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LC III/FPU Issues

A friend from Apple writes to clarify the LC III/FPU issue raised a while back in TidBITS #169.

I understand the following to be the case:

  • If there is no FPU on the motherboard, and none on the card, no problem.
  • If there is no FPU on the motherboard, and there is one on the card, the system uses the one on the card, albeit at 16 MHz.
  • If there is an FPU on the motherboard, and there is none on the card, the system uses the one on the motherboard, at 25 MHz.
  • If there is an FPU on the motherboard, and there is one on the card, the system uses the one on the motherboard, at 25 MHz.

The FPU on the motherboard, since it is physically linked to the CPU, takes priority, in a manner of speaking. Removing a 16 MHz FPU from a PDS card and placing it in the 25 MHz socket on the motherboard will likely cause unexpected results. The 16 MHz part will probably crash the system. In other words – DON’T DO IT!

If an LC card with an FPU crashes an LC III, I would first look at other incompatibilities with the card by contacting the vendor. I contacted Technology Works, whose cards for the LC that include an FPU will work in the LC III, but the software for the cards is not yet ready. I imagine the case is similar with cards from other companies.

— Information from:
Pythaeus

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