80-Bus News

19 100 REM Demo program for light pen

110 GOSUB 1000 :REM Initialise the code 120 GOSUB 2000 :REM Get coordinates

130 PRINT “Row=";ROW;” Col=";COL

140 GOTO 120 :REM Loop

1000 REM KKK KELEKEKEEEER EEE EEEERERE EERE EREEELEREREREE TERRES

1001 REM Subroutine to Initialise the USR function 1002 REM 8YHERERHEKRRERKKAE HEE R ERR ER EERE IEEE MAREE EERE EERE

1003 REM

1010 ADDR=-128 :REM Machine code Put here

1020 DEF USRO=ADDR :REM ***define USRO function *** 14030 FOR I=ADDR TO ADDR+39 :REM POKE loop

1040 READ A

1050 POKE I,A :REM Set byte

1060 NEXT I

1070 RETURN

1080 REM ****Machine code subroutine ****** 1090 DATA 219,178,15,56,251 ,62,27,211,177,219 14100 DATA 178,15,56,251,62,80,211,177,219,178 1110 DATA’ 7,56, 251,219,177, 103,219,178, 7,56,251 4120 DATA 219,177,111,229,42,5,1,227, 201

1130 REM 2000 REM FEXREEK REEL ER EEEH EKER ELE ERE EKER REE EER ERE

2001 REM Subroutine to Read back coordinates from LPEN 2002 REM KLEE EEE EERE ERE EE ER EEREEEEREE LEE ERE EER EEERER ERE

2003 REM

2005 MAXCOL=80 :REM Screen size

2010 OFFSET=1 2 :REM Character offset 2020 COL=USRO(ROW) :REM Get coordinates 2030 ROW=INT(COL/256) :REM Isolate ROW number 2040 COL=COL AND 255 :REM Isolate COL number 2050 COL=COL-OFFSET :REM Allow for offset 2060 IF COL>=O0 THEN RETURN :REM return if all ok 2070 COL=COL+MAXCOL :REM Not,so adjust COL 2080 ROW=ROW-1 :REM ..and correct ROW 2090 RETURN :REM return ROW and COL

Shown below is an example of how the subroutine 2000 can be altered to increase the accuracy of the returned value. Here line 2020 obtains four successive readings from the Light Pen. These are averaged in the next line, and then a check is made that all four lie close to the average value. (If you leave out the check and C1-C4 vary widely you may find strange values like Col=129 appearing!). The routine might be improved by including a small delay between the calls to USRO. Only two calls with a reasonable delay between them may be enough....experiment!

2000 REM KEELER KK LERKELERERE EER EEE EE EER EERE EEEEEEEEREREERE

2001 REM Subroutine to Read back coordinates from LPEN 2002 REM KEREKEEKRLEREEKEE EK LEEEKEERELEEEREEEKEELELEERELELEEERE

2003 REM 2005 MAXCOL=80 :REM Sereen size 2010 OFFSET=1 2 :REM Character offset

2020 C1i=USRO(A) : C2=USRO(A) : C3=USRO(A) : C4=USRO(A) 2021 COL=(C1+C2+03+C4) /4 :REM Average four readings 2022 IF ABS(COL-C1)>2 OR ABS(COL-C2)>2 THEN GOTO 2020 2023 IF ABS(COL-C3)>2 OR ABS(COL-C4)>2 THEN GOTO 2020

2030 ROW=INT(COL/256) :REM Isolate ROW number 2040 COL=COL AND 255 :REM Isolate COL number 2050 COL=COL-OFFSET :REM Allow for offset 2060 IF COL>=0 THEN RETURN <:REM return if all ok 2070 COL=COL+MAXCOL :REM Not,so adjust COL 2080 ROW=ROW-1 sREM ..and correct ROW

2090 RETURN :REM return ROW and COL