Atari CX70

Atari CX70 Light Pen
Cover of the CX70 operator's manual (1981), with the pen drawn in the foreground
Manufacturer	Atari, Inc.
Type	Light pen
Release date	1981
Discontinued	1981
Price	US$74.95
Interface	Atari controller jack
Compatible	Atari 400, Atari 800
Successor	CX75 Light Pen
Model	CX70 (CA016311)

The Atari CX70 Light Pen is Atari's first light pen for the Atari 400 and Atari 800. Pressed against the screen of a CRT television, it lets a program read the position of the electron beam under the pen tip, for menu selection, games and drawing. Atari announced it in June 1980 at US$74.95 but sold it only for a few months in early 1981, and few were shipped. It came with the CX4124 demonstration cassette and the Atari Light Pen Operator's Manual, C016367.

History

Atari showed the CX70 at the Summer Consumer Electronics Show in Chicago in June 1980. Creative Computing reported that it read the coordinates of the television's electron beam as the beam passed, plugged into any of the four controller jacks and listed at $74.95. The comp.sys.atari.8bit FAQ says Atari shipped it in the winter of 1981 and that it had been discontinued by Atari's price list of 1 May 1981. It adds that some, if not all, units were marked "Not For Retail Sale".

ANALOG Computing followed the delays. In May 1981 it reported that the first release had been held back by a software problem, the pen not working on 400s, and that quality problems had been found after a few hundred were shipped. In September 1981 it said that only 200 had gone out before Atari found that the internal switch was poor, with about half not working. The writer, who had one, found its accuracy imperfect, its switch unreliable and some colours, such as light red, hard for it to read.

Atari's next light pen was the CX75, announced in June 1983 and introduced in January 1984 with the AtariGraphics cartridge. Gibson Laboratories developed it for Atari.

How it works

The tip of the pen has a window that lets light from the screen reach a phototransistor, and a switch that closes when the pen is pressed against the screen. When the pen sees the brief flash as the beam passes under it, the computer captures the horizontal and vertical scan position. Atari's manual gives the registers as 54284 (horizontal, $D40C) and 54285 (vertical, $D40D), with copies at 564 and 565 ($234 and $235). They update 60 times a second while the pen is over a bright enough area.

CX70 position and switch readings (operator's manual)

Reading	Location	Values
Horizontal position	564 (54284)	74 to 227, or 0 to 5
Vertical position	565 (54285)	16 to 111
Tip switch, jack 1	PEEK(632)	14 pressed, 15 released
Tip switch, jacks 2 to 4	PEEK(633) to PEEK(635)

Atari's first BASIC example converts a horizontal reading Q to a GRAPHICS 23 column with X=Q+154-228*(Q>10), which maps 74 to 227 onto 0 to 153 and 0 to 5 onto 154 to 159, and takes 16 from the vertical reading to give rows 0 to 95. Appendix A prints a different sequence, adding 154 and then subtracting 128 if the reading is below 10; applied to readings of 74 to 227 it gives 228 to 381, so the program line is the form to use. The computer reads the position whether or not the tip switch is pressed. If there is too little light, the registers keep their last values, so a program can repeat an earlier selection if the switch is pressed while the pen is pointing away from the screen.

Atari's manual lists the pen's limits. Pens vary in sensitivity. The pen is more sensitive to blue than to green, and to green than to red, and more sensitive in the middle of the screen than near the corners. Accuracy rises with screen brightness; at low brightness the readings drift and become noisy. A light pen needs a CRT television or monitor.

Which jacks it works in

The manual says the pen fits any of the four controller jacks and that the demonstration programs expect it in jack 1. The FAQ says the Atari 400 supports a light pen only in jack 4, so much light pen software cannot be used on a 400.

Demonstration cassette

The CX4124 cassette holds PALLET, a light pen painting program: an 8K version on side 1 and a 16K version with more features on side 2. Both load with CLOAD from an Atari 410 Program Recorder, with the Atari BASIC cartridge fitted. A copy of the cassette and a listing of its program are preserved on the Internet Archive.

Care

Atari's periodic maintenance section says:

• Clean dust from the window in the tip occasionally with a soft cloth, to keep the pen sensitive. Do not use paper towels, which scratch the window.
• Keep the pen in a cool, dry place.
• Do not use detergents or cleaners on the pen.
• Do not clean the window with rough or sharp objects.

Testing and troubleshooting

Atari's first programming example plots a dot wherever the pen is pressed against the screen, and makes a quick test of a pen in jack 1:

1 REM DEMO PROGRAM #1, LIGHT PEN WITHOUT "PENBUF"
10 GRAPHICS 23:SETCOLOR 4,12,8:SETCOLOR 0,7,14:COLOR 1
20 IF PEEK(632)=15 THEN 20
30 Q=PEEK(54284)
40 X=Q+154-228*(Q>10)
50 Y=PEEK(54285)-16
60 PLOT X,Y
70 GOTO 20
80 END

Atari's checklist for a pen that does not respond is:

1. Check that the program was typed in correctly.
2. Check that the plug is fully seated in the controller jack.
3. Check for dust or dirt in the tip.
4. Check the luminance in the program's SETCOLOR statement, and raise it if it is too low.
5. Check the television's brightness setting.
6. Check the tip switch. With the pen in jack 1, press it against the screen and type PRINT PEEK(632); the result should be 14. Lift it and repeat; the result should be 15.
7. If step 6 fails, plug the pen into jack 2 and repeat using PEEK(633). If it now works, the jack is faulty; if not, the pen's switch is.

The manual tells owners not to take the pen apart but to send it to an Atari Service Center.

Documentation

The operator's manual is hosted and described on the Atari CX70 Documentation page.

Related pages

• Atari 400
• Atari 800
• Atari 410
• Atari CX77