UNLIMITED
AD-A240 676
RSREMEMORANDUM No. 4479
ROYAL SIGNALS & RADARESTABLISHMENT
A BI1-DIRECTIONAL BAUC')T/ASCIICODE CONVERTER
Author: L Anderson .
4' 1991'
PROCUREMENT EXECUTIVE,MINISTRY OF DEFENCE,
0z RSRE MALVERN,
2 WORCS.
0
91-11287
UNLIMITED
CONDITIONS OF RELEASE
0105885 303454
........... DRIC U
COPYRIGHT (c)1988CONTROLLERHMSO LONDON
.......................... DRICY
Reports quoted are not necessarily available to members of the public or to commercialorganisations.
UNCLASSIFIED
RSRE. MEMORANDUM No. 4479
A BI-DIRECTIOI1AL BAUDOT/ASCII CODE CONVERTER
L. ANDERSON
MAY 1991
SUMMARY
A unit has been developed to interface between a
terminal generating BAUDOT Code at 75 BAUD and a
terminal generating ASCII Code at 9600 baud,
facilitating bi-directional serial communication. This
report describes the hardware and software design; and
includes the Operating Instructions.
Copyright©
Controller HMSO London1991
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CONTENTS
1/ INTRODUCTION
2/ GENERAL
3/ HARDWARE DESCRIPTION
4/ SOFTWARE DESCRIPTION
4.1. MICROCONTROLLER 1, ASCII TO BAUDOT CONVERSION
4.1.1 MAIN PROGRAM (ASCIIBAUDOTCONVERT)
4.1.2 SERIAL INTERRUPT SERVICE ROUTINE
4.1.3 BAUDOT SERIAL TRANSMISSION
4.2. MICROCONTROLLER 2, EAUDOT TO ASCII CONVERSION
4.2.1 MAIN PROGRAM (BAUDOTASCIICONVERT)
4.2.2 EXTERNAL INTERRUPT SERVICE ROUTINE4.2.3 TRANSMISSION OF ASCII CHARACTER-/2
5/ CONCLUSIONS Aoesso ForNTIS GPA&I '
DT C TA 9
6/ ACKNOWLEDGEMENTS u i,
!Dltt "r c a
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LIST OF FIGURES
2.1. SYSTEM SCHEMATIC
3.1 CIRCUIT DIAGRAM
4.1.1 ASCII TO BAUDOT CONVERT.
4.1.2 ROUTINE TO INITIALISE THE PORTS.
4.1.3 SERIAL BUFFER INTERRUPT SERVICE ROUTINE.
4.1.4 ROUTINE TO CONVERT FROM ASCII INTO BAUDOT THEN
TRANSMIT.
4.1.5 ROUTINE TO OUTPUT AN UPPER CASE SHIFT CHARACTER.
4.1.6 ROUTINE TO OUTPUT A LOWER CASE SHIFT CHARACTER.
4.1.7 ROUTINE TO SERIALLY OUTPUT EACH BAUDOT CHARACTER.
4.1.8 ROUTINE TO GENERATE A 6.7mS TIMING INTERVAL.
4.1.9 TIMER 0 INTERRUPT SERVICE ROUTINE.
4.1.10 ROUTINE TO GENERATE A START PULSE.
4.1.11 ROUTINE TO GENERATE A LOGIC 1.
4.1.12 ROUTINE TO GENERATE A LOGIC 0.
4.1.13 ROUTINE TO GENERATE A STOP PULSE.
4.2.1 MAIN ROUTINE.
4.2.2 ROUTINE TO INITIALISE THE PORTS.
4.2.3a EXTERNAL INTERRUPT 1 SERVICE ROUTINE.
4.2.3b EXTERNAL INTERRUPT 1 SERVICE ROUTINE.
4.2.4 ROUTINE TO EXTRACT 5 DATA BITS FROM THE BAUDOT
SERIAL STREAM.
4.2.5 ROUTINE TO SAMPLE THE BAUDOT SERIAL STREAM.
4.2.6 ROUTINE TO GENERATE A 6.7mS TIMING INTERVAL.
4.2.7 TIMER 0 INTERRUPT SERVICE ROUTINE.
4.2.8 ROUTINE TO TRANSMIT AN ASCII CHARACTER.
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APPENDIX
A BAUDOT CODE (CCITT ALPHABET NUMBER 2).
B BAUDOT CODE DATA BYTE TIMING WAVEFORM
(CCITT ALPHABET NUMBER).
