Lewis A, Thompson W51FQApplied Research Laboriltor iesThe Universit y 01 Texas at AustinAustin TX 78711

RTTY with the H8- a Heath-based RTTY system

(l TRS or FIGS) and end-of­line detection can be doneautomatically by the com­puter. Complex fun ctionssuch as text storage, auto­mat ic CQ, and Morse codeiden tification can easily beimplemented in the com­puter's so ftware.

Amateur RTTY ConventionsAmateur RTTY in the

high frequency (HF) rad iospectrum (1 through 30MHz) uses frequency-shiftkeyi ng (FSK) to transmitBaudot c ha rac te rs. EachBaudot character consistsof a sta rt bit, five data bits,and 1 VJ stop bits. Baudotcode consists of two sub­sets of characters, l TRSand FIGS. l TRS consist s ofall the uppercase alpha­betic characters and FIGScons ists of the numbers andm iscellaneous punctuationcharacters. The high sta teor " mark" is by conventio nthe higher frequency of thetwo FSK signa ls. A frequen­cy sh ift of 170 Hz has re­placed the o ld standard of850 Hz. Rates of both 60and 100 words per m inuteare now in use but 60 wpmis st ill dominant. ' A popularmeans to generate FSK is to

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t ro lli ng a rad iote letype(RTTY) a mateu r stat ion .

Fig. 2. Phase locked loop FSK demodulator.

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~'06 73 Magazine· August, 1980

FSK Computer InterfaceThe serial word fro m the

demod ulator is convertedto a valid pa ra llel word andsent to the computer by theFSK computer interfaceshown in Fig. 3. The UART(U3) is hardware -pro-

phase locked loop outputto TTL voltage levels.

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Control interrupt program listing.

(phase) as the input signal.This forcing voltage is theactual output of the phaselo ck ed loop . The poten­t iomete r shown in Fig. 2 isused to adj ust the ze ro­input vco frequency to ava lue of 2100 Hz . A fre­quencv counter should beused for this adjustment.The device U2 converts the

Fig. 3. FSK computer interface.

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signal within 400 Hz of itsnominal vco frequency .Without a narrow band­width filter, near-freq uencysigna ls will cause signif i­cant receiving e rro rs. Theeas iest place to add a re­ceive r filte r is between theaudio o utput of the rece iv­er and the RTTY interface.The filter of Fig. 1 has abandwidth of 300 Hz cen­tered at 2210 Hz and a volt­age gain of 10 . Each stageshould be adjusted sepa­rately using an acc uratesigna l gene rator a nd an out­pu t ind icator. In o peration,input 1 is connected to thereceiver audio output andoutput 2 is connected tophase Jocked loop demod­ulator input.

Phase Locked Loop FSKDemodulator

The demodulator shownin Fig. 2 conve rts aud io fre­quencv shifts into se ria lbinary data . Within deviceUt . a phase detector sendsa voltage to a vco. forcingit to the same frequency

RTTY Input FilterThe bandwidth of the

SB-300 receiver is 2.1 kHz(350 through 2450 Hz). Ap­pro ximately 300 Hz o fbandwidth is requi red to re­ceive 60 wpm, 17o-Hz sh iftRTTY.J The phase lockedloop demod ulator used inthe RTTY interface unit willattempt to lock onto any

feed aud io FSK into the mi­crophone input of a single­sideband (SSB) suppressedca rrier rad io transm itter.

System DescriptionThe RTTY system de­

sc ribed in thi s a rticle con­sists o f the fol lowing eq uip­ment: 1) Heath H8 comput­e r, 16 K m e mory. serial(H8-5) and parallel (H8-2) in­terface boards; 2) Heath H9video terminal; 3) dual GE5121 cassette tape decks; 4)Heath 5B-300 single-s ide­band receiver; 5) Heath5 B-401 si ng le-side ba ndtransmitte r, and 6) aut hor­const ructed RTTY inte r­face.

No ha rdware modifica­tions were done on any ofthese equipments. Thisminimal system will receiveand tra nsmit RTTY on a llauthorized HF bands. A tun­ing aid and inverted codeswitc h are incl uded to im­prove reception. All trans­mit and receive functionsare controlled from the H9terminal as follows :CTRl-A - Transmit Morsecode identificationCTRl-B -Go to transmitmode (turn on transmitter)CTRl-C- Transmit buffer(CQ o r message)CTRl-O - Force l TR5 codeduri ng receive modeE5C KEY -Go to rece ivemode (turn off transmitter)

Shift characters are auto­matically transmitted whencalled fo r by Baudot con­vention. After 72 charac­ters, line feed. two ca rriagere turns, and the l TRS codea re a utomatica lly trans­mitted . A 28-character type­ahead buffe r is included inthe transmit program toprevent loss of text.

