binary devices are cheaper and more reliable

• electrically, we need a wire for each bit
• with three wires we could have 8 different combinations (2³=8)
• three wires can carry a 3 bit binary number

• at any given moment the three wires contain one number
• all wires are examined simultaneously

• we'd need about 75 different combinations
• an 8 wire circuit has 256 different possibilities
• we'd need 7-8 bits (7-8 wires) to carry one character at a time

• is called a BUS
• for example, DATA BUS can transmit data (numbers, characters)

• One wire could carry a complex signal as a series of pulses
• Each pulse is a "1" or a "0"
• The bits of a number have fixed time slots
• It would take 10 timing intervals to transmit a 10 bit number
  • serial data is slower, but often avoids more complex and expensive cables and conenctors
  • it's also more compact (one wire takes the place of 10 wires)

• greatly simplifies the wiring
• needs more complicated circuits (need to watch the timing too)
• usually there are two wires (one to send and one to receive data)
• it's harder to send signals in both directions over a single wire.
  • (digital data on the telephone lines? Actually we send tones over the single wire
  • (each direction uses a different tone)
  • (actually, touch tone telephones have 12 possible tones which is really base 12 logic)
  • may actually be an optical fiber rather than a copper wire

• At the heart is a microprocessor (or computer chip)
• Virtually useless by itself
  • need to add memory (stores numbers and instructions)
  • need forms of input (keyboard, mouse, serial data port)
  • need form of output (monitor screen, disk drive, printer)
• computer itself only does very simple, primative tasks
  • rapidly and accurately
  • many simple tasks can perform a complex operation, still quite simple
• Usually an Operating System (like WINDOWS) takes control and has instructions for almost all of the ordinary activities
  • Your applications programs perform your specific computation
  • You applications program is generally written in a programming language

• computer accepts an instruction from memory
• computer performs that instruction
• computer advances to the next instruction
  • usually stored at the next memory location
• each memory cell or location is assigned an address
  • a unique numerical identifier
  • typically a big number since we have millions of memory locations
• each memory cell contains information
  • could be numerical data
  • could be a numerically coded instruction

ADDRESS BUS

• The microprocessor shares an address but with memory and many other accessories
• The microprocessor places a number on the address bus
  • all devices are designed to stay quiet unless that's its unique address
  • that way we don't need different connections to each different item

CONTROL BUS

• A small group of wires are used to tell what should be done and when
• For example, a "1" on a READ wire tells memory to send its contents (data)
• A "1" on a WRITE wire tells the memory location to receive and store data

Then we need a DATA BUS

• A group of wires that allow a binary number to be sent
• The smallest grouping is a byte or 8 bits
• the earliest Apple-II's and IBM-PC's sent data in 8 bit words
• the next generation microprocessors moved to 16, 32 and to 64 bits
• (an older machine would need 4 steps to send a 32 bit number)

• Has a microprocessor
• Has some support circuits of various kinds
• Has a large row of parallel wires forming these three busses
  • Has sockets connected to the busses
• The rest of the computer plugs into these sockets
  • A board or several boards of memory (add as much as you can afford)
  • A board that handles graphics and drives the monitor display
  • A board that controls the disk drives
  • A board that control sound
  • (Usually the output ports are built into the mother board)
• In this form a computer can be
  • customized (mix and match)
  • upgraded, part at a time
  • made by many manufactures
  • equipped with special features (by adding a special board)
    • in the lab, the ability to measure analog voltages is important
    • most PC's sold for offices and home use don't need that feature
    • (example: I brought in a board that controls stepping motors for a monochromator.)

• The PC is open architecture using boards from many sources
• Apple has always favored closed architecture
  • they made all the parts
  • they made it difficult or impossible to modify
  • their patent policy kept others from supplying alternative parts
  • they didn't really support laboratory voltage measurements

• In the 1960's a few minicomputers were available that could control lab equipment
  • The PDP-8 and PDP-12 for example
  • These were 3-4 times the size of a modern PC
  • The input and output was probably a teletype (keyboard and printed output)
  • Memory was probably 2-8K (thousands of words)
  • Cost was probably $10-20K (a great car was probably $3000 for comparison)
  • But this allowed computerized data collection and computerized control
  • Not a very widespread trend however.
  • BGSU Chemistry's first computer was a NOVA-II (1972) purchased for physical Chemsitry Lab with an NSF Grant.
• The early Apple-II's (1970's)
  • lowered the cost ($2000) but were even less powerful
  • still, you could add some voltage measuring devices
  • you could add simple data collection
• The IBM-PC (1984)
  • was the first really powerful, low cost machine (by the day's standards)
  • within a few years there were large numbers of "clones"
    • machines following the same IBM architecture
    • with interchangeable boards and components
    • using the same software
    • provided great competition, lowering prices and raising performance
• Actually there were a number of other small computers made at the same time
  • often targeted as more specialized fields, especially for controlling instruments
  • Hewlett Packard, Varian Instruments, Wang
    • the early HP Diode Array Spectrometers ran only on HP computers
  • many instruments (FT-IR for example) came with built-in computers
  • Each machine had its own hardware and programming
  • Gradually all of these vanished or were transformed into IBM-like clones
    • it was generally more economical not to build and maintain peculiar devices
    • most instruments began to use standard IBM-clones for data handling
• The Apple Macintosh came out a few years later
  • It had little impact on the lab market
  • It did however drive the IBM software into "Windows"

• it is possible to design an instrument that is controlled by its own microcomputer
• this would read a keyboard (replacing switches) and provide brief messages (replacing meters)
• often a regular PC is still used for handling the data and for asking the user what options he wants to operate today.

Computer memory (cost and availability) greatly changed computer design

• for early computers memory was very expensive (8K more memory could double the cost of an early Apple-II)

• RAM- Random Access Memory (every location can be read with equal speed)
  • compare to a magnetic tape-- to reach data at the end you need to fast forward through lots of tape
  • -- almost all computer RAM memory is also Read and Write memory
    • compare you buy recorded CD's and can't change the content
    • you can also buy recordable CD's ( write once, read many times = WORM)
    • and we now have rerecordable CD's
• ROM -- Read Only memory (data stored once and it remains)
  • EROM (also EPROM) -- Erasable Read Only memory
    • has a quartz window
    • you can erase everything with UV light
    • you can store a new set of data (P=programable)

SERIAL DATA

• The IR controller on a TV or VCR
  • light provides a binary signal
  • obviously there's only one "wire" or signal path
  • we need to transmit complex numbers one bit at a time (serial)
• You can record the signal with a light sensor and a storage oscilloscope
  • you can see a series of pulses
  • in the example here the "1" is a relative large pulse
    • both in amplitude and in width)
  • the "0" is a shorter pulse (less voltage change)
    • it is also only about half as long
  • the example is a 32 bit pulse string
    • probably half of these bits are identification
    • since you may have several IR clickers, how does the TV avoid the CD commands?
    • again, device ignores any data string that does not contain its own identification code

• We are not trying to teach you to design digital electronic circuits
• We are not trying to teach you to design, fix or critique computers
• We want you to have enough background to understand how devices work (in a general sense)
  • The limitations and the possible applications
  • The basic language of the field (computers used with instrumentation)

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