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CHARACTER WRITING PROBLEMS
CONDITION
POSSIBLE CAUSE
19. Characters don't write on the screen.
1.
Duplex switch in "full" position when it should be in "half".
1.
DAV latch (1C616) - No DAV signal.
3.
DAV priority encoder (1C61 1A, C, D, lC610D, lC619B).
4.
Three-state enables (1C6088, C, D).
5.
RAM WE. OC if DAV timing pulse (lC737A).
6.
RAM F. OC if DAV riming pulse (1C7198, D).
7.
DAV from lC719C, lC725A, lC713C.
8.
Special character generator l@04, 1C405, 1C406 latched on
(A1 or A2 high).
20. Wrong characters write on the screen
1.
Keyboard encoder 1C402.
2.
Keyboard buffer 1C403.
3.
Bus wires interchanged.
4.
Bus wires shorted to + or ground.
5.
Bus buffers 1C603, 1C620.
6.
Shorted bus foil.
7.
BAUD rate set wrong.
21. Screen filled with same character.
1.
RAM counters not counting:
tc722o, D.
tcs02.
tc219F, tc217D.
2.
Keyboard output enable always on:
P402-8 always logic 0.
Check DAV latch and three-state enables
tc616. tc608. tc610. tc619.
3.
Parallel input enable always on: Check items listed under 2
above.
4.
UART ilDE enable always on: Check items listed under 2
above.
22. Same line is repeated 12 times.
1.
+12 counter not cbunting up.
2.
1 c 51 9,l C 5 11 C, rc s 15 8, r C 716 A , r C 71 5C .
23. Blocks of characters are repeated left to right.
1.
2.
+4 counter not counting.
1c513, lc5148, lcs10C.
24. Writes two characters.
1.
Full Duplex key set for half duplex (released) instead of full
duplex (pushed in).
25. Characters write erratically.
'1.
Wrong baud rate.
26. Misses characters coming from the computer.
1.
Refresh interrupt not enabled at the rear pqnel baud rate
switch. Connect the gr6en wire to lug 2 of SW2 (for BAUD rates
great€r than 600).
2.
At 110 baud, the computer may not be programmed lor two
stop bits.
27. Characters write in more than one position on the screen.
1.
Address encoder. 1C512, lC526A, lC526D, & lC527B.
2.
RAM address lines shorted or open.
3.
Counters not counting up (see "Conditions 22 & 23" above).
4.
Check the counter cascade controls. lC73O,1C724 (+20 +
+ 4, + 4 + + 12, +QQ e
+ 12, etcl.
28. Writes control characters.
' t.
2.
Control character decoder. lC703C, lC703B, lC707C, and
tc702.
Control character detector. lC617A.
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Troubleshooting