H8-2 Parallel InterfaceBoard

The FSK co mpute r inte r­face is connected to one ofth ree pa ra lle l ports ofHeath's H8-2 pa rallel inter­face board. The port ad­dress jumpers for the se lec t­ed pa ral le l port should beconnected for 1008. O n thatport. connect the logi cju m p e rs a s fo ll ows : 1 )A, - A 2, open; 2) 6 1 - 8 2.

open; 3) (1 - (2, shorted,and 4) E, -E 2, shorted . Theoutput of the FSK compute rinterface (Fig. 3) is co n­nected to the parallel portat Pl 0l by a ribbon cableterm inated with the Heath­supplied 2S·p in connector.

SSB Receiver andTransmitter Operation

Both the SSB rece iverand tran smitter mu st beoperated in the upper side­ba nd (USB) mode for cor­rect pola rity receptio n andtransm ission. The receivershou ld have less than 100HzfH drift after wa rm-up.Most SSB transm itters a renot designed to operate a tthe 100 % duty cycle re­quired of RTTY. The man­ufac tu rer's specifications

low-pass filter and then tothe microphone input ofthe SSB radio transmitter.The second elect ronicswitc h is cont rolled by the" reader o n/off" line fromthe H8-1 parallel interfaceboa rd. This a rra ngement a l­lows the co mpute r to turnthe transmitter on and offusi ng the tran smitter ' sv o ic e -o p e ra t e d sw itc h(VOX) and also to transmit aMorse code ident ifi ca tionsigna l. The 3-kHz low-passfilter converts the squarewave output of U7 to a sinewave. The 1k-Q potentiom­eter sho uld be ad justed toprovide the same outputleve l as the m icro phonenor mally used with t hetransm itter. The comple tefilte r a nd le vel ad jus tpotentiometer shou ld bewell shie lded to preventst ray noise from beingtransmitted .

grammed for 5-bit wordsand 1 'h sto p bits . TheUA RT's clock. U4, is ad­justed for a f requency thatis 16 times the RTTY baudrate. For 60 wpm, the fre­quency of the clock is 727Hz . W hen the UART de­tects and loads a validBaudot word. it signals theco mpute r on the OAV (da taavailab le) line. After thecompute r accepts the par­a lle l word, it signa ls backo n the RDAV (reset d ataavailab le) line , allowing theUA RT to load a nother wordin its buffer. The Pll inputc ircuits , including U9 andpa rt of U8, a llow rece ivingof norm al a nd inve rtedsigna ls and provide a need­ed tuning aid. When a RT TYsigna l is correc tly rece ived,the mark LED should ligh tdu ring idle moments be­tween characters and bothshou ld light during con­tinuous transmiss ion.

In the transm it mode, theUA RT signals the compute rto load a word using theTBMT (trans mitter bufferem pty) line. After the com­pute r has loaded a Baudotword, it signa ls the UA RT tobegin serial tr ansm ission us­ing the OS(da ta strobe) line .The UART adds the correctnumber of sta rt (1) and stop(1 %) b its and then transm itsthe word serially at a baudra te of o ne-sixteent h itstransmit clock rate. These ria l tra nsmit signa l op­e rates an electron ic switc hin U6 that keys a two-fre­quen c y osc il lato r (US).When t he se ri a l out line islow, the switch is open andUS should be adjusted bythe 10k·Q potentiometer tooscil late at 42S0 Hz. Whenthe seria l line is high, theswitch is c losed and USshould be ad justed by theSk-Q potent iometer to os­ci lla te at 4S90 Hz.

The J-K fl ip-flop device.U7, divides the output fre­quency of US by two to pro­vide a symmetrica l sq uarewave at the correc t markand space freq uencies. Theo utput of U7 goes th roughan e lectronic switch to a

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~ 108 73 Magazine" August. 1980

RTTY receive program listing.

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shou ld be consulted to de­termine a safe maximumpower limit. The SB-401transmitter is presently be­ing operated at a plate cu r­rent of 150 mA rather thanthe SSB-recommended 250rnA

RTTY Interface SoftwareThe speed requi rements

as well as the lack of com­plex mathematical expres­sions allowed the interfacesoftware to be written in as­sembly language. The soft­ware is divided into threeseparate ly assembled pro­grams, plus Heath's consoledr iver program. listings ofthe three RTTY programsare included in this ar ticl e.

