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Chapter 3.
BASIC KEYBOARD UNIT
Whenever the blinking cursor appears on the screen, the computer transfers data from the
keyboard to the screen memory.
Keyboard data ;s transferred by the interrupt routine to the screen memory each time a new
key is struck. Only after a carriage return is the keyboard data transferred to the operating
program, and then a whole line is transferred at once.
There are two exceptions to this, neither one of which causes the cursor to blink. One of them
is the use of GET, which will be discussed in a later section, and the other one is when
the keyboard data is accessed directly using machine language programs.
The PET keyboard has been optimized for use as a computer keyboard, though the organization
,
is similar to that of a typewriter so a touch typist does not feel totally out of place.
However, some important changes have been made:
1. Because of the high use of numbers and calculations with the computer, a calculator-like
number pad has been added to the right of the main keyboard.
2. The number pad has all of the mathematical operators in a form that is normal for BASIC.
3. The various keys for screen movement and editing are located on the numeric pad.
4. The characters which are normally the shift of the numbers on a standard keyboard no
longer require shifting. These characters are quite often used in BASIC, and it is
convenient to have them available without shifting.
5. All standard characters are unshifted, so that a complete 64-character graphics set is
available by use of the shift keys. These graphics give the PET significant line drawing ability.
PET
KEYBOARD
The keyboard consists of 73 keys, including two shift keys, either one of which may be pressed to cause
the upper or shifted characters displayed on the keyboard to be operational. Lower characters
are always used unless one of the two shift keys is pressed simultaneously. Each key has
a thin, transparent plastic film covering the keytop which should be removed. This protection was
left in place to protect the keys against scratching dUring shipping. To remove the film, carefully
peel it off by uSing the sticky side of a piece of masking tape so as to avoid scratching the keytops.
There are 64 printed characters on the keyboard with 64 upper case, or shifted characters on
the same keys. The rest of the keyboard consists of function characters. Some of the functions
are obvious: like character return or cursor right and left. Reverse on allows all subsequent characters to
be diplayed in reverse field· black on white.
The reverse key ;s operational on a memory basis. From time to time the key is struck, the
function is operational until it is terminated by a RETURN pressed or printed, or by pressing reverse-off
(the shifted reverse key). This concept of reversal of function, up and down, right and left is carried
through to the function keys, so that the complementary functions are usually combined, with one being
the shift of the other.
The keyboard is scanned using a 6520 PIA, a four line to ten line decoder and the interrupt
routine from the CRT controller. Each time the Interrupt occurs from the CRT, the keyboard is scanned
using a left to right scan. The keyboard is organized on a 2 x 5 row matrix with the matrix
being repeated 8 times across the keyboard. To implement noise protection and N key roll
over, the keyboard scan routine keeps the final value of the last scan in a buffer.
Until that key is released, no other keyboard scans are acknowledged unless a later scanned key
Is struck. The later scanned key is then considered to be the next key closure. The algorithm does not
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