1. What's in a Terminal?
An Architectural Comparison
Richard Thomson
https://ComputerGraphicsMuseum.org

2. A Brief History of Electrified Serial Communication
• Telegraph (~1840s)
• Teleprinter (~1900s)
• Teletype (~1930s)
• Dumb Glass TTY (~1960s)
• Discrete Logic Terminals (~1970s)
• Microprocessor Terminals (~1980s onward)
All dates are approximate for the purposes of discussion and not
meant to indicate historical firsts

3. Telegraph (~1840)
• From the Greek: "write at a distance"
• Two symbols (dot and dash, or dit and dah)
• Presence or absence of electric current
• Introduced the terminology of "mark" and "space"
• Symbols combined into groups to encode glyphs (characters)
• Morse Code character encoding
• Transmission speed limited by human operator
• Baudot (5-bit) character encoding established (~1870s)

4. Teleprinter (~1900)
• Receive the message electronically and print directly
• Eliminates the need for a human operator to interpret the signal
• Transition to encoding messages on paper tape
• Messages are transmitted and received in a machine friendly signal
• Operators key in messages into punched paper tape
• Keyboard message punch and message printer are separate machines
• Electricity provides the power, but mechanisms provide all the
interpretation of signals
• Baudot character encoding

5. Teletype (~1930)
• Teletype combines the keyboard, printer and paper tape
mechanisms into one machine
• Sometimes the paper tape mechanisms are omitted
• Many variations of mechanisms and combinations of equipment
tailored to the needs of different customers and scenarios
• Electric powered, but mechanically driven
• Baudot character encoding

6. Dumb Glass TTY (~1960s)
• Teletypes are reliable workhorse machines, but...
• They are noisy
• Long-running monitoring requires a constant supply of paper
• They aren't a best fit for interactive computing
• ASCII (7-bit) character encoding introduced in 1963
• First generation of CRT terminals
• Simple discrete control logic to manage the display of text on the screen
• Promoted as quieter replacements for noisy teletypes
• Baudot, ASCII, EBCDIC character encodings

7. Discrete Logic Terminals (~1970s)
• Users desire more control over the screen:
• Erase characters, lines; insert characters, lines; multiple "pages" of display
• Large batch systems require bulk transmission
• "Forms" are edited locally inside the terminal; contents are sent as a block
• Terminal becomes a state machine:
• Input from the keyboard or host transitions between states
• Edit operations become a sequence of states executed by the terminal

8. Microprocessor Terminals (~1980s+)
• Microprocessor increases local functionality
• Many chips of discrete logic replaced with a handful of chips
reduces cost
• Features are mostly a function of firmware
• Terminals become a commodity, more similar than different

9. Raster Video Terminal Architecture
• Video Refresh Control
• Display Memory
• Control Logic
• User Input
• Communication Logic
• Host Computer I/O

10. Video Refresh
• Vertical sync every frame
• Vertical blank between frames
• Horizontal sync every scanline
• Horizontal blank between
scanlines
• Read data from display memory
• Convert to glyphs
• Convert glyphs into pixels
• Supply pixels for scanline

11. Video Refresh is Synchronous
• Digital systems are synchronized with a clock
• Fastest changing signal in a system determines minimum clock rate
• Raster pixels are typically the fastest changing element in a terminal
• The "dot clock" determines the maximum screen resolution
• The scanline buffer avoids high frequency access to display memory for
video refresh
• Character/attribute generator may feed the scanline buffer,
different terminals have different configurations
• Typical to see the "dot clock" as the master clock,
divided down for the rest of the system (CPU, memory, etc.)

