IBM 2250 3 User manual

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IBM System/360 Component Description
IBM 2250 Display Unit Model 3
IBM 2840 Display Control Model 2
This document presents detailed information about IBM 2250
Display Unit Model 3/IBM 2840 Display Control Model 2 pro-
gramming, operation, and special features. The material is
presented with the assumption that the reader has read the
IBM System/360 Principles of Operation manual, Form
GA22-6821.
The following publications may also be of interest to the
reader:
• IBM System/360 Component Description:
IBM 2250 Display Unit Model 1, Form GA27-2701.
• IBM System/360 Operating System, Graphic Programming
Services for IBM 2250 Display Unit, Form GC27-6909,
File No. 8360-03
Form GA27-2721-1

Second Edition (March, 1971)
This edition, Form GA27-2721-1, incorporates information
contained in Technical Newsletter N27-2934 pertaining to the
Display Copier attachment. This edition obsoletes Form
GA27-2721-0 and Technical Newsletter N27-2934. Changes are
indicated by a vertical line to the left of the change.
Copies of this and other IBM Publications can be obtained through IBM Branch Offices.
Address comments concerning the contents of this publication to:
IBM Corporation, Product Publications, Dept. 528, Kingston, New York 12401
© International Business Machines Corporation 1967, 1971

INTRODUCTION •
2250 OPERATIONS
General
Displays
Vectors and Points .
Characters
Light Pen.
Alphameric Keyboard.
Programmed Function Keyboard
2840 OPERATIONS WITH ATTACHED 2250's
Orders
Order Fonnat
Graphic Mode Orders .
Character Mode Orders
Control Mode Orders .
Light Pen Mode Orders
Display Regeneration .
Display Regeneration Timing
2840 OPERATIONS WITH THE CHANNEL
Interface Operations .
Commands .
Write Buffer Command
Read Commands .
Figure 1 Attachment of 2840-2/2250-3 Configuration
to System/ 360 •
Figure 2 Example of a 2250 Display •
Figure 3
Functional Sections of a 2250-3
Figure 4
Display Area Coordinate Addressing System.
Figure 5 Examples of Display Area Coordinate
Address Modification by the 2250 .
Figure 6 Character Set and Code Assignments .
Figure 7 Character Grid Coordinate System
Figure 8 Strokes that Fenn the Letter "A"
Figure 9 Character and Line Spaces
Figure 10 Fiber Optic Light Pen .
Figure 11 Alphameric Keyboard.
Figure 12
Programmed Function Keyboard
Table 1
Character Display Characteristics •
Table 2
Mode Orders.
Table 3
Displayable Vectors Per 2250-3 at a Regener-
ation Rate of 40 cps
Table 4
Displayable Vectors Per 2250-3 at a Regen-
eration Rate of 30 cps
Table 5
Displayable Characters Per 2250-3 at a
Regeneration Rate of 4-0 cps
5
7
7
8
9
10
12
12
15
17
17
17
17
20
21
23
25
26
31
31
32
33
34
5
7
8
9
9
11
12
13
13
14
14
16
11
19
28
29
30
CONTENTS
Control Commands
Sense Command
36
38
Instructions • 39
Test I/O • 39
Halt I/O . 39
Examples of 284-0/2250 Operations 39
Example 1. Displaying an Image on One 2250 39
Example 2. Nonnal Light Pen Operation - 2250 Equipped
with Programmed Function Keyboard . 41
Example 3. Alphameric Keyboard Operation 42
Graphic Design Operations 42
Entities • 42
Buffer Subroutines 43
Input by Light Pen Search 43
Input by Light Pen Tracking. 43
Feedback by Light Pen Tracking History. 43
CONTROLS AND INDICATORS. 53
Basic Operator Controls and Indicators 53
Metering . 54
APPENDIX A: HEXADECIMAL -DECIMAL CONVERSION 56
APPENDIX B: STATUS-SENSE COMBINATIONS 61
GLOSSARY
INDEX
Figure 13
Figure 14
Figure 15
Figure 16
Figure 17
Figure 18
Figure 19
Figure 20
Figure 21
Figure 22
Table 6
Table 7
Table 8
Table 9
Table 10
Table 11
Table 12
64
65
ILLUSTRATIONS
Functional Sections of the 2840-2 and a
2250-3
18
Read Manual Input Command Response . 35
Programmed Function Keyboard Overlay
(Top View)
36
Buffer Program Example Display
40
Sequence of Routines in Light Pen Tracking
History Programming Example •
47
Flow Diagram of Example Program
49
Point Pattern Used in Example Program
51
Vector Pattern Used in Example Program
52
2250-3 Operator Controls and Indicators .
