BBC Micro Issue 4 - factory disc interface fitted.
Everything was OK until I added Econet, then the Beeb wouldn't start up, just a flashing cursor at the top left hand corner of the screen.
Looking at the scope, everything was reasonable except that the NMI line to the 6502 was very busy indeed. I removed the disc controller IC and made link S9, but no difference. I then removed IC91 and made S2 and the problem vanished and the Beeb booted up with no disc or Econet enabled.
I then removed S9 and reinstalled the disc controller and got DFS back.
The source of the NMI was not the 6854 ADLC, as its IRQ line was high during this time. By process of elimination with a thermal camera, it turned out that IC27 was bad - no-one would have known until Econet was installed. I removed IC27 (7438 - yup, old school TTL) and replaced it, then removed S2 and reinstalled IC91 and the Beeb booted up successfully with CTRL+N+BREAK bringing up the usual BBC boot screen with Econet station ID. Two out of the four NAND gates tested as bad, one for the strobe on the printer port and the other for Econet.
Everything was OK until I added Econet, then the Beeb wouldn't start up, just a flashing cursor at the top left hand corner of the screen.
Looking at the scope, everything was reasonable except that the NMI line to the 6502 was very busy indeed. I removed the disc controller IC and made link S9, but no difference. I then removed IC91 and made S2 and the problem vanished and the Beeb booted up with no disc or Econet enabled.
I then removed S9 and reinstalled the disc controller and got DFS back.
The source of the NMI was not the 6854 ADLC, as its IRQ line was high during this time. By process of elimination with a thermal camera, it turned out that IC27 was bad - no-one would have known until Econet was installed. I removed IC27 (7438 - yup, old school TTL) and replaced it, then removed S2 and reinstalled IC91 and the Beeb booted up successfully with CTRL+N+BREAK bringing up the usual BBC boot screen with Econet station ID. Two out of the four NAND gates tested as bad, one for the strobe on the printer port and the other for Econet.
Statistics: Posted by maniacminer — Wed Feb 14, 2024 12:56 am