am79c974 Advanced Micro Devices, am79c974 Datasheet - Page 132

• Manufacturer Part Number: am79c974
• Description: Pcnettm-scsi Combination Ethernet And Scsi Controller For Pci Systems
• Manufacturer: Advanced Micro Devices

them to multiplex logic that can map these outputs to in-
terrupt controller inputs by means of software.
REQ and GNT (pins 123, 124, 126,
and 127)
The Am53C974 uses pins 127 and 124 (REQ and GNT)
for bus master arbitration, while the Am79C790 uses
pins 126 and 123. The Am79C974 uses pins 127 and
124 (REQA and GNTA) for bus arbitration from the SCSI
controller, and it uses pins 126 and 123 (REQB and
GNTB) for arbitration for the Ethernet controller. The
motherboard can connect both pairs of signals to its bus
arbitration logic through jumpers or zero Ohm series re-
sistors. One pair of resistors or jumpers would be omit-
ted for a SCSI only product, and the other pair would be
omitted for an Ethernet only product. Both pairs would
be inserted for a product with both SCSI and Ethernet.
If the motherboard uses a PCI controller that allows a
limited number of bus masters, you may have to omit
one PCI slot or define one slot to be slave only for a prod-
uct with both SCSI and Ethernet on the motherboard.
IDSEL (Pins 9 and 10)
The Am53C974 uses pin 9 for IDSEL, which is a chip se-
lect for the PCI configuration space, while the
Am79C790 uses pin 10 for IDSEL. The Am79C974,
which has two separate configuration spaces, uses pin
9 (IDSELA) to access the configuration space for the
SCSI controller and pin 10 (IDSELB) for the Ethernet
controller. IDSEL is used only during configuration cy-
cles, which are defined by the command on the C/BE
lines during the address phase.
A typical way to generate the IDSEL signals on a
motherboard is to connect a separate address line to
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AMD
P R E L I M I N A R Y
Am79C974
each of the IDSEL pins of the various PCI devices and
expansion slots. Since a typical PCI motherboard will
not have more than 10 PCI devices and slots, two of the
high order address lines can be connected directly to
pins 9 and 10 of the AMD device. The Am79C970 will
respond only to configuration cycles when pin 10 is ac-
tive and will ignore configuration cycles when pin 9 is ac-
tive. The Am53C794 will respond only to configuration
cycles when pin 9 is active.
Clocks (Pins 60, 97, and 99)
The Am53C974 requires two clock inputs: SCSICLK1
(pin 60) and SCSICLK2 (pin 97). These clock inputs can
be driven by the same clock source. The Am79C790 re-
quires either a crystal connected to the XTAL1 and
XTAL2 pins (97 and 99) or an external oscillator con-
nected to XTAL1. The Am79C974 requires both a clock
input for SCSICLK (pin 60) and either a crystal con-
nected to the XTAL1 and XTAL2 pins (97 and 99) or an
external oscillator connected to XTAL1.
To satisfy all of these requirements, the motherboard
can connect pin 60 to pin 97 through a jumper or zero
Ohm resistor. For the SCSI only product, the oscillator
or crystal should be omitted and the jumper inserted.
For the Ethernet-only product the oscillator or crystal
should be inserted and the jumper omitted. It does not
matter whether or not the SCSI oscillator is included.
For the combination SCSI plus Ethernet product, the
crystal and the SCSI oscillator should be included, and
the jumper must be omitted.
Figure F-1 illustrates these connections.