Symptom: Startup screen appears, but no cursor.
Target : C-64 (all models)
Possible sources of failure
- CIA -> missing interrupts
- CPU -> does not execute interrupt routine
Before checking anything, remove any modules from the expansion
port. If the problem is now solved, your module applies such a
heavy load on the /IRQ line that it drops below the trigger
level. At the risk of causing you a sleepless night: Your module
seems to be seriously damaged!
Check pin 21 of the CIA #1 (the one next to the keyboard
connector) which is the /IRQ output. You should be able to detect a
periodic signal with either a logic tester (both the HI and LO
LEDs lit) or with an oscilloscope (the signal should be of rectangular
shape and the frequency - independant from PAL or NTSC version - 60Hz).
Most likely you will not detect correct /IRQ signals. You
should now swap the two CIAs, if they are socketed. If you can
now see the blinking cursor (and have a defective serial bus or
user port :), the CIA is damaged, buy a new one. If the CIAs are
not socketed, replace CIA #1. After exchanging the broken
CIA your C-64 should work again.
In the rare case that the /IRQ signal at pin 21 of the CIA #1
seems to be alright, check for it at pin 3 of the CPU, too. If
you cannot find it there, check for haircracks.
If - which is very rare - the /IRQ signal can be detected at the
CPU, then it may be that it is damaged, replace it.
IRQ signal well-shaped?
If you happen to have an oscilloscope, check the amplitude of
the /IRQ signal and also pay attention to its (hopefully rec-
tangular) shape. It may be that the amplitude is too low for
triggering interrupts; either the CIA or the CPU may be damaged.
Updated: May 18th, 1999
Created: January 1997
Status : NOT VERIFIED!
Site copyright © 1997 by Marc-Jano Knopp
This document is part of MJK's Commodore 64 & LCD Page
Brought back to life by Peter Schepers, Dec 10, 2005 because I really liked this site!