NuBus slots
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NuBus in the Quadra 950 provides all the benefits found in the Quadra 900:
- Provides higher power
- Accommodates oversized NuBus cards
- Supports some of the NuBus '90 features
Higher Power
------------
The power supply in the Quadra 950 computer is designed to provide
additional current on the +5V outputs for the NuBus slots, compared with
the current specified in "Designing Cards and Drivers for the Macintosh
Family," second edition. The Quadra 950 has enough power to support a
total of two 25-watt cards and three 15-watt cards.
Oversized NuBus Cards
---------------------
Each NuBus slot can accommodate either a standard NuBus card or an
oversized card. The oversized card is the same length as a standard NuBus
card, but it is 2 inches taller. Each NuBus slot in the Quadra 950
computer includes a card guide; to install an oversized card, the user
removes the card guide.
NuBus '90 Features
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NuBus '90 is the 1990 proposal for revision of the IEEE standard for the
NuBus (IEEE STD R1196-R-1990). Apple has a representative on the panel
designing the new standards, and has had since its inception. The
specifications for NuBus '90 are incomplete and still under development,
but Apple has implemented the portion of the specifications that are
completed. Apple will be updating the implementation as the specifications
expand and stabilize. The NuBus slots in the Quadra 950 computer provide
the following new features described in that proposal:
- Low current at +5 V is available on the new STDBYPWR pin when main power
is off and the AC cord is plugged in.
- New signals /TM2, /CLK2X, and /CLK2XEN support block transfers at double
the standard rate (20 MHz). The Quadra 950 computer allows double-rate
block transfers between NuBus cards, but doesn't support double-rate
transfers to or from the main memory (this remains at 10 MHz).
- NuBus '90 defines new signals SB0 and SB1 for a serial bus on the
formerly reserved pins A2 and C2. The serial-bus signals are bused and
terminated, but the main circuit board doesn't drive them.
- NuBus '90 defines new signals /CM0, /CM1, /CM2, and /CBUSY to support a
cache coherency protocol. Pins on the NuBus connector are assigned to
those signals, but the Quadra 950 system doesn't support them.
Processor-Direct Slot
---------------------
For maximum performance, the processor-direct slot (PDS) is connected
directly to the MC68040 microprocessor by way of the system bus. Possible
applications for a PDS expansion card include cache memory, a video frame
buffer, a DMA-based I/O controller, expansion memory, or even an additional
MC68040 microprocessor. When a PDS card is installed, slot $E is
unavailable for a NuBus card.
A PDS card can have memory locations in the upper part of the RAM memory
space or in the space assigned to NuBus slot $E. If the card uses slot $E
addresses, it must decode all addresses in both the slot space and the
super slot space, responding to any access to an unused location with a TEA
on the processor bus to indicate an illegal address.
A typical PDS card maps into the NuBus space and works with the system
software's Slot Manager. Such a card must contain a NuBus declaration ROM
and must notify the NuBus controller that it is using the NuBus space by
asserting (pulling low) the signal /PDS.SLOT.E.EN on the PDS connector.
WARNING: A PDS expansion card for the Quadra 950 computer must be designed
to work with the MC68040 microprocessor; PDS cards designed for computers
that use the MC68020 or the MC68030 won't work in the Quadra 950 computer.
A PDS card for the Quadra 950 computer has the same dimensions as a NuBus
card and can include a back-panel connector. When the PDS card is
installed, it occupies NuBus slot $E, reducing the number of available
NuBus slots from five to four. Any PDS card designed to operate in the
Quadra 950 must be capable of meeting the 33 MHz timing specification for
the Motorola 68040. This requires shorter output delay times than the 25
MHz timing. Otherwise, PDS connector pinouts, size and placement are
identical among the Macintosh Quadra 900, 700, and 950.
IMPORTANT: The signals on the PDS connector are connected directly to the
MC68040 with no buffers. Therefore the address, data, and clock lines on a
PDS card must present capacitive loads of not more than 40 pF. The control
lines must present capacitive loads of not more than 20 pF.