BAUDOT-MACHINE INTERFACE WITH MICROCOMPUTERS
Sengoda G. Gan...n, Merwyn Bruhn• and William C. Dixon
Northam Illinois University
Th is paper describes an interface technique for Baudot machines with microprocessor based
computer systems. The hardware for Interrupt 110 and software drivers is analyzed in detail.
INTRODUCTION
The exponential growth of microprocessors has stimulated considerable interest in the area of
digital peripherals. Modern computer systems can be built with peripherals such as line printers,
video terminals. and floppy disk drives. Extensive design changes have been made on the line
printers over the past three decades to improve the1r speed and data handling capabilities. Modern
line printers are designed to deal w1th about 600 wpm in ASCII code whereas the old model,
Baudot pnnters. can operate only at a maximum rate of 1 00 wpm in Baudot code. These Baudot
machines are cheaper and are fast enough for many applications. Programmable 110 ports play an
important role in the interface design. These ports are designed to be compatible only with the
modern teletypes and therefore. hardware change s and additional circ Uitry w ill be needed for
implementing Baudot machine-based microcomputer systems. Th1s paper descnbes an interrupt 110
interface technique for an 8080 based computer system with a Te letype Model 1 5 KSR Baudot
machine.
INTERFACE
The computer system. Fig. 1 . cons1sts of a bas1c system, a teletype model 1 5 KSR machme,
and an USART based-Interface circuit. The bas1c system IS Implemented with Intel 8080
microprocessor. The system consists of 512 bytes of memory with two pages. The first page of
the memory map is implemented with an Intel 1 702 UVPROM. The firmware in the UVPROM
consists of power up rout1ne, USART Initialization routine, teletype keyboard input service driver,
and look up table for code conversion. The RIW memory in the second page of the map is for data
and user onented program storage.
The teletype handles data in Baudot code. The keyboard consists of 31 keys representing 57
Baudot characters. The characters are classified into two different categories. All the numeric keys
are called 'letters' and the alphabetic and special characters are called ' figures '. The keys can be
used in two different modes. If a key is pressed follow1ng a ' letter' key, then 1t IS treated as a letter
character. A key-press is interpreted as a figure character if it follows a 'figure' key. The Baudot
characters are represented by a 5 bit code and a listing of the various Baudot key code assignments
is given in Fig. 2. The important aspect of the teletype interface is that the processor deals with
parallel data transfer and that the Baudot mach1ne handles the data transfer in serial mode. The
keyboard and the printer of the teletype machine can be operated independently with an average
speed of 60 wpm. The speed can be 1ncreased to a maximum of 1 00 wpm with a proper drive gear
selection. The printer magnet requires a signal with a cu rrent capability of at least 60 ma. The
USART's incoming data from the teletype must be compatible with TTL-signal requirements. These
USART and printer requnements can be achieved by an external power supply and buffenng
transistors as shown 1n F1g. 1 .
The interface between the teletype and the computer system IS by an Intel 8251 USART. The
functions of USART are two-fold. It converts the parallel data from the processor into a serial data
and the serial data from the Baudot machine into a parallel data. Also, it establishes a hand-shake
between the teletype and the processor to compensate for the slow operation of the Baudot
machine. The data fram1ng speed of the USART is called Baud Rate and is exclusively controlled by
67
The frequency of the time, Baud Rate Generator, is determined by the speed of the Baudot
machine. For example, for a machine speed of 60 wpm, Table 1, the bit length is 22 ms and the
Baud rate is 45 bps 11/22x1o· 3). The Baud rate generator drives both the TXC and RXC of the
USART so that the USART transmits and receives the data at the same speed. The timer speed~s
increased by a factor, Baud Rate Factor, of 16 for efficient data framing . Since the CTS input of the
USART is always enabled, whenever the processor fills up the transmit buffer or the TTY transmits
data to fill up the receiver buffer, the USART starts data framing synchronously. An interrupt 1/0
technique is used for transmitting data from the teletype keyboard to the computer system
whereas, a software hand-shake technique is used for transmitting data to the printer. The
interrupt 1/0 is achieved by driving the INT input of the processor with RXRDY signal of USART.
Whenever a key is pressed, the receiver buffer in the USART will be filled up with the key code. The
buffer will then activate the RXRDY signal which in turn interrupts the processor. Upon a qualified
INT input, the processor will generate an INT A signal. Since the INT A output of the system is pulled
high at 1 2 volts, the controller places a RST • 7 instruction to switch the processor into the keyboard
input routine located in trap· 7. In the output mode, the processor places an 8 bit data word in the
transmit buffer and stays in a loop. In the wait loop, the TXE status of the USART is monitored by
the processor. The processor leaves the loop only when the TXE status is detected as active.