53
2840-2 Operator Panel
54
TABLES
Displayable Characters Per 2250-3 at a Regen-
eration Rate of 30 cps 30
Status Byte Bit Assignments • 32
Commands Used with the 284-0-2 33
Sense Bit Assignments 38
Buffer Program Example 41
Sample Entity Routine
42
Feedback by Light Pen Tracking History,
Example Routine 44

IBM 2840 Display Control, Model 2

The IBM 2250 Display Unit Model 3 (Frontispiece)
is a cathode-ray-tube (CRT) display console which
can be attached to a System/360 via the IBM 2840
Display Control Model 2. Along with the capability
of displaying graphic or alphanumeric information,
the 2250 offers man-machine interaction through its
light pen and two keyboards. Using these facilities,
a programmer can furnish computer-aided design
capabilities whereby the 2250 user can create,
modify, and add graphic and alphanumeric data into
the system through the display screen. This exten-
sion of the System/360 data processing power is
useful (1) for handling the graphic information asso-
ciated with scientific and engineering applications,
and (2) for providing faster and more effective
retrieval and graphic expression of management and
business operating data.
Programming requirements for the 2250 differ
from other I/O devices in that the 2840 Display
Control has a buffer with logical capabilities that
require programming. A buffer program consisting
of buffer orders and data to be displayed can be
developed either in final image form or as a frame-
work to accept data (to be provided at object time)
by the CPU program and to be transmitted via a
channel to the 2840-2. Under control of this buffer
program, the 2250-3 displays graphic images in the
form of lines, points , and alphameric characters.
Using the logical capabilities of the buffer and the
light pen, programmed function keyboard, and
alphameric keyboard, the programmer can design
and implement his own tracking, sketching, or dis-
play manipulation techniques with minimum CPU
interaction.
A buffer program consists of orders interleaved
with data. The three major groups of orders are
Graphic, Character, and Control. When decoded,
an order sets a mode of operation which will be in
effect until another order is encountered. All data
in between is processed in that mode. Hence, an
order requesting absolute vectors will put the
2250/2840 in Absolute Vector mode, and all following
data will be treated as the absolute X, Y end points of
vectors to be displayed until the next order is en-
countered. Available in conjunction with this basic
principle is the ability to control light-pen responses
in the 2840/2250. These orders can condition the
2840/2250 to accept light-pen detects until another
light-pen order resets the condition. A light-pen
detect on any displayed information between these
two orders is then accepted and passed on to the
program for processing. In addition, immediate
action orders allow direct transferring, movement of
INTRODUCTION
data and/or addresses, and storage of the deflection
registers into a buffer location.
Each 2250-3 can operate up to 2,000 ft. from the
2840-2, allowing access to the computer from the
user's normal working area. Furthermore, sharing
of the common control unit (the 2840-2) by several
2250-31s results in more economical configurations
for the multiconsole environment.