C RS-232C SERIAL DATA BYTE TIMING WAVEFORM.
D BAUDOT CHARACTER SERIAL STREAM SAMPLING.
E OPERATING INSTRUCTIONS.
F PHOTOGRAPHS OF THE INTERIOR/EXTERIOR OF THE APPLIQUE
UNIT.
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1. INTRODUCTION
In support of Operation Granby, IS5 Division of RSRE
was required to produce a self-powered applique unit
which would interface between a terminal generating
Baudot characters at 75 baud (see Appendix A & C), anda terminal generating ASCII characters at 9600 baid
(see Appendix B), providing bi-directional ser I1
communication.
Due to the immediacy of the requirement, and the need fora flexible and low cost solution, an industry standard 8-
bit Microcontroller was chosen to perform all data
processing and interface control between the applique unitand the two terminals. This flexibility in design, resulted
from the microc',ntroller's internal software control of all
data handling functions and external interfacing protocols.
The unit was powered from a 240V AC supply, with twoexternal 25-way D-type Cannon connectors: one to transmit
and receive Baudot code characters and the other to
transmit and receive ASCII code characters. A manual RESET
button was also provided to initialise the unit on power
up.
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2. GENERAL
Details of the system configuration showing the
interconnections between the unit and the two
terminals are to be found in Figure 2.1. The unit
contains two distinct processing paths. These are:-
1. The conversion from ASCII
generated characters into Baudot
characters.
2. The conversion from Baudot
generated characters into ASCII
characters.
Due to the different baud rates of the two terminals,
it was necessary to implement a scheme to provide
simultaneous transmit and receive capabilities. Thi;
was achieved by routing the data transmit line from
each terminal to a different microcontroller for
conversion into the opposite serial format, each
microcontroller operating asynchronously and
independently of the other.
It can be seen from Figure 2.1 that mi-rocontroller 1
converts from ASCII into Baudot and microcontroller 2
converts from Baudot into ASCII, indicating that each
microcontroller transmits a different type of serial
data stream from that which it receives, and at a
different baud rate. Each microcontroller was
configured to be interrupt driven on receipt of an
ASCII or Baudot character. The ASCII characters were
received and transmitted via a dedicated software
configurable serial port, resident on chip, while the
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Baudot characters were received and transmitted via a
microcontroller port pin, configured in software to
act as a pseudo serial port; where the received
characters were identified using waveform sampling
routines and the transmitted characters were produced
using Pulse Width Modulation (PWM) routines.
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3. HARDWARE DESCRIPTION
Figures 2.1 & 3.1 show the system hardware employed.
The hardware operates from a single 5 Volt supply,
derived from an 240V to 5V AC-DC converter. From
Figure 3.1 it is evident that the converter contained
two basic functional elements:-
1. The RS232 serial interface (MAX232)
These MAX232 devices are RS232 communications
interfaces, used to generate the positive a,:a negative
logic levels required for true RS232 communication
from a single 5V supply. Therefore, with the aid of an
on chip charge pump, the MAX232 generates -10V to
represent a Logic 1 and +1OV to represent a Logic C
(see Appendix B).
2. 80C51 INTEL MICROCONTROLLER
This is a 8-bit processing device which contains, 8K
bytes of EPROM and 256 bytes of user addressable RAM.
It also has four 8-bit bi-directional ports, Port 3
having a dedicated software programmable serial
interface and a number of external interrupt pins,
where each of the 32 individual port pins are uniquely
addressable.
To generate the two different baud rates required for this
application, namely 75 and 9600 baud, an 11 592 MHz crystal
was used, to allow the internal timer which generated the
appropriate data bit timings to be loaded v*Uth a convenient
divide ratio.
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To receive and transmit characters at different baud rates
would have required each microcontoller to contain two
dedicated software configurable serial interfaces. However
each microcontroller had only one dedicated software
configurable serial port. Therefore to solve this problem,
and provide a flexible and re-configurable solution, a
second serial interface was simulated, using an external
interrupt pin (INTI), which would under software control:-
I. On Receive:-
Be interrupted by the START bit of the
Baudot character serial stream, sample the
serial stream and extract the 5 data bits
(see Appendix A) at 75 baud.
2. On Transmit:-
Generate a Baudot character serial stream
using software controlled Pulse Width
Modulation (PWM) at 75 baud.
