Keyboards; Scan Codes; Break Codes; Keyboard Cable - IBM 3279 2A Maintenance Information

1.3 Keyboards
1.3.1 Scan Codes
Pressing a key generates a seven-bit scan code that
is presented serially to the control unit. The scan
code for a specific key position is a1ways the same.
The control unit translates each scan code into the
appropriate character or function.
1.3.2 Break Codes
Several keys, including the reset key, produce a
second scan code (called a 'break code') as the key
is released. The break code for the reset key is
ignored by the control unit.
1.3.3 Keyboard Cable
This cable carries the control and data signals
between the keyboard and the display unit. It also
provides the +5 V supply for the keyboard (see
Figure 6-12 for details).
1.3.4 System Upshift
System upshift cannot be selected from the
keyboard; it is controlled by the application program
and applies to all keyboard types. When the system
is in upshift mode, the control unit (3274 or 3276),
or IDPA, processes only specified characters.
Characters other than those specified may be
entered by an operator using the override capability.
1.3.5 Keyboard Type Identification
The keyboard logic card contains four pairs of pins
that must be jumpered to indicate the type of
keyboard (see Figure 6-11).
1.3.6 Keyboard Lock
The keyboard can be 'locked' (that is, keyed
characters are ignored by the control unit). Specific
data, for example a 'reset' scan code, may still be
accepted.
The operator is made aware of keyboard lock by a
change in the action of the keyboard clicker. The
clicker normally clicks once for each key depression,
but stops clicking after keyboard lock. If the
operator has chosen to turn off the clicker for
normal operation, then- keyboard lock will cause it
to click at each key depression.
An )( is displayed in the Operator Information
Area when the keyboard is locked.
3279 MIM
1.3.7 Numeric Lock (Feature)
The numeric lock feature is selected by installing a
keyboard jumper in position 1 of the keyboard 1D
jumper block. This feature limits the characters
that can be entered into a numeric field to:
o
through 9
period (.) or comma (,)
minus (-)
The DUP key remains active. Depression of any
other key while the cursor is in a numeric field
causes the keyboard to lock with the symbol
)( *
NUM in the operator infonnation area.
Press RESET to clear the locked condition.
1.4 Tools and Test Equipment
The following tools and test equipment are required
for maintenance of the 3279.
Item
Insulated probes·
General logic probe
Miniprobe**
GLP extension cable
Key top puller
Keylock retaining nut wrench
MR C test card
SLT jumper (3)
SLT pins···
Adjusting tool
Metric tools
Alignment mask
Isopropyl Alcohol
Lint-free cloth
IBM Part
Number
1749249
and
1749250
453212
453718
453605
9900373
4418787
1742659
452655
453443
1864853
1749235
4423472
2200200
2108930
For use with standard CE meter
This probe is for use on low voltage
(up to 15 volts)
For use with jumpers on some top
card connectors
Chapter 1. General Description and Location
1-9