RTTY Recei¥er ProgramThe receiver program

uses the conso le driver andthe O P programs. An excel­lent description o f Heath'sconsole driver is conta inedin issue No. 2 of Remark .)

The RTT Y software e nt rypoin t shou ld be the sta rtof the re cei ve program(041.150 offse t oc ta l) to ef­fec t por t in itialization. Fig,4 shows a fl owchart fo r thereceive program. Both theconso le termina l's se ria lport and the RTTY inter­face's para llel port are ini­tia lized at the begi nning ofthe program. The commandword. 0478. sent to the con­trol port (101sl tu rns thetransmitter off. The CIP pro­gram places d ifferent num­bers in a sc ratc hpad (042/000) based upo n w hic hCTR l key inte rrupts occur.The receive p rogram in itial­ly zeros this location a ndthen tests it fo r a branchto the transmit routine eachtime a new character loopis performed. After testingfor a transm it programbranch, the first bit o f thecontrol input port is testedto determ ine if a newcha racter has been loaded

in th e RTTY interfa c eUA RT. If a new c harac te r ispresent, it is loaded into thecomputer, thereby freeingthe interfa ce UART tosearch for the next cha r·acter. Afte r loading a cha r­ac te r, the routine checkswhethe r it is in the l TRS(D=1) mode. If it is in thel TRS mode, the new cha r­acter is compared with theFIGS code.. If the new cha r­acter is the FI GS code, theFIGS mode is set (D =0) andthe program loops back fo ra new cha racter. If the newcharacter is no t the FIGScode, its numeric va lueforms the lower 8-bit ad­dress at which the Baudotc ha rac t e r's equi val entASC II va lue is sto red. Thisva lue is sent to the terminalv ia the conso le dri verroutine SWCHAR for dis­play. The same logic is fo l­lowed for the FI GS codebranch. The FI GS cha rac­ter's numeric va lue is added

to a n o ffset (378) to form thelower 8-bit address for entryinto the conversion table.When a CTRl-D is keyed,the ClP program sets D=O,forcing the receive programinto the l TRS mode.

RTTY Transmit ProgramThe t ra ns mit p rog ra m

consists o f t hree separaterout ines: transm it, CQ, andMorse ID. The transmit rou­tine togethe r with the CI Pa nd console d rive r pro­gra ms genera te Ba udo tcode from the terminal. Aflowcha rt fo r the tran smitroutine is shown in Fig. 5..The program begins byzeroing the sc ratc hpad andthen turning on the trans­mitter. Both the LTRS/FI GSregister (D) and the co lumncount register (E) are zeroedbefore the scratc hpad istested .. The scratchpad istested for either a branc h tothe Morse ID routine or theCQ ro utine. If bo th tests are

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RTTY transmit program listing.

9000. I~~\loI0.1~~9000.1~7

&0'.'';'79001. ''53&o~.IOO

900:.100 87';' _&4:.182 "' - oc.&4:. ""

~, 00'&4:.'87 ::";J ",00I~.11l L~ -";:.113 ~-" ;:.11':1 e7~

_.. ;:

";:.Il" ~, 00'";:.In JI ;: - oc,9oO;:.1~ 37,;. OOJ";:.':7 3';: '00 -0-0;:.1:;: '" 'M -";:.I~ l7';' IU:";:.n:" :1:0: '" oc,..~. ,~;: '" '" OC,&0:0:. , ~" 17:;&0:0:. '06 :;7';' '"&o:o:.,-,e ~, '" OC,&0a. 1"3 J I" - OC,&0•• 1% ~, '" OC,

042 .' 6 1 ~,

IM2 . 16 2 '" ," OC,042.16" ~, '"" 2 . ,"7 ~, '" oc,9002 . I n ." ...&4;:.17 ~ JI" ...:; oc,&0z, 17'7 ~ -";:.;:ell ~,

"'" •• ;:el2 :"" •• ;:el~ 370> 81.&o2.~ ::0;: .17 oc,0-0;: •• '8 8'6 00'0-0 ••• '. : ," ...: oc.&4•• ;:1' ~,

804_ •• 16 ;IIO-O •• ;:P· ::".. ~0-0;:.;:;:' = ;:... oc..... ;:.;:.~ r... _(1000 z, ;:;:,;. : I~ ':'I: oc,"," •• ;::1 :,0:; ~, oc,~ ~ 14:"~ ss P;:~ ~ l7" eee

( ,".~"", J the mai n prog ram for a n­other c ha racter. The I F testsimply return s to the mamprogram for another c ha r­a cter. The space testbranches within the subrou­tine to a point where theco lu m n co u nt is inc re­mented, and a return to thema in program is executed.

