The Apple I's onboard video memory was capable of displaying text only. With the addition of the GT-6144 Graphics Board, the Apple I could display primitive black and white graphics.
INTERFACING A SWTP GT-6144 GRAPHICS KIT TO AN APPLE I MICROPROCESSOR
The STWP graphics board displays a picture made of a 64 X 96 array of rectangles. The schematic I've enclosed is pretty straigt forward. The .006 MFD capacitor connected between pins 6 and 7 of the 74123 controls the length of time that must elapse between each output of date to the GT-6144. I've been unable to load the date faster than the .005 capacitor will allow.
The rectangles are turned on by sending a horizontal co-ordinate followed by a vertical co-ordinate to the GT-6144. This is clearly explained in the instructions included with the GT-6144. The co-ordinates are stored in location D00A which in basic would be -12278 and would be accomplished by the following: 10 POKE -12278,X where 'X' is hte co-ordinate to be written to the 6144.
The GT-6144 is a kit, and took me about 4 hours to build. The instructions are very good, and my board worked the first time I applied power to it. There is no power supplied with the kit, so you must either build your own, or buy their kit which is sold seperately.
The Gt-6144 is either connected to a TV monitor or to a standard TV thru a video modulator just as you must do with the APPLE 1.
The commands that can be acomplished by the GT-6144 are 1) turn on a rectangle 2) turn off a rectangle 3) display screen as white on black 4) display screen as black on white. The PIA must be set up before you can write to the GT-6144. It must also be set-up after you press the 'RESET' button on your APPLE 1. I get around this problem by making the first statement of my programs an exit in to the subroutine attached to this sheet. This routine not only sets up the PIA but also clears the screen of any rectangles left over from the last program. or the random rectangles that are always present right after powering-up the board.
If you plan to program the graphics in Basic, rember that Basic is very slow, and therefore your graphics will not respond very fast. If you use the POKE command shown above, the following values for X will cause the following actions:
X=0 to 63
send horizontal cordinate and set up to set off rectangle
X=64 to 127
send horiztonal cordinate and set up to set on rectangle
X=128 to 223
send vertical co-ordinate and set on or off rectangle as previously defined in the last horizontal co-ordinate sect
X=240
display screen as black on white
X=241
display screen as white on black
This information is taken from the schematic for the hook up of the Apple1 to the SWTP GT-6144 graphics board, refer to it while reading this, especially on the PIA pins , 23, 34, 35, 37 and 38.
The chip on the left is for the PIA for the graphic board. All pins marked with a * are connected to the corresponding pin on the keyboard PIA, pins 1, 20, 21, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36. This is listed at the bottom of the sheet on the left side. Pin 22 goes to the address line A3.
The capacitor on the 74123, between pins 6 and 7. On the bottom of the sheet on the right hand side says, ì you may have to play with this capacitor a little. The capacitor governs the length of time that must elapse between each output of data to the GT-6144.
The 12 pin molex connector on the GT-6144 board is shown on the right side of the sheet.
The information right below the 12 pin molex connector has the 6820-6520 PIA pins.
Richard Drennan
4590 Oberlin Avenue
Lorain, Ohio 44053
May 5, 1978
Hi Joe.
I finally got my graphics board to work!!! I'm sending along the information
on the graphics that I've been promising you for the last several months.
The system uses the SWTP GT-6144 Graphics board kit which can be had for around
$100.00 . I'm sending you the following: 1) some sales liturature about the board
2) a schematic of how I hooked up the board (it's actually very simple) 3) a
diss-assembled copy of a subroutine needed to set up the graphics PIA and to clear
the graphics screen and 4) some notes to help anyone who might like to try to hook
up the GT-6144. I did not go into much detail about programming with the GT-6144
because the is covered very well in the instructions included with the GT-6144.
If there is anything that I can do to help anyone attempting to use my hook-up,
I'd be glad to help them.
Well, thats it for now. Hope that the information on the graphics board will
be of some help to someone out there.
Sincerely,
Richard Drennan
SUBROUTINE TO SET UP GRAPHICS PIA AND CLEAR SCREEN
IN BASIC - CALL 768
0300- 48 PHA
0301- 8A TXA
0302- 48 PHA
0303- 98 TYA
0304- 48 PHA
0305- A9 00 LDA #$00
0307- 8D 0B D0 STA $D00B
030A- A9 FF LDA #$FF
030C- 8D 0A D0 STA $D00A
030F- A0 2C LDA #$2C
0311- 8D 0B D0 STA $D00B
0314- A2 00 LDX #$00
0316- A0 00 LDY #$00
0318- 8A TXA
0319- 8D 0A D0 STA $D00A
031C- 98 TYA
031D- 09 80 ORA #$80
031F- 8D 0A D0 STA $D00A
0322- C9 FF CMP #$FF
0324- 10 04 BPL $032A
0326- C8 INY
0327- 4C 1C 03 JMP $031C
032A- 8A TXA
032B- C9 3F CMP #$3F
032D- 10 04 BPL $0333
032F- E8 INX
0330- 4C 16 03 JMP $0316
0333- 68 PLA
0334- A8 TAY
0335- 68 PLA
0336- AA TAX
0337- 68 PLA
0338- 60 RTS
0339- FF ???
033A- FF ???
033B- FF ???
033C- FF ???
033D- FF ???
033E- FF ???