The 2840-2 can control the operation of up to four
2250-3 Display Units (Figure 1). Light-pen tracking
can be performed simultaneously by the user of each
2250 with no interference to System/360. Attachment
of the 2840-2 to System/360 (CPU) and CPU main
storage is via either a selector or multiplexor chan-
nel; it uses one of the eight control unit positions on
the channel interface. The channel provides the
2840-2 with the data to be displayed and with the
control information necessary to direct the operation
of the 2840-2 and associated display units. Buffer
storage in the 2840 stores digitally coded images for
each attached 2250-3. The buffer enables image
regeneration as well as message composition from
the 2250 alphameric keyboards; this allows the
2840-2 and attached 2250-3's to operate concurrently
with the computer system, freeing the CPU and the
channel for other functions. Buffer storage areas
are program-assignable for any attached 2250-3 and
can be varied under program control. The 2840-2
controls the operation of each attached 2250-3. By
means of shared circuitry and interleaved operations
in the 2840-2, each 2250-3 can be operated independ-
ently, and different images can be generated simul -
taneously on each display.
The basic 2250-3 (without special features) pro-
vides the ability to display graphic information in
System/.360
r----
2250
Model 3
One 2840
can control
up to four
2250 Model 3
Display Units
2250
Model 3
Up to 2000 feet
IJp to 2000 feet
; Selector or Up to 7
I Multiplexor !--------------(additional
I Channel control units
Figure 1. Attachment of 2840-2/2250-3 Configuration to System/360
Introduction 5

absolute or incremental mode in the form of points
or straight lines in any direction; it can also display
alphameric characters (alphabetics, numerics, and
special symbols). In addition, a light pen is provided
with the basic 2250-3. For increased intercommu-
nication between the user and the controlling program,
two keyboards are available as special features:
Alphameric Keyboard - Provides a typewriter-like
keyboard with which the user can perform editing
functions and compose messages consisting of letters,
numbers, and/ or special symbols for entry into 2840
buffer and CPU main storage.
Programmed Function Keyboard - Provides com-
munication between the user and the computer. The
keyboard consists of keys, indicators, and sensing
switches for use with replaceable descriptive over-
lays. The function of each key and indicator, which
is program-defined, is identified to the program by
the overlay coding and to the user by symbols on the
overlay. The program associated with the overlay
code and the selected key then directs the requested
operation. For example, as a result of a key de-
pression, the program might direct the computer to
enlarge, reduce, or delete the image displayed by the
associated 2250.
When a 2250 is not in the same room with the 2840
and the CPU, a telephone should be near the 2250 so
that the 2250 operator can communicate with the CPU
installation.
The basic 2840-2 contains a 32, 768-byte core
buffer, order mode control, and a character gener-
ator. The character generator can translate one
System/360 eight-bit byte representation from an
6
alphameric character into a sequence of signals
which, when converted to analog deflection signals
by the 2250-3, cause the character to be drawn on
the 2250-3 CRT display area. A standard character
set of 63 alphabetics, numerics, and special symbols
is provided; two character sizes are program-
selectable. The basic 2840-2 can attach to, and con-
1
trol, two 2250-3's. A special feature, Display
Multiplexer, is available for increased 2840-2
attachment capability:
Display Multiplexer -Allows attachment of two
additional 2250 Model 3 Display Units to the 2840-2.
A maximum of one display multiplexer feature can
be installed on one 2840-2, allowing attachment of
up to four 2250-3's.
Display Copier Attachment - Permits attachment
of an IBM 2285 Display Copier to the 2250. The
publication Component Description, IBM 2285
Display Copier, Form GA27-2730, contains a
functional description of, and operator procedures
for, the 2285. The 2285 is a free-standing, non-
programmed device; it provides, under 2250
operator control, 8-1/2 by 11-inch paper copies of
the 2250 display image. Each copy consists of a
black image on a light gray background. The 2285
is located beside the left edge of the 2250 reading
board; in this position, the 2285 controls, indicators,
and hopper (copy receptacle) are easily accessible
to the 2250 operator. Basic power for the 2285 is
provided by the 2250. Analog signals are switched
from the 2250 to the 2285 during the paper-exposure
portion of each copy cycle.