If a ll three tests a re in­va lid. XM IT the n tests theLTRS/FI GS reg ister. If thep rogra m is in the LTRSmode , the new c ha racter iscompared to 1008. Apositive result means thenew c haracter is a n ASCI ILTR and its Baudot equiva­lent is transmitted . If thecompa rison is negative, thenew c haracter is a FIG. andthe Baudot FIGS shiftc ha racte r (338) mu st betran smitte d b e fo re thec haracte r. After the Baudo tc ha rac te r is tr ansmitted ,

invalid , t he progra m getsthe next character fro m thetermin al usmg a consoled rive r r o u t in e ca lledSRCHAR. If the ASCI I c ha r­acter is not the ESC keycode, then the programcall s the subro utine XMIT;otherwise, it branches to therece ive r program, A flow­c ha rt of the XMIT subrou­tine is shown in Fig. 6. Afterbeing ca lled, this subrou­tin e immediately echoesthe c ha rac ter back to thete rm inal fo r d isplay usi ngthe console d river subrou­tine SWCH AR. XM IT thencompares the ASCII c har­acter with the character forcarriage re turn (CR), linefeed (IF), or space. If a ny ofthese tests is val id, the validc ha racter is converted toBau do t and transm itted .The CR test sets the col umncount to zero and returns to

0' "... )

( • • r~•• )

1n .

( ."v.' )

I t...... h'-' I

I cOO. -".. ..uOO. II "'..&<OJ ••

Fig. 6. Transmit subroutine.

r- - - - - -'''-(......-;:..........>"..'---- ----,<OO( .-,/

~ 110 73 Magazine. August, 1980

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( ,...... )

I ,OAO "~OO' IL' ''. ' ''0I

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. ...1' u ••,...••HJ. . .. '0..... '....,

It"o .'UOO' • I<...,. 0....". ..

II <ALL ,n.o I

I OUL " "0 I

1'0'0 L'.I <00. I

I . 'C' "" 0 I

( •• N ... )

LI>& O L'.'<1'0.

Fig. 7. Utility transmit subroutine.

.I'U , •••'"~

the l TRS/FIGS mode mustbe shifted to FIGS before aretu rn to the m ain prog ramis executed. When the pro­gram is in the FIGS mode.the l TRS code (37s) mustprecede the next l TR to betransmitted . After any print­able cha racte r is transmit­ted . the colum n counter (E)is incremented before aretu rn to the main prog ra mis executed . The util ity sub­ro u t in e s SPFIGS a n dSPlTRS shown in Fig. 7 han­dIe the l TRS/FI GS conve r­sions and SSE ND subro u­ti ne does the actua l char­acter transmitting. If uponreturn to the m ain programthe colum n count is equa lto 72 (width of a standardTTY termi nal). a subro utine

73 Magazine . Augus t, 1980 111 ~

( IlU' ) ( O" .T )

I N._.... I'-"0

U .. L 0'-'

'u.' .,,,T. ..... '''l.

,-~T...... ' T.. .

l ' otCOU'u . ,.. . ..00' _ . ... I

J..

y>"'-'"~"" ........~••T

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~ " I_ _ '0

I CAl.L T......" I

I C.LL ""'CO II OOTO I

T...... "

Fig. 9. Morse ID program.

t . .......,'O"TO"'~O

Ot.",t.,• •

.t. "'t"'.O'u " 0 " I TO

'u .n .... COOl

". ........ TO

TU ....... ) ILOAD ...00

' . tt"t..

.0""'"'£0 00 TO .0«1<,

110.1t. . ..O" .

... .. '••e

OUT"'" . 0..n '''L

,."". ' ......

' ~L '"fl L y • • T

'.t...... T....00. 0'0'0".

Fig. 8. CQ transmit program. Fig. 10. Control interrupt p rogram.

SPLFC R is called. SPLFCR is encountered in the but­transmits line feed, two car- fer, a branch to the Morseriege returns, and the LTRS ID routine is executed.shift character. The two car- After each column count ofriage returns allow mechen- 72. the SPLFCR subroutineleal printers time to return is called . A special, man­to co lumn zero before text ually-executed routine ,starts again. After execut- LOAD BU FF ER (beginninging SPLFCR, the program at 044n40J, is used to ini­branches back to "se t LTRS tially load the buffer. Thiscode" to beg in anot he r routi ne sequentially loadsloop through the program . ASCI I cha racters from the

The CQ routine, whose terminal into memory unti lfl owchart is shown in Fig. 8, a period is sent. The periodfi rst ca lls $PLFCR then sets is sto red and then the pro­the column count to zero . gram branches to the re­ASC II cha racters are then ceive program . The CQ buf­sent seq ue ntia lly to the fer is p laced at the end ofXMIT subrouti ne until a the RTTY program so thatperiod character is en- the length of the buffer iscountered. When a period limited only by the memory

~ 112 73 Magazine· Aug ust, 1980

size (less stack req uire­ments).

