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TRANSCRIPT
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
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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~
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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 OUL " "0 I
1'0'0 L'.I <00. I
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( •• 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
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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.,• •
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'u .n .... COOl
". ........ TO
TU ....... ) ILOAD ...00
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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 ~