GENERAL
Each 2250-3, under control of a 2840-2, generates
images on the 12-inch by 12-inch usable display
area of a 21-inch cathode-ray tube (CRT). An image
can be composed of straight lines (vectors) , points,
standard characters (in two sizes), and special char-
acters formed with vectors and points (Figure 2).
A visible display is produced when an el ectron
beam in the CRT strikes the phosphor-coated CRT
screen, causing the portion of the coating struck by
the beam to glow briefly. Normally, the glow fades
within a fraction of a second, too soon for the human
eye to carefully perceive and identify the image. For
this reason, the display must be redrawn continuously
Figure 2. Example of a 2250 Display
2250 OPERATIONS
(regenerated) at a rate that will cause the display to
appear steady and stationary to the observer. Re-
generation is performed automatically, under control
of a program in the 2840. The 2840 accomplishes
regeneration by continuously retransmitting control
and display data to the 2250; this data can be modified
during regeneration by the 2840, as directed by the
buffer program and/or the CPU program, to update
or change the display. The 2250 also performs
various nondisplay services for the user by providing
the interface between the user and the problem pro-
gram with the following devices:
1. Programmed function keyboard . Provides
keys and overlays for user communication
2250 Operations 7

to the program and indicators for program
communication to the user.
2. Alphameric keyboard. Enables the user to
change, edit, or create character displays.
3. Light pen. Supplies the buffer address of
a vector, point, or character at which the
user is pointing a pen-like device. This
information can be used for operations as
determined by the program, by the alpha-
meric keyboard, or by the programmed
function keyboard. The light pen thus
enables the user to enter and manipulate
graphic information.
4. Audible alarm (single-stroke buzzer).
Enables the program to inform the
operator that action is required.
2840 Via Interface
Control to Channel 2840
Buffer 2840 Line Buffer
Reg
,------A---,
'
Indicator Data
Stroke Codes
Character
Stroke
Deflection
Character and
Cursor Keys
END and
CANCEL Keys
Audible
Alarm
Notes:
key,
overlay
codes
r --- ,
I
Programmed I
I Function I
: Keyboard I
L _____ .J
1. Dashed blocks represent
special feature functions.
2. Heavy lines represent
data flow.
r --- 1
: Alphameric I
I Keyboard I
L--- __ J
Figure 3, Functional Sections of a 2250-3
8
Intensity
The functional sections of the 2250 are shown in
Figure 3. The functions represented by solid blocks
are provided in the basic units, whereas those re-
presented by dashed blocks are available as special
features. Heavy connecting lines represent data
flow; the light lines represent control signal routing.
DISPLAYS
Information positioning on the 2250 display area is
controlled by a display program resident in the 2840
buffer. This buffer program is prepared by the main
CPU and is sent to the buffer via a standard I/O
channel operation. The program specifies electron
beam deflection to horizontal (X) and vertical (Y)
coordinates on a virtual square grid composed of
Coordinate
Data
Beam Bits
Main
Deflection
Assembly
Control
Various I
~---'
X-Y
Coordinates
Operator
Panel
2840 Control
,----A---,
•
Light Pen
Detect
light
Pen

possible electron-beam~deflection end points. This
grid covers (logically) the 12-inch by 12-inch display
area on the face of the CRT; it comprises 1, 024
equally spaced X positions and 1, 024 equally spaced
Y positions (Figure 4).
Positioning data in the display program selects
the X and Y coordinates for each element of a 2250
display (each point, line end point, and character
area centroid). This same data can also control the
IBM 2280 Film Recorder. The grid of addressable
coordinates for a 2250 or 2280 device is called its
"raster". The space between two sequentially ad-
dressable lines on the raster is called a raster unit.
A 2250 raster unit represents 1/1, 023 of the image,
whereas a 2280 raster unit represents 1/4,095 of the
same image; this reflects the difference in address
resolution.
The data format in the 2840 provides for the
4, 096-by-4, 096-position grid of the 2280 and 2282
film units. The 2250 maintains program compati-
bility with the film units by disregarding the two
low-order bits (binary) of the 4,096-by-4,096 X and
Y coordinates. For example, a binary configuration
1111 1111 1111 (4, 095 decimal) in the 2840 is inter-
preted by the 2250 as 1111111111, or 1,023 decimal.