Microcontroller l's function of receiving ASCII
characters at 9600 baud, converting to Baudot and re-
transmitting to the Baudot terminal configured to 75
baud caused a data bottle neck. If the length of
message expected could have been defined and would
never exceed the available Ram space, a FIFO buffer
could have been used to store the characters awaiting
transmission, until the processor was free. Since
there was no constraint on the message length this was
not possible. So the approach adopted was to use the
Clear To Send (CTS) line from the ASCII terminal in
conjunction with the large character buffer built into
the terminal. The CTS line was controlled to hold-off
receipt of more characters until the processor had
carried out the conversion and re-transmitted the
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character at 75 baud. Control of the CTS line was
exercised via a buffered port pin with an additional,
external, transi&tor used to provide sufficient
current drive capability.
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4. SOFTWARE DESCRIPTION
The software controlling the converter function
resides in three progrars: one program resident in
microcontroller 1, which converts from ASCII into
Baudot; one program resident in microcontroller 2,
which converts from Baudot into ASCII; and the third
program which defines all user constants and Eprom
look-up tables, which resides in both
microcontrollers. This system Firmware resides in the
8K bytes of EPROM within the microcontrollers.
4.1 Microcontroller 1 : ASCII to BAUDOT Conversion
The program listings are contained in Appendix E & G, with
the Structured Design Methodology (SDM) charts contained in
Figures 4.1.1 to 4.1.13.
4.1.1 Main Program Routine (ASCIIBAUDOTCONVERT)
Details of the main program routine are shown in
Figure 4.1.1. On RESET the processor jumps to the
starting address of the main routine via the reset
vector (located at address OOOOH in the on-chip
Eprom). Program exec.ution begins by configuring the
microcontroller's ports to the desired input/output
conditions (see Figure 4.1.2). The serial port is then
configured to allow serial communication to and from
the ASCII terminal (see Appendix A).
As the program is interrupt driven, the processor only
responding if a serial interrupt occurs, the serial
interrupt flag must first be activated and enabled.
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The main program routine then enters an infinite loop
where its only action will be to vector to the
appropriate serial interrupt service routine (ISR) on
receipt of an ASCII character.
4.1.2 Serial Interrupt Service Routine (SERISR)
When a serial interrupt occurs the processor vectors
to the serial interrupt service routine and tests for
character validity (see Figure 4.1.3). On receipt of
a valid ASCII character the processor inhibits, by the
assertion of the CTS line, the ASCII terminal from
transmitting any further characters from its buffer.
Conversion from ASCII into Baudot then occurs, using
the ASCII value of the received character to address
its Baudot equivalent in Eprom (see Figure 4.1.4).
Appendix A shows that the Baudot characters are
divided into either upper or lower case, to achieve
more than 32 (25) possible characters. To switch
between these cases a special character, either a
Lower Case Shift character (L.C.S.) or an Upper Case
Shift character (U.C.S.), has to be issued prior to
the transmission of a character. Failure to transmit
the appropriate special case character will result in
the Baudot terminal decoding the received character
incorrectly. As an example consider the character
stream "ABClA", where nothing has been sent prior to
this. The transmission would then be of the form:-
L.C.S. IA IB IC 1UC.S. j11 L.C.S. IAI
Therefore the processor determines whether a case shift
character is necessary prior to the character (see Figure
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4.1.4 to 4.1.6) and outputs the serial stream to the
destination Baudot terminal.
Finally the processor clears the CTS line, allowing the
ASCII terminal to send another character from its buffer.
4.1.3 Baudot Serial Transmission
As each microcontroller contained only one dedicated
serial interface, but was required to transmit
characters at a different baud rate from those it
received, it was necessary to simulate a second using
a port pin and software controlled PWM routines (see
Figure 4.1.7). The modulation scheme used Timer 1 (see
Figure 4.1.8 & 4.1.9) to generate a strobe at half
data bit intervals (6.65mS at 75 Baud), defining
sequential bit intervals which marked the transition
between:-
1. Start bit and Data Bit DO.
2. Data Bits D(x) and D(x+l) where x = 0..3.
3. Data Bit D4 and Stop Bits.
As each bit forming the Baudot serial stream is sent,
(see Figures 4.1.10 to 4.1.13) the appropriate logic
levels are output dependinq whether a Start, Stop or
Data Bit is required.
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4.2 Microcontroller 2 BAUDOT to ASCII conversion
The program listings are contained in Appendix F & G, with
the Structured Design Methodology (SDM) charts contained in
Figures 4.2.1 to 4.2.10.
