Input/Output Devices

These are of two types: Character Devices transfer one
character of data at a time. Block devices transfer data in blocks of
characters.

There are two ways of connecting I/O devices to the rest of the system. The
first is the tree method (used in large systems). The CPU is connected
via the memory to one or more channels, each connected to one or more control
units, each connected to one or more I/O device. The second method is where
memory and I/O devices are fed by the CPU via a common bus and identified by a
unique address. Most micros use this system.

The Magnetic Tape Drive

The tape drops in two loops to either side of the drive so that it can pass
at a constant speed across the heads. The reservoirs are triggered by lights and
photo-sensors. The standard tape is 9 track, 1/2 inch wide (9 parallel tracks
running along the tape - 8 are for data and the last is a parity bit).

Tape density is defined in terms of bytes/inch (the standard is 1600 bytes
per inch, the maximum is 6250 bytes per inch). The standard tape is 2400 feet
long. Between blocks stored on tape is the inter-record gap (0.6 inches). This
is the smallest possible distance to allow the drive to stop and start again.

The Magnetic Drum

Each track is circular and has its own read/write head. A drum can't be
removed from the hardware as it is in a sealed container. Typically there are
hundreds of tracks. They are expensive devices because of the number of
read/write heads. The time taken to find any block on any track depends on:

• the time taken to select a particular read/write head (which is
  negligeable),
  
• the rotational speed of the drum (quoted in terms of the average rotational
  delay for half a revolution).

Read/write heads are prevented from
touching the drum only by the turbulent rotation air layer, so the power can't
be turned off without risking damage to the magnetic surface. On each track
there is an index point to mark the beginning ot the track.

A block of data on the disk consists of an address marker, a count area
containing the record number and the data length (and perhaps a key length) and
the actual data area. There is a gap between each of these. If a stored key is
associated with each block, the key is placed between the count and the data
(with associated gaps). With increasing block size, the drum is more efficiently
used. A block size of 8000 bytes is within 10% of the maximum.

A key is a string of characters that describes uniquely a record. The drum
drive can be given a search command to look for a particular key. There is
software which tells it which track the key is on.

Pack of Exchangeable Discs

The tracks on the discs consist of concentric circles, but they do not extend
to the centre of each disc (due to the decreasing circumference - each track
must contain the same amount of data).

Records/blocks are written exactly as on the drum. The comb moves in and out
and, if there are N recorded disk surfaces, at any position
the comb can select any one of N tracks. The delay is mostly
the time taken to move the comb (the arm access time). In terms of
storage versus cost, discs are cheaper and of greater capacity than drums.

Printers

There are various types of printers:

1. Matrix printers (typically 120 to 200 characters per second).
   
2. Slow speed, high quality printers. These are again of two types:
   
   
   
   • Daisy wheels. Typically 40 characters/second. Bold characters are achieved
     by overprinting the text once.
     
   • High quality matrix printer. Typically 40 characters/second. Uses hammer
     wires but goes over each character four times (leaving not a series of dots, but
     blacked-in lines). They also have draught quality (one pass for every character
     at 180 characters/second). Bold characters are achieved by overprinting with a
     horizontal displacement of typically 0.004 inches (giving a total of 8 passes
     for each character). Lists of characters that can be printed are stored in
     fonts.
   
   
3. Ink-jet printers. These squirt tiny droplets of coloured inks at the paper
   which dry within seconds.
   
4. Laser-jet printers. These are similar to photocopiers (quite often they are
   hybrid photocopiers/scanners/printers). A low power laser is used to charge the
   paper with a small electrical charge wherever a black dot is required. The paper
   is then passed over a toner tray containing toner (fine black powder) which is
   then attracted to the paper wherever it is charged. The toner is then fixed in
   place with a heater which melts it onto the paper.

Punched Cards

Every character was a series of 12 bits of holes or
unpunched areas in the card. The last few columns showed the position of the
card in the sequence in case the whole stack of cards is dropped - it makes
putting them back in order easier.

Paper Tape

5 or 8 channels running in parallel along the tape for
recording data by punching holes. With 5 channels, there is a letter shift,
which is a special sequence of 5 bits which enables all the characters after it
to be interpreted as letters, and figure shift, after which everything is
interpreted as numbers. Editing is done by splicing the tape or by leaving
regularly spaced blank sections which can then be filled with amendments.

Data Transmission

Full duplex transmission occurs where the
computer echoes back characters typed in from the keyboard to the screen. In
Half duplex transmission, characters typed appear on the screen via a
keyboard-screen connection only, and are sent down the line to the computer
separately.