Zenith Z-100 PC series Service Manual page 289

Page
4.3
Troubleshooting
Table 4.1. Serial 1/0 Component Failures
SYMPTOM SUSPECTED PARTS
No Output U338, pin 18
U309, pin8
U351 , pins 6,8,11
U322, L322, C322
Data Bus Fault U348
U338, pin 15
No Output U347, U350, U351
COM1,COM2
U316, pin 31
U339,pin8
J307-J312
Wrong baud rate U322, L322, C322
COM1,COM2 U316
THINGS TO CHECK
Active-low ACE chip select.
1/0
write control signal.
Output signals.
Check for base frequency
of 1.8432 MHz.
Check for proper signal gating.
Pin 15 determines data
direction transfer.
Check signal flow through
drivers and receivers.
Software controlled interrupt
enable.
COM 1 interrupt signal.
Check for proper IRO
selection. COM 1 uses IR04 (J309),
COM 2 uses IR03 (J308).
Check for base frequency of
1.8432 MHz.
Replace ACE