4.2.1 Main Program Routine (BAUDOTASCIICONVERT)
Details of the main program routine are shown in
Figure 4.2.1. On RESET the processor jumps to the
starting address of the main routine via the reset
vector (located at address OOOOH in the on-chip
Eprom). Program execution begins by configuring the
microcontroller's ports to the desired input/output
conditions (see Figure 4.2.2). The serial port is then
configured (see Appendix B) to allow serial
communication to and from the ASCII terminal once a
character has been received from the Baudot terminal
and converted.
Although this program, like the previous, is interrupt
driven, the interrupt which flags the processor indicating
that a Baudot character has been received is the external
interrupt pin (INT1) and not the serial interface. This pin
is configured in software, to sample the incoming serial
stream from the Baudot terminal, and interrupt when the
Start bit of each serial character packet is detected (see
Appendix B). As part of the main program routine the
external interrupt 1 is both enabled and activated.
The main program routine then enters an infinite loop where
its first action will be to vector to the external
interrupt 1 service routine when the detection of the Start
bit causes the processor to be interrupted.
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4.2.2 External Interrupt 1 Service Routine (INTI_ISR)
When the Start bit, which marks the beginning of a
serial character packet from the Baudot terminal,
causes an interrupt, the processor vectors to the
interrupt l's ISR (see Figure 4.2.3). The processor
then samples the serial stream in the middle of each
bit frame and extracts the corresponding 5 data bits
which constitute a Baudot character (see Figures 4.2.4
to 4.2.6).
As an example, take the receipt of Baudot character
"A" which has bit pattern 00011. The corresponding
sample waveform would be as found in Appendix D. From
this diagram it is clear that the processor first
aligns itself to the middle of the Start bit, labelled
SO, then takes 5 successive samples separated by the
incoming serial bit rate (13.3mS) to extract the
Baudot character "A".
When the Baudot terminal transmits characters it also
transmits, depending on whether upper or lower case is
required (see Appendix A), one of two case shift
characters. If any of these characters are retrieved
the processor waits for the next serial packet, which
is the actual character, and retrieves the
corresponding 5 data bits. It then uses these data
bits and the case shift character to address Eprom and
extract the ASCII equivalent character. If no case
shift character is received the processor converts the
current 5 data bits into the corresponding ASCII
equivalent character without waiting for the next
serial packet. This decision process can be seen in
Figure 4.2.3.
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4.2.3 Transmission of ASCII Character (PRTCHR)
In this system where microcontroller 2 receives at 75
baud and transmits at 9600 baud, no data buffer is
required. This is because the converted character
would be transmitted onto the ASCII terminal before
(even if the worst case of 2 Baudot characters, each
being separated from the other by the bit rate
(13.3mS) is anticipated) the next character from the
Baudot terminal, could possibly be received.
Therefore having converted the received Baudot character
into its ASCII equivalent, the character is loaded into the
serial buffer (SBUF) and transmitted (Figure 4.2.8).
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5. CO1NCLUSIONS
In rapid response to an Operation GRANBY requirement,
IS5 Division has produced a compact, low cost,
reliable applique unit to allow direct connection
between an ASCII terminal and a Baudot configured
terminal.
This equipment can be easily reconfigured to operate at any
baud rate by minor software alterations without the need
for any hardware re-design.
6. ACKNOWLEDGEMENTS
The author would like to thank all merics of the E.W.
System Techniques section for their assistance and
advice during the design and assembly of the applique
unit.
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APPENDIX A
CCITT alphabet number 2 (the Baudot code)
BIT BIT BIT BIT BIT LOWER UPPER
1 2 3 4 5 1 CASE CASE
0 0 0 0 0 no action no action
1 0 0 0 0 T 5
0 1 0 0 0 CR CR
1 1 0 0 0 0 9
0 0 1 0 0 SPACE SPACE
1 0 1 0 0 H
0 1 1 0 0 N
1 1 1 0 0 M
0 0 0 1 0 LF LF
1 0 0 1 0 L
0 1 0 1 0 R 4
1 1 0 1 0 G
0 0 1 1 0 I 8
1 0 1 1 0 P 0
0 1 1 1 0 C
1 1 1 1 0 V
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CCITT alphabet number 2 (the Baudot code)
BIT BIT BIT BIT BIT LOWER UPPER
1 2 3 4 5 CASE CASE
0 0 0 0 1 E 3
1 0 0 0 1 Z +
0 1 0 0 1 D who are you
1 1 0 0 1 B ?
