Atari 800XL

TROUBLESHOOTING (Continued)
PORT PADDLE
U22
PIN
BUTTON
U23
PIN
1 0
14
0
4
1
1 15
1
5
2 2
12 2
8
2
3 13 3
g
The waveform should vary from the sawtooth waveform
shown in Figure 2 to the pulse waveform shown in Figure 3.
If the waveform is missing, check the port connector, the
paddle control and check 1C U22 by substitution. If a button
is not functioning, use the chart to determine which pin the
button is connected to and use a logic probe to check the
pin while the button is pressed. The logic probe reading
should go from High to Low when the button is pressed. If
the reading does not go Low, check the button switch and
the port connector. If the logic reading is correct, check the
PIA tC (U23) by substitution.
Figure 2 Reference
JOYSTICK PORT
The following Basic program can be used to check the
operation of the joystick ports. Plug a joystick into the port
being tested, type in and run the program.
10 PRINT, "JOYSTICK", "BUTTON"
20 FOR P = 0 TO 1
30 PRINT "PORT"; P + 1, STICK (P), STRIG (P)
40 NEXT P
50 FOR T = 1 TO 200: NEXT T’
60 GOTO 10
On the Monitor screen the number 1 under BUTTON should
change to 0 when the button is pressed. The number 15
under JOYSTICK should change to the following for each
position of the joystick:
UP 14 LEFT 11
DOWN 13 RIGHT 17
If the joystick is not working properly, check the logic
readings on the PIA 1C (U23) and GTIA 1C (U17) pins that are
used for the Joysticks. Use the following chart to deter¬
mine which pin is affected by each joystick function.
PORT 1 PORT 2
JOYSTICK 1C
PIN
1C
PIN
UP
U23 2 U23 6
DOWN U23 3 U23
7
LEFT U23 4 U23 8
RIGHT U23 5
U23 g
BUTTON U17
8
U17 g
The logic reading should go from High to Low when each
function of the joystick is used. If the logic readings are
correct, check 1C U17 if the button is not working, and
check 1C U23 if the position functions are not working. If
the logic reading is not correct check the components con¬
nected to the pin with the incorrect reading and check con¬
nectors J5 or J6 for good conenctions.