Display Screen; Field Attributes; Display Screen Indicators; Cable Adapter - IBM 5251 Series Maintenance Information Manual

DISPLA V SCREEN
Field Attributes
Field attributes control how data in a field will be
displayed on the display screen. They control display
blinking, intensity, underscoring, column separation,
reverse image, and no display. The application program
controls field attributes. After power on, field attributes
will default to normal intensity, no blinking, no
underscoring, no column separators, and / or no reverse
image when the screen is refreshed. Field attributes
occupy one character space position; this position is the
first character of the field.
Display Screen Indicators
These indicators appear on the display screen. A bright
square indicates the on condition, and a line indicates
the off condition. The controller controls all the
indicators except the System Available indicator. Power
on resets all the indicators.
When the display screen indicators are on, they indicate
the following:
• System Available - indicates that the display station
has received a poll or a command. This indicator will
turn off if the display station does not receive a poll
or command within 200 ms.
• Message Waiting - indicates that a message is
waiting for the operator.
• Keyboard Shift - indicates that the display station is
in upper (numeric) shift.
• Insert Mode - indicates that the Ins key has started
an operation.
• Input Inhibited - indicates that the system cannot
process a keyboard scan code.
A cursor, which is controlled by the M PU or controller,
indicates where the next keyboard character will be
shown on the display screen.
CABLE ADAPTER
The cable adapter consists of the hardware (drivers,
receivers, and logic) on the planar board; this hardware
handles the incoming and outgoing communications
between the controller and the display station.
Note: A system cable connects the display station to
the controller. See the System Cable section in the
Maintenance Procedures section of this manual and the
Cable Thru section in the Features section.
The 16-Bit Frame
Communications between the controller and the cable
adapter at the display station are carried by 16-bit
frames that are sent to and from the controller as
needed. The system cable carries no signal between
frames.
To understand what is to follow, it is necessary to
describe briefly the phase encoding used for sending
data on the system cable. The data is phase encoded
so that a transition occurs during each bit time (a 0 to 1
transition is used for a 0 and a 1 to 0 transition is used
for a 1). Because the transition divides the bit into two
parts, the term half bit will be used in the description
that follows. (The data is sent through the system cable
in the form of a four-level signal. Refer to the system
documents for more information on the phase encoding
process.)
Theory 5251 M 1M 4-9