Thus, each display element is positioned by the 2250
at a set of 1,024-by-1,024 2250 coordinates that are
virtually equivalent to the set of 4, 096-by-4, 096 film
unit coordinates. The maximum shift in the image
caused by this conversion is three-fourths of a 2250-
raster unit (three 2280 raster units), a shift that is
not noticeable to the user (Figure 5) .
1023 (FFF)-----------------.
X=0256 (400)
y 0512 (BOO)------, Y~0512 (BOO)
Axis
0256 (400)
012B (200)
0064 (100)
0000 (000)1-1-.....--..---..l~----r--------t
0000Io 12B 0256
(OOOll (200) (400)
0064
(JOO)
Note:
0512
(BOO)
X Axis
Numbers in parentheses are hexadecimal equivalents of
1023
(fFf)
the coordinates as they appear (in 12-bit farm) in the 2840 buffer.
Figure 4. Display Area Coordinate Addressing System
2250
Coordinates~
\ 0255
2048 (BOO)
0000+---+-~---t
0000 1028 2048 4095
(404) (ROO) (FFF)
0256 0257 025B 0259
0514 L.L..Lil-l-...J-l-l-..l-l-1-+4-l-+4-1--< 2056 (BOB)
y
Notes:
LL...l_Jl-l--~-1--1--1-l----l-4-l----l-4[/I-+--' 2054
0513 ~-l-l>--!--l-l-++--l-+-+-1-+-+-1~_,._. 2052 (B04)
0512 ~4->--!--1-1>--i-~H-+-l-++-l-i-< 204B (BOO)
0511 L..l.._l_L.-A_l_l-l-...l_Jl-l--1--1-1--1--1~ 2044 (7FC)
0510 a-'11"4---1-4-1-+4-l-+-+-+-.i-+-+-+-4-t 2040 (7F8)
1020
(3FC) 1024
(400) 1028 (404)
,,..___ _____ x 1032
(408) 1036
(40C)
~Film Unit
Coordinates
1. X Position specified by '.1840 duto.
2, e .. Position selected by 2250 from this data.
3. Numbers in parentheses are hexcidecimol equivalents.
Figure 5. Examples of Display Area Coordinate Address Modification
by the 2250
NOTE: Subsequent use of the term "raster unit" in
this publication refers to 1/1,023 of the image.
Also, this 1,023-by-1,023-raster unit grid is called
the "reference" grid.
Vectors and Points
During vector or point display operations, position-
ing data from the 2840 directs electron beam move-
ment (deflection) on the 1,023-by-1,023 raster unit
display area. The 2840 first sets the 2250 mode of
operation (in this case, to display vectors or points).
It then transmits a set of positioning data to the 2250
for each vector or point to be displayed.
Each set of positioning data addresses one X, Y
coordinate to which the electron beam is to be re-
positioned. Beam deflection is always from the
previously addressed coordinate, where the beam is
currently positioned, to the new coordinate. If
vectors are specified by the 2840, the beam is turned
on as it is being repositioned, displaying a line be-
tween the current position and the new coordinate
specified; if points are specified, the beam is turned
on after it has been repositioned, displaying a point
at the new coordinate. Points plotted 4 or more
raster units apart can be distinguished by the user
as distinct points.
2250 Operations 9

The 2250 can also "position" the electron beam
without causing a visible line or point to appear on
the display. This capability is used (1) to select a
starting location for displaying characters and (2) to
start the display of a new set of vectors. Each set
of positioning data from the 2840 contains a beam
control (blanking) bit, which specifies whether the
2250 is to display (unblank) or is not to display (blank)
the resulting vector or point.
Control signals from the 2840 specify not only the
type of operation (vector or point) to be performed
but, also, a coding format for positioning data that
will be used during the operation. Positioning data
can be in either of two basic coding formats, absolute
or incremental.
