Archived - Macintosh LC III: FPU Configurations with PDS Cards (7/93)
A Macintosh LC III (Performa 450) can potentially have two FPUs: one on the
logic board, and another on an expansion card. This article describes the four
possible combinations of FPU (on logic board and on add-on card) in a Macintosh
LC III.
* No FPU on the logic board, and none on the expansion card. No problem.
* No FPU on the logic board, but one on the expansion card. The system uses
the one on the card -- at 16 MHz.
* An FPU on the logic board, but none on the expansion card. The system uses
the one on the logic board -- at 25 MHz.
* Two FPUs: one on the logic board, and one on the expansion card. Both FPUs
are on the processor bus. There's no way to predict which one will respond
to a given FPU cycle, and no way to assign priority.
Apple does NOT recommend removing an FPU from an expansion card and installing
it on the logic board. Expansion card FPUs are 16 MHz, but the logic board
requires a 25-MHz FPU.
If, after installing a card with an FPU, your system crashes, contact the
vendor of the card to determine if there are any known incompatibilities
between that card and the Macintosh LC III.
Article Change History:
07/13/93 - Revised to include Performa 450.
06/24/93 - Revised in response to input from Apple Engineering.
* No FPU on the logic board, but one on the expansion card. The system uses
the one on the card -- at 16 MHz.
* An FPU on the logic board, but none on the expansion card. The system uses
the one on the logic board -- at 25 MHz.
* Two FPUs: one on the logic board, and one on the expansion card. Both FPUs
are on the processor bus. There's no way to predict which one will respond
to a given FPU cycle, and no way to assign priority.
Apple does NOT recommend removing an FPU from an expansion card and installing
it on the logic board. Expansion card FPUs are 16 MHz, but the logic board
requires a 25-MHz FPU.
If, after installing a card with an FPU, your system crashes, contact the
vendor of the card to determine if there are any known incompatibilities
between that card and the Macintosh LC III.
Article Change History:
07/13/93 - Revised to include Performa 450.
06/24/93 - Revised in response to input from Apple Engineering.
Last Modified: Feb 19, 2012
