Weather Satellite decoder from Maplin. Consists of at least three separate projects: The analogue Receiver, the analogue to digital Decoder, and the 8bit Input Port for connecting it to the CPC. The Decoder project contains source code for BBC B and for Amstrad CPC.
I/O Port Address
The recommended I/O address (used in the CPC example source code) is Port F8F0h (ie. shorten Link 1 on the Maplins 8bit Input Port).
Bit7 Dotclock (probably derived from E.O.C.) (end of ADC conversion) Bit6 Sync (used as hsync) (the sample program doesn't seem to do any vsync handling?) Bit5-4 Unknown/unused Bit3-0 Luminance
The decoder outputs some control signals and an 8bit luminance signal. That data can't be squeezed through the 8bit input port, so some of the luminance bits are left unconnected (the CPC's color palette couldn't display them anyways).
Source Code
Here is a commented version of the original sample code...
--- basic code ---
5 MEMORY 30000:MODE 2 10 LOAD"wefax1.obj" 20 INPUT "enter horizontal resolution 1-4";resh 30 IF resh>0 AND resh<5 THEN POKE <resmod>,resh ELSE CLS:GOTO 20 40 CALL <entryoint> 50 CALL <rerun> 60 GOTO 50
--- asm code ---
io_port equ 0F8F0h temp equ 40000 lum equ 40001 xreg equ 40002 yreg equ 40004 hxreg equ 40006 blkadd equ 40008 org 0A028h ;=41000 decimal entrypoint: ld a,00h ld [hxreg+1],a call 0BC0Eh ;SCR_SET_MODE rerun: ld hl,160-1 ld [xreg],hl ld hl,200-1 ld [yreg],hl ld ix,bytead+15 ;\ ld a,0fh ;ink index ; ld [temp],a ;ink index ; colset: ; ld a,[ix] ;ink address ; ld b,a ; init ink 0..15 ld c,a ; ld a,[temp] ;ink index ; call 0BC32h ;SCR_SET_INK ; ld hl,temp ;ink index ; dec [hl] ;ink index ; jp m,wtfrm ;lop done ; dec ix ;ink address ; jp colset ;lop next ;/ wtfrm: call 0BD19h ;MC_WAIT_FLYBACK call 0BD19h ;MC_WAIT_FLYBACK loop1: ;--- y loop --- di ld bc,io_port line: ;\ in a,[bc] ;[port] ; bit 6,a ;[port].bit6 ; jr z,line ;/ enline: ;\ in a,[bc] ;[port] ; bit 6,a ;[port].bit6 ; jr nz,enline ;/ ld d,0Ah ;\ delay: ; delay dec d ; jr nz,delay ;/ loop2: ;--- x loop --- di resmod equ $+1 ;\horizontal resolution ld d,2 ;<-- MODIFIED (1..4) ;/ ld bc,io_port smpl: ;\ in a,[bc] ;[port] ; bit 7,a ;[port].bit7 ; jr nz,smpl ;/ ensmpl: ;\ in a,[bc] ;[port] ; bit 7,a ;[port].bit7 ; jr z,ensmpl ;/ dec d jr nz,smpl getlum: in a,[bc] ;[port] ;\ and a,0fh ;[port].bit0-3 ; ld [lum],a ;/ rra ;\ rr b ; rra ; rr b ; rra ; rr c ; rra ; rr b ; ld d,0 ; rlc b ; rr d ; rr d ; rlc b ; rr d ; rr d ; rlc c ; rr d ; rr d ; rlc b ; rr d ;/ ld a,[xreg] rra jr nc,nolft or a ;cy=0 rr d nolft: ld [hxreg],a ld a,d ;pixel value ld [temp],a ;pixel value ld hl,5000h ;\ ld a,[yreg] ; srl a ;\ ; srl a ; ; srl a ;/ ; ld e,a ; ld d,0 ; ld b,8 ; mult: ; add hl,hl ; jr nc,noadd ; add hl,de ; noadd: ; djnz mult ; ld [blkadd],hl ;/ ld a,[yreg] ;\ sla a ;\ ; sla a ; ; sla a ;/ ; and a,38h ; calc vram address ld h,a ; ld l,0 ; ld bc,[blkadd] ; add hl,bc ; ld bc,0c000h ;vram base ; add hl,bc ; ld hl,[hxreg] ; add hl,bc ;/ ld a,[temp] ;pixel value ;\ ld ix,xreg ; bit 0,[ix] ; jr nz,plot ; draw pixel or a,[hl] ;merge pixels ; plot: ; ld [hl],a ;/ ld bc,0001h ;\ ld hl,[xreg] ; or a ;cy=0 ; sbc hl,bc ; next x jr c,nexy ; ld [xreg],hl ; jp loop2 ;/ nexy: ;\ ld hl,160-1 ; ld [xreg],hl ; ld hl,[yreg] ; or a ;cy=0 ; sbc hl,bc ; next y jr nc,newlin ; ei ;frame done ; ret ;return to basic ; newlin: ; ld [yreg],hl ; jp loop1 ;/ bytead: ;color palette (increasing luminance on green monitor) db 0,1,2,4, 5,6,8,10, 12,14,16,18, 20,22,24,26
Scanned Articles
The Weather Satellite receiver consists of three Maplin projects:
- Weather Satellite Receiving System – Part 1 - Receiver - project 18 (not yet scanned, does somebody have it on paper?)
- Weather Satellite Receiving System – Part 2 - Decoder - project 20.a (pdf)
- Amstrad 8bit Input Port (for use, among others, with the Satellite Receiver) - project 20.b (pdf) (see here for details and commented schematic)
Judging from their name, the following maplin projects may be also related:
- Weather Satellite Down Converter – Part 1 - project 22 (not yet scanned, does somebody have it on paper?)
- Weather Satellite Down Converter – Part 2 - project 23 (not yet scanned, does somebody have it on paper?)
- Weather Satellite Prediction Table - project 24 (not yet scanned, does somebody have it on paper?)