Absolute positioning data specifies the actual X,
Y coordinates to which the beam is to be deflected.
Each group of four eight-bit absolute data bytes
addresses one coordinate on the reference grid (i. e. ,
x = 0512 ' y = 1016).
Incremental positioning data specifies the amount
and direction of beam deflection relative to the cur~
rent beam position. Each pair of eight-bit incre-
mental data bytes specifies one increment (up to
X = +63 or -64 , Y = +63 or -64, a displacement of
0. 74 inch) of beam deflection. For example, if the
current beam position on the reference grid is
X = 0512, Y = 1016, and if a pair of incremental data
bytes specifies X =+20, Y = -40, beam deflection
will be to position X = 0532, Y = 0976 on the refer-
ence grid. Thus, the ±X, ±Y incremental value is
added to the absolute value of the current beam posi-
tion, resulting in a new absolute value for the new
beam position.
When incremental data causes the beam to move
outside the reference grid area but when a total dis-
placement of 1,023 raster units beyond the perimeter
in the X or Y direction is not exceeded, the vectors
and/or points so displaced will be blanked. The X,
Y deflection registers will contain the value of a
wrap-around position. Unless the displacement
limit of 1,023 raster units is exceeded, the displaced
beam can be returned to the normal grid area; then,
displaying will resume when positioning data specifies
an unblanked deflection that is entirely within the
normal display area.
When a portion of a display is blanked because
of a beam displacement condition, the 2840 program
can specify an absolute positioning operation to the
2250,which will reset the displacement blanking con-
dition. However, if the first absolute data bytes
received by the 2250 following this positioning
operation specify an unblanked vector, a line will
be drawn from the wrap-around position to the
specified location.
Electron beam deflection to the previously ad-
dressed coordinate can still be in progress when
10
the next coordinate data is received. When the
deflection currently in process is completed, the
blanking bit is sent to the intensity control section,
and the new X, Y coordinates are sent to the main
deflection section.
The main deflection section applies X and Y
analog values for the current beam position to the
deflection coil of the CRT until new positioning data
is received. When the new data is received, the
analog values start changing to reflect the new
position. As the analog values change, the beam
moves, causing the image to be displayed. If the
blanking bit specifies a blanked vector or point, the
beam moves without being displayed. If the blanking
bit specifies an unblanked vector or point, the
electron beam is deflected and unblanked, as
required, to form a vector or point as previously
specified by the 2840.
The X, Y position registers in the main deflection
section always contain the absolute X, Y address of
the current beam position in digital form; the 2840
can retrieve this data and the blanking bit, rec on-
structing the most recent positioning data.
Characters
A standard set of characters can be displayed in
either of two sizes by a 2250; this set consists of
63 alphabetics, numerics, and special symbols
(Figure 6) . Any characters that are not in this set
can be created with vectors and/or points.
In Character mode, the X, Y coordinate (on the
1, 0 24-by-1, 024 reference grid) at which the electron
beam is currently positioned becomes the center
point of a basic-size or large-size character area.
The 2840 specifies the character size, which is
maintained throughout one Character mode operation.
The beam must be positioned by the program to a
starting coordinate by a blanked point or vector be-
fore a character display operation is started. The
character area is divided into a grid format of 8X-
by-8Y addressable points of which 7X and SY are
used (Figure 7). Character grid points do not coin-
cide with the 1, 024-by-1, 024 main deflection grid
points. Characters are drawn in this area with a
series of high-speed deflections, or "strokes". An
average of six such strokes is required to form one
character. Each stroke end point is specified by
an X, Y character grid coordinate sent from 2840 to
the character deflection section (Figure 3). This
section converts each coordinate to X and Y analog
signals, which are applied to the high-speed char-
acter stroke deflection coil of the CRT.
The main deflection system and the character
deflection system operate independently. The main
deflection system maintains the current beam posi -
tion (the center point of the character grid) by