0 0 1 0 1 S I
1 0 1 0 1 Y 6
0 1 1 0 1 F
11 0 1 x
0 0 0 1 1 A
1 0 0 1 1 W 2
0 1 0 1 1 J BELL
1 1 0 1 1 UPPER SHIFT UPPER SHIFT
0 0 1 1 1 U 7
1 0 1 1 1 Q 1
0 1 1 1 1 K
1 1 1 1 1 lower shift lower shift
Note: Unallocated combinations are used for specificnational symbols and should, therefore, never be usedfor international transmission
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APPENDIX B
RS-232C serial data-byte timing waveform
Format : Baud Rate : 9600 (Pulse width =lO4uS)Data Bits : 8Start Bits: 1Stop Bits : 1
Logic 0 ~ 1- --- (> +3v)
Logic 1 (< -3v)
Time=0
Start bit
DO
D1
D2
D3
D4
D5
D6
D7
Stop Bit
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APPENDIX C
CCITT alphabet number 2 (Baudot code) data-byte timingwaveform
Format Baud Rate : 75 (Pulse width = 13.3mS)Data Bits : 5Start Bits: 1Stop Bits : 1.5
Logic 0 ----- > +3v)
Logic 1 (< -3v)
Time=O
Start Bit
DO
D1
D2
D3
D4
Stop Bits
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APPENDIX D
EXTRACTION OF BAUDOT CHARACTER
BAUDOT SERIAL CHARACTER PACKrET FOR 'A'
0.1 SECONDS
-7 -13.3m S
13.3mS
S0~ S1~ S2 S 3k S4~ S5~
START BIT 5 BIT 4 BIT 3 BIT 2 BIT I STOPBIT BITS
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APPENDIX E
OPERATING INSTRUCTIONS
[1]. Connect the +240V supply to the 3-pin plug on therear of the box housing, using the white mainscable and switch ON.
[2]. (a). Connect the shorter of RS232 cables (Female D-Type connector on both ends), to the TOP Male D-Type connector (labelled ASCII) on the front ofthe Box Housing.
(b). Connect the other end of this shorter cable tothe device which is generating the ASCIICharacters, for example the ELF.
[3]. (a). Connect the longer of RS232 cables (Female D-Typeconnector on one end and Male D-Type on theother) to the BOTTOM Male D-Type connector(labelled BAUDO[T]) on the front of the BoxHousing.
'b). Connect the other end of this cable to the devicewhich is toreceive and transmit the BAUDOT Characters.
[4]. (a). The ASCII Character generating Terminal must beconfigured to(1). BAUD RATE of 9600 Baud.(2). 8 Data Bits.(3). One Stop Bit and One Start Bit.
(b). The BAUDOT Character generating Terminal must beconfigured to(1). BAUD RATE of 75 Baud.(2). 5 Data Bits(3). One Start Bit and One and a Half Stop Bits.
[5]. When the cable connections have been made Pressthe RESET Button on the front of the Box Housing.The Unit is now operating.
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APPENDIX F
PHOTOGRAPH SHOWING THE EXTERIOR OF THE APPLIQUE UNIT
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PHOTOGRAPH SHOWING THE INTERIOR OF THE APPLIQUE UNIT
UCL
UNLSSFEDF
REPORT DOCUMENTATION PAGE DRIC Reference Number (if knownl)......................... ....
Overall security classification of sheet ......................................... UNCLASSIFIED...............................................(As far as possible this sheet should contain only unclassified Information. Ifit Is1 necessary to enter classified Information, fte field concernedmust be mariked to Indicate the dIssification eg (R), (C) or (S).Originators Reference/Report No. MnhYa
Originators Nam and LocationRSRE, St Andrews RoadMalvern, Worcs WR14 3PS
Monitoring Agency Name and Location
Tite
A BI-DIRECTIONAL BAUDOT/ASCII CODE CONVERTER
Conference Details
Abstract
A unit has been developed to interface between a terminal generating BAUDOT Code at 75 Baud anda terminal generating ASCII Code at 9600 Baud, facilitating bi-directional serial communication. Thisreport describes the hardware and software design; and includes the Operating Instructions.
Abstract Classification (U,R,C or S)
U
Descriptors
Distribution statement (Enter any limitations on the dsrrbution of the document)
UNLIM ITEDon"4
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