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

INTENTIONALLY BLANK