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onboard DRAM require 5 bus clock cycles with the bus error
reported in the current cycle. Write accesses to onboard DRAM
require 2 bus clock cycles.
Burst read accesses require 8 (5-1-1-1) bus clock cycles with the bus
error reported in the current cycle. Burst write cycles require 5
(2-1-1-1) bus clock cycles.
ROM Cycle Times
The ROM cycle time is programmable from 4 to 11 bus clock cycles.
The data transfers are 32 bits wide. Refer to the Single Board
Computers Programmer's Reference Guide.
SCSI Transfers
The MVME177 includes a SCSI mass storage bus interface with
DMA controller. The SCSI DMA controller uses a FIFO buffer to
interface the 8-bit SCSI bus to the 32-bit local bus. The FIFO buffer
allows the SCSI DMA controller to efficiently transfer data to the
local bus in four longword bursts. This reduces local bus usage by
the SCSI device.
The first longword transfer of a burst, with snooping disabled,
requires:
Each of the remaining three transfers requires one bus