SOFTWARE
Upon power up, the system starts with the first page of the memory. The USART is a
computerized 1/0 port. Therefore, it must be programmed before the system is used for data
transmission. The initialization routine is located from 000 0009 to 000 0258 locations of the
UVPROM memory. In this routine, the stack pomter, the register pairs B and Dare Initialized as a
first step. The reg1ster pair B is used a pointer for the storage of object program in the R/ W memory.
The object program is loaded into the system in Baudot octal code through the TTY keyboard. The
register paid D is used as a pointer for the storage of the text {Source) in R/ W memory in Baudot
code. Once the memory pointers are initialized, the mode of the USART is programmed. The mode
programming includes framing details such as number of start and stop bits, the data size, and the
Baud rate factor. Usually, the number of start bits is chosen to be 1 and stop bits as 2 . For Baudot
teletype machines, the data size IS 5. A Baud rate factor of 16 is chosen for eff1c1ent data fram1ng .
Data transmission does not start until a command word is programmed into the USART. The
command word enables both the transmitter and receiver of the USART. At the end of the
initialization, the processor enables the ruption and waits for interruption in halt mode. The flow
chart for the power up routine and the initialization program are as in Figs. 3 and 5 .
The keyboard input is by interrupt 1/0 operation. When a key is pressed, the key code is loaded
1nto the receiver buffer os USART. The USART will then sw1tch the processor 1nto the keyboard
input routine 1000 0708 · 000 18291. In this routine, the processor emptys the USART, stores the
data in the text memory using the register pair D. and echos the data back to the printer. The
processor waits in a loop reading the status of the USART. As soon as the transmission is
complete, the processor starts the object program storage. Before getting into the discussion of the
object program storage, it is important to compare and contrast the object program and text. The
object program is in object code, ' figures' , whereas the text is any Baudot character. The object
program is an user-oriented driver, possibly, for the text. Whenever an object program is boot
strapped through the keyboard, the keyboard service routine just echos it back as a text and
decodes the object program into binary code. In the object program decoding logic, the processor
takes three successive octal digits and converts them into an 8 bit binary word using the look-up
table (000 0308 • 000 04 71. In order to boot strap a word of an instruction, one has to generate an
8 bit binary word. This binary word can be stored in R/W memory using the reg1ster pair B as the
pointer and'S' key as the strobe. The flow chart and a complete listing of keyboard routine are as m
Figs. 4 and 7. Once the object program is stored, it can be executed by simply entering the address
of the starting location of object program and pressing the 'G' key.
68
If it is desired to dump the text of any part of the memory, a memory dump program must be
boot strapped through the keyboard. When the text is dumped, carriage and line feed options are
not needed because these characters are part of the text and are built into the text when it is
manually keyed into the memory.
CONCLUSION
The system discussed in this paper is good enough to be a complete terminal if one is not too
conce rned about the speed. Also, this system does not have any utility program such as assembler.
If one wishes to use the Baud ot machine as one of the peripherals in the computer system, there
must be some provisions for code conversion from Baudot to ASCII code. This can either be done
by a look up table or by a character generator. If the peripherals tn the system require various
clocks, a programmable interval timer such as Intel's B253 may be used.
REFERENCES
Intel Corporation. Intel 8080 Microcomputer System User's Manual, Santa Clara, California,
1977.
Hill-burn J .L., and Julich, P.M ., M icrocomputers/Microprocessors: Hardware, Software, and
Applications, Englewood Cliffs, N.J .• Prentice Hall, Inc. , 1 976.
Bruhns, M ., Analysis of Programmable 110 Ports, Northern Illinois Un1versity, DeKalb, Illinois. 1980.
Words Per
Minute
60
66
75
100
Data
Rate
45
50
57
75
bps
bps
bps
bps
Table 1.
Characters
Per Second
6
6.6
7.5
10
Bit
Pulse Width
22 ms
20 ms
18 ms
13 . 5 ms
Stoo
Pulse Width
31
30
25
19
ms
ms
ms
ms
Operations
Per Minute
368
400
460
600
Physical Characteristics of Teleprinter Speeds
for Baudot Machines
69
•
8080
BASIC
SYSTEM
'tr,....--------3-w-R u~~
-..j
~R xRDY
+12vRESET
1KJ'\.
