May June 1980 - Commodore Computers

MAY/JUNE. MAY/JUNE, 1980. ISSUE 4.
COMPUTE.
113
order data bit, so I let programming convenience
override generality, and output the highest order
bit always as a mark, followed by sufficient marks to
act as stop bits and marks until the next data come
along.
along.
Table 2 Output parameters
A. The frequency output by the 6522 shin shift register is given
by the formula
Frequency = System clock frequency,
4 •* (contents ts of T2 low + 2)
assuming that the shift register contains 01010101 or 10101010. 10.
The PET sys tem clock appears to be spot on 1 MHz,
The PET system clock appears to be spot on 1 MHz,
and we calculate
use T2 low
For frequency
use T2 low
2225 Hz
6E hex -= 110 decimal answer mark
2025 Hz
79 hex -= 121 decimal answer space
t270 1270 Hz
C3 hex R = 195 decimal originate mark
1070 Hz
E8 hex '"" = 232 decimal originate space
If desired, frequencies may be halved by usingg 00110011
in the shift register, and divided by four by using 00001111.
Depending on the equipment used, , a certain amount of
inaccuracy can be tolerated. I found no equipment that
would give errors in the following ranges:
69 to 71 instead of 6E 8B55 to D3 instead of C3
76 to 80 instead of 79 DA to 10 F8 instead of E8
B. The timing loop as given by the present machine e code yields yictds
Bit time = 1284 XR + YR - 1257
clock cycles between successive loads of 6522 T22 low,
give or take one cycle due to the difference in execution
times between n mark and space caused by the branch at 0347.
With the e PET's one microsecond dock clock period, period. we have
XR = - INT( Bit time + 1257 ) ,
1284
1284
YR = _ Bit time + 1257 - 1284 XR
XR
For baud rate
one data bit time XR YR SB S8
110 0
9091 1 us.
08
OF( ( = - 15)
16 (=22)( - 300
3333 us.
03
93( ( = . 147)
07
4% error seems to be tolerable. For for 110 I ill baud, we can
range from XR.YR XR, =07,CO= to 08,60; for 300 baud we can
range from 03,70 to 10 03,B8.
1 GOTO 1000 Bl00
2 LIST 99
3 SS="8.03 S$; S14: 11 4: CHEEP PRINT MASTER WITH -. ~
YR
-.DISASSEMBLER"
~DISASSEMBLER~
4 RETURN
5 LINE FUNCTION: 23 LIST STYLE: : 43 BOLL ROLL -. ~
-nPAPER:50, ~PAPER:50,63 63-66 - 66
BAUD RATE
6 LINE FUNCTION: 55 PAGE FORMAT FORHAT FROM -. ~
i85-86: ~8S - 86 : 56 CRLF FROM 87-88
7 P=LEN(PS):IF P:LEN(P$) : P THEN FOR P0=1 PO:1 TO P:
-.P=ASC(MID$(PS,P0,l) ~P:ASC (MID$(P$ , PO,l)) ) ;GOSUB8: :GOSUB8 :
-.NEXT ~NEXT P0: : RETURN
8 POKE 10 IB23,P: 23,P:EYS(M0) SYS(M0) :RETURN:REM : RE~! POKE -> .,
-dB23,0:SYS{M0) :RETURN:REM FILL -.
.,1023 , O: SYS(MO):RETURN : REM FILL ~
.,NUL LS
-.NULLS
10 GOSUB 70:GOSUB : 3:PRINT 3 : SS:INPUT S$:INPUT -. ~
-."LIST .,"LIST_~~~- «F=(F+(Q=3 4} ) AND1:C=C-(F*Q=5 8) : NEXT
25 IF PY+OPV-PB PY+C>PV-PB THEN TI.IEN GOSUB 41
26 F=1:FOR F:1:FOR J=Y TO Z:Q=PEEK(J):F=(F+(Q=34
Z:O=PEEK(J) :F:(F+(O;J4
i) .,)IAN01
)AND1
27 IF Q*F=58 O·F=58 THEN GOSUB GaSUS 45:GOSUB :GOSUS 46:
:
-.GOSUB -.cOSUB 39:SS=" 39:S$. " :":GOTO 30
28 GOSUB GaSUB 32:IF PL+LEN(S$)+LEN(A$) (£5) (AS) >= >:., -i
.,PH-PR THEN GOSUB 45 : GOSUB 46:
-.PH-PR THEN GOSUB 45:GOSUB 46:
-.GOSUB ,casus 39:S$=" 39 : S$="
29 SS=SS+A$ S$=S$+AS
30 NEXT J:GOSUB J : 45:GOSUB 45:GOSUS 46:GOSUB 46 39:
-.GOSUB -.cOS US 39:GOSUB 40:GOTO 16
32 AS="":IF A$="":IF Q>127 0 >127 THEN AS=CHRS(114}:
A$=CIIR$(114):
-.Q=Q-128 "0"'0-128
33 IF Q>63 0 >63 THEN A$=A$+CHR$(115):Q=Q-64
A$. A$+CHR$(l 1S ) :0=0-64
3344 AS=AS+CHRS(Q-64*(Q