12. Beehive B100: Discrete Logic on Steroids
• Introduced 1976
• 12-inch display
• 80x24 character screen
• 5x7 characters in 7x9 matrix
• Advanced cursor control
• Protected fields
• Block transmission

13. B100
Logic
Board

14. Received ESC Codes
0
Aux Page
Send
I
Page
Send
[
Start
Field
A
Cursor
Up
J
Clear
EOS
]
End
Field
B
Cursor
Down
K
Clear
EOL
b
Keyboard
Enable
C
Cursor
Right
P
Delete
Char
c
Keyboard
Disable
D
Cursor
Left
Q
Insert
Mode On
i
Line
Send
E
Clear
Screen
R
Insert
Mode Off
l
Start
Blink
F
Cursor
Position
W
Format
Mode On
m
End
Blink
H
Cursor
Home
X
Format
Mode Off
0 1 2 3 4 5 6 7
0 NUL DLE SP 0 @ P ` p
1 SOH DC1 ! 1 A Q a q
2 STX DC2 " 2 B R b r
3 ETX DC3 # 3 C S c s
4 EOT DC4 $ 4 D T d t
5 ENQ NAQ % 5 E U e u
6 ACK SYN & 6 F V f v
7 BEL ETB ' 7 G W g w
8 BS CAN ( 8 H X h x
9 HT EM ) 9 I Y i y
10 LF SUB * : J Z j z
11 VT ESC + ; K [ k {
12 FF IS4 , < L l |
13 CR IS3 - = M ] m }
14 SO IS2 . > N ^ n ~
15 SI IS1 / ? O _ o DEL

15. Implied State Machine Functionality
• Cursor navigation implies cursor state position maintained
• Page/line send implies scanout of display memory to comm port
• Clear to EOS, EOL implies clear counters or clear start position
• Insert mode implies moving blocks of data in display memory
• Start/end blink implies blink attribute stored somewhere
• Start/end field implies protected attribute stored somewhere
• Control functions can be typed locally or received from host

16. B100
Block
Diagram

17. B100 Block Diagram
Clock Generation
Crystal
Oscillator
Dot Position
÷ 7 DPC
Char Position
÷ 96 CPC
Char Height
÷ 9 CHC
Char Line
÷ 29 CLC
Baud Clock
Selection
Baud Clock CK1

18. B100
Block
Diagram
Host
Comms
UART
Transmitter
Port
Selection
Circuit
UART
Receiver
Data Input
Buffer

19. B100
Block
Diagram
Display
Refresh
Blink/Low
Circuit
Video Shift
Register
Memory
Address
Manipulation
1920x1 Bit
Blink
1920x1 Bit
Protect
1920x7 Bits
Data Refresh
Memory
Character
Generator CHC
Composite
Video Driver
Video
Driver

20. B100 Block Diagram
Control Logic State Machine
PLA
Input
Selection
Mode
Programmable
Logic
Array
Command
Decoder
Operation
Mode
Mode
Sequence
Sequence Operation
Sequence
Counter
Operation
Register
Roll Counter
BCURL Buffered
Cursor Line
BCURP Buffered
Cursor Position
Roll
CURL Cursor Line Reg. CURP Cursor Position Reg.
Cursor Line
Cursor Position
Video
Cursor
"Off
Screen"
Circuit
BUS
BUS

21. State
Machine
Flow
Chart I

22. State
Machine
Flow
Chart II

23. State Machine Flow Chart Detail

24. Digital Equipment Corporation VT100
Microprocessor Control for ANSI Compliance
• Introduced 1978
• 12-inch display
• 80/132x24 character screen
• 7x9 character matrix
• Blink, bold, reverse, underline
• Intel 8080A CPU
• Supports many ANSI sequences
• Hardware expansion ports

25. VT100
Logic
Board

26. Received ANSI ESC Codes
0 1 2 3 4 5 6 7
0 NUL DLE SP 0 @ P ` p
1 SOH DC1 ! 1 A Q a q
2 STX DC2 " 2 B R b r
3 ETX DC3 # 3 C S c s
4 EOT DC4 $ 4 D T d t
5 ENQ NAQ % 5 E U e u
6 ACK SYN & 6 F V f v
7 BEL ETB ' 7 G W g w
8 BS CAN ( 8 H X h x
9 HT EM ) 9 I Y i y
10 LF SUB * : J Z j z
11 VT ESC + ; K [ k {
12 FF IS4 , < L l |
13 CR IS3 - = M ] m }
14 SO IS2 . > N ^ n ~
15 SI IS1 / ? O _ o DEL
A
Cursor
Up
K
Erase in
Line
m
Select
Rendition
B
Cursor
Down
c
Device
Attributes
n
Status
Report
C
Cursor
Right
f
Hor/Ver
Position
q
Load
LEDs
D
Cursor
Left
g
Tab
Clear
r
Set
Margins
H
Cursor
Position
h Set Mode x
Request
Params
J
Erase in
Display
l Reset Mode y
Confidence
Test
Commands can take one or more
parameters to define behavior.
Non-ANSI sequences not shown.