The flowchart for theMo rse ID ro ut ine is shownin Fig. 9. This program gen­erates Morse code char­acters by interrupting theaudio output of the RTTYinterface usi ng the "readeron/off" control line of theH8-2 paralle l port. (A com­ple te d iscussion of th isMorse code storage tec h­nique can be found in Ref­erence 4.) Each Morse cha r­ac ter is represented fromleft to right, one bit pe r e le­ment, using 0 for dot and 1for d ash. At the end of thecharacter, a 1 is p laced as astop bit; remaining bits are

filled wit h zeros. A com­plete alphanumeric conver­sian table in octal appearsin the source code listing ofthe Morse 10 program. Aword space is generated byusing two all -zero char­acte rs . The number ofMorse characters must belo aded in 043/201 in octa l.The prog ram sequentiallyloads characte rs fromTAB LE into the accu mu­lator . Each cha racte r isshifted left using ADD Aone place per loo p and thenthe carry bit is tested. If thecarry bit is not set, a dot istransmitted. If the carry bitis set, a dash is transmitted .When only zeros remain in

Partial Parts List- LM565CN phase locked loop - National Semiconductor- LM31 1H- National Semiconductor- AY-3·1 01 5 UART - General Instruments- LM555CN - National Semiconductor- CD4066AE- RCA- SN7473N- Texas Instruments- SN7400N- Texas Instruments- SN74LS365 - Texas Instruments- LM741CN-National Semiconductor-85.5 mH (463 turns of No. 34 enamel wire on 1811P-A400·3B7pot core-pot core from Ferroxcube)

the accu mu lator, a dash­le ngth letter space is gen­erated before the next char­acter is loaded . The subrou­tine COD EGEN uses a sub­rou tine DlY to generate dotand dash lengt h time de­lays. DlY is part of Heath'sH8 fro nt pane l monitorroutine (in ROM) and usesthe interrupt clock togenerate time de lays inm il l iseconds equa l to twicethe v al ue of t he ac­cumula tor.

Once the CQ buffer andthe Morse 10 tab les havebeen loaded, the complete

U1U2U3U4,U5U6U7usU9U10,U11L1,L2

RTTY object program canbe dumped onto a new tapep reserv i ng t he custo mtab les.

Control Interrupt Program

The f low diagram fo r t heCIP is shown in Fig. 10.Heath's H8 console d riverprogram has interrupt pre­cessing for CT Rl A, B, C, 0,0, P, Q, and S. CTRL A, B, C,and 0 are av ailable for userrou t ines. W hen a CTRl Athrough 0 interrupt occurs,an identi fying number isplaced in $CSlCTl (040/252) and then a branch is ex-

ecuted to the user interruptprogram found at the ad­d ress la beled $C IS. Theeight least significant b itsof the address of the in ter­r u pt pr o gr am (0008)are placed in $( 15 (040/250).and the ei ght m ost sign ifi ­cant b its (0448) are placedin $C IS+1 (040/251). The in­terrupt program tests the lo­cat ion $CSlCTl to deter­mine whi ch o f t he f ou rCT Rl interrup t s has oc­cu rred. A fter either sett ingthe l TRS code (CTRl-D) ormoving one of the threenum bers to a sc ratc hpad,

$CSlCTl is zeroed and a re­turn exec u ted.

Testing

The co mplete RTTY in­terface may be tested bytape record ing the m icro­phone o utput o f the inter­face o n an audio cassetterecorder and t hen playingthat signa l back through t hein terface receiver. Perfectcopy should resul t if every­thi ng is correct. The po lari­ty o f the system m ay beve r i f ied b y th e cor rectreception of amateur RTTYsig na ls (usual ly found near14.1 MHz ) in t he USBmode. •

References1. RTTY Handbook. , 73, Inc.,Peterborough NH 03458, 1978, p.17.2. Ibid, pp. 65-66.3. Robert Furtaw, ed., "Using theH8 Console Driver," Remark., No.2, Heath User's Group, St.Joseph MO 49085, pp. 16-17.4. L. Krakauer, "Eff icient Stor­age of Morse Character Code,"Byte 14, pp. 36·38, 1976.

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73 Magazine. August, 1980 113 ~