0
...
DATA BUS
6.!\.
PRINTER
MAGNET
~v
FIG.1
A BAUDOT MACHINE-BASED TERMINAL
Bot Numb4rt
54321
0
1
c...
Uppet (Fituros) CoM
Communiea1tona
csw..b rdl
Boll Svt~em ITWX)
Stoclo Market
We~ thor
c~
-
Bl>nk
E
Lino fted
A
B!lr,k
3
Uneftod
Blank
3
Line Fted
3
00000
00001
00010
00011
4
5
6
7
00100
00101
001 10
00111
Space
$pece
Sp..:e
(Apool
(A poll
8
9
10
11
0 1 000
01001
010 1 0
01011
Car Ret
0
R
J
Cat Ret
SWRU
4
Bell
12
13
14
IS
0 I 100
0 11 0 I
011 10
011 11
N
F
i
K
(
16
T
s
..s
18
19
10000
10001
10010
1001 I
2
3 /4
2
20
21
22
23
10100
1 0101
10110
10111
H
•
•6
6
0
1
0
1
24
25
11000
1 1001
11010
11011
9
518
6
Fi<jures
•\
2
'-I
lower
(L otion)
11
26
27
28
~;}
30
31
1 1 I 00
11 I 0 I
11110
1 1 I 11
s
I
u
c
z
L
w
y
p
a
0
B
G
Figures
M
X
v
Lttlers
-
8
7
•I
6
0
1
9
?
6
Figurn
.
I
Lotttn
8
3
LlneFHd
t
Space
Bell
I
7
l
Cat Ret
Car Ret
4
Bell
4
718
114
118
112
...
s
,
"
0
-•
5
•
"2
•
I
3/8
9
FiturH
I
Letters
•
LAtten
-----· L . . . . - -
Figure 2.
Baudot key code Assignment
....,
N
FIG 3
INITIALIZATION
FIG. 4
FLOW CHART FOR TTY INPIIT
000
001
002
003
004
005
006
007
010
011
012
013
014
015
016
017
020
021
022
023
024
025
Instruction
LXI SP
000
Initialize the stack pointer
070
071
072
073
074
075
076
077
100
101
102
103
104
lOS
106
107
110
111
112
113
114
115
116
117
120
121
122
123
124
125
126
127
130
131
132
133
134
135
136
002
LXI B
000
001
LXI D
200
001
HVI A
302
OUT
001
HVI A
047
OUT
001
El
HLT
Jlf>
021
Initialize the object memory
pointer
Initialize the text memory pointer
Set USART mode
Enable the transmitter and
receiver
Enable Interruption
000
-...1
(,.}
FIGURE 5.
030
031
032
033
034
035
036
037
040
041
042
043
044
045
046
047
001
003
007
007
012
Initialization Program
Baudot 3
Baudot 7
Baudot 4
004
020
005
025
006
026
000
027
023
023
002
Baudot 5
Baudot 6
Baudot 0
Baudot 1
I
Baudot 2
FIGURE 6.
Baudot to Binary look-up table
Instruct ion
IN
000
STAX D
OUT
000
IN
001
ANI
004
JHZ
075
000
LDAX D
IHX D
PLISH D
PLISH H
HVI 0
010
LXI H
026
Input the USART
Store in memory
Echo it on TTY
Read USART
Status
Stay in the
loop until TXE•l
Retrieve the data
Increment pointer
Save the pointer
Set the table size
000
INX H
INX H
DCRD
JZ
145
000
CHP H
JNZ
115
000
INX
I«JV
POP
POP
HOV
HOV
RAL
RAL
H
A, H
H
0
H,A
A,L
Jump if 'letter'
Move binary code
137
140
141
142
143
144
145
146
147
150
151
152
153
154
155
156
157
160
161
162
163
164
165
166
167
170
171
172
173
174
175
176
177
200
201
202
203
Instruction
RAL
ANI
370
ORA H
I«JV L,A
RET
POP H
POP D
CPI
Check if 'L' key
022
JNZ
156
000
HOV C,L
RET
CPI
Check if 'H' key
024
JNZ
165
000
I«JV B, H
RET
CPI
Check if 'S' key
005
JNZ
176
000
MOV A,L
STAX B
INX B
RET
CPI
032
RNZ
HOV L,C
I«JV H,B
PCHL
FIGURE 7. TTY Keyboard input routine
Check if 'G' key