Commodore 1541 - Appendix F: VIC-1541 vs. 1541 Drives; General Differences Overview; Reference Designator Mapping; Theory of Operation Differences

General
APPENDIX F
VIC-1541(1540) vs. 1541 DISK DRIVES
This appendix lists the differences between the VIC-1541 (also
known as the 1540) and the 1541 Disk Drives. If you are
maintaining a 1541, read this appendix in its entirety before
attempting any procedure in this manual. Basically, COMMODORE
combined certain discrete components which were present on the
VIC-1541 Disk Controller PC Board and combined them in custom
IC's. When they did this, they renamed the disk drive the 1541
and changed the reference designators. Functionally, there is
very little difference between the two versions.
Reference Designators
When servicing a 1541, always cross reference all reference
designators using the table in this appendix. The designators in
Sections 2 thru 9 apply to the VIC-1541 only and must be converted
to their equivalent 1541 designator before attempting any
maintenance procedure. In addition,when a procedure references a
pin number, use the cross references given in this appendix. If
confusion arises, compare the VIC-1541 Disk Controller schematic
with that of the 1541 .
• FAILURE TO CROSS REFERENCE DESIGNATORS OR
PIN NUMBERS MAY RESULT IN DAMAGE TO YOUR
1541.
Theory of Operation
Functionally, there is very little difference between the VIC-1541
and the 1541 disk drives. Listed below are the differences as
they apply to Theory of Operation:
1. Timing Circuit
UC9,Yl,Cl0,Rl and R2 on the VIC-1541 are replaced by Yl on
the 1541. Extra bypass capacitors have also been installed.
Yl on the 1541 produces a 16 MHz square wave. The remainder
of the circuit is the same.
2. Computer Circuit
UA2,UA3,UB2 and UB3 have been replaced by UB2 on the 1541.
To accomodate this change, UC6D and UC5E have been added to
the Address Decoder circuit. These gates functionally OR the
Low RAM (pin 1) and the High RAM (pin 2) outputs of UC7.
This causes UB2 to be selected when addresses OOOO(H) thru
07FF(H) are present on the address bus. Since A13 and A14
are not decoded, images of this RAM appear at three other
address ranges: 2000-27FF, 400-47FF and 600-67FF. In
addition, a minor change to the Reset circuit was made. UDlD
and UDlE on the VIC-1541 is replaced by UD3B on the 1541.
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