27. VT100 Components

28. VT100 Functional Block Diagram

29. VT100 Detailed Block Diagram

30. VT100
Microprocessor
Block Diagram

31. VT100
Microprocessor
Block Diagram

32. VT100 Video Processor Block Diagram

33. VT100 Hardware Expansion
• Advanced Video Option (AVO)
• Standard Terminal Port (STP)
• Graphics Connector
• Card Cage

34. VT100
Logic
Board
Expansion
Locations

35. Advanced Video Option
• Provides more display memory for 132x24 character mode
• Provides more attribute memory
• ROM sockets for overlay/expansion of program memory
• Daughter board attached to motherboard

36. Standard Terminal Port
• Options can intercept and change I/O between terminal and host
• Usually a daughter board attached to motherboard
• Allows an option to extend ESC sequences without modifying
VT100 ROMs

37. Graphics Connector
• Provides direct access to the digital video signal of the VT100
• Typically connected to an external card mounted in the card cage
• Can be used to capture the video output as a digital signal,
such as might be used by a screen grab style printer

38. Card Cage
• The VT100 main board is mounted in a card cage chassis
• Room is available for an additional card with rear panel connector
access
• Card edge provides power only, no direct communication with the
main board

39. Hewlett-Packard 264x
• Range of terminals 1975-1981
• 5-inch by 10-inch display
• 80x24 character display
• Microprocessor control
• Tape drives for local storage
• BASIC language variant for
local processing
• Graphics variants

40. Card Cage Chassis

41. Rich Set of
Received ESC Codes
0 1 2 3 4 5 6 7
0 NUL DLE SP 0 @ P ` p
1 SOH DC1 ! 1 A Q a q
2 STX DC2 " 2 B R b r
3 ETX DC3 # 3 C S c s
4 EOT DC4 $ 4 D T d t
5 ENQ NAQ % 5 E U e u
6 ACK SYN & 6 F V f v
7 BEL ETB ' 7 G W g w
8 BS CAN ( 8 H X h x
9 HT EM ) 9 I Y i y
10 LF SUB * : J Z j z
11 VT ESC + ; K [ k {
12 FF IS4 , < L l |
13 CR IS3 - = M ] m }
14 SO IS2 . > N ^ n ~
15 SI IS1 / ? O _ o DEL
• Subset of 2648A codes shown
• Text control
• Graphics primitives
• Graphics attributes
• Automatic plotting of user data
• Tape device control
• Most commands take parameters
• No ANSI support!
• Tektronix 401x emulation

42. HP 2648A System Architecture

43. Modular Design
• CPU card
• Memory card
• Display card
• Graphics display card
• Keyboard interface card
• Host communications card
• Tape interface card
• Additional slots open for expansion
• Reminds you of... a personal computer! (because it is)

44. Family of Terminals from a Single Design
• HP 2640A (1975): simple character terminal
• HP 2644A (1975): character terminal with tape drives
• HP 2641A (1976): APL character terminal
• HP 2645A (1976): replaced 2644A
• HP 2648A (1977): graphics terminal w/tape drives
• HP 2647A (1978): 2648A with BASIC in ROM
• HP 2642A (1980): 2645A w/external 5.25" floppy drive
• HP 2647F (1982): 2647A w/external 5.25" floppy drive
• 60,000 units shipped by 1981

45. Is it a Personal Computer or a Terminal?
Personal Computer
• Programmability expected
• Serial comms optional
• Local storage expected
• Character display expected
• Graphics display optional
• Expansion expected
Terminal
• Programmability optional
• Serial comms expected
• Local storage optional
• Character display expected
• Graphics display optional
• Expansion optional

46. Learn More About Terminals!
• https://Terminals-Wiki.org
• Your one-stop shop for terminal info!
• Browse by manufacturer
• Browse by year of introduction
• Find links to documentation
• Find pictures
• Contribute!
• Wiki edits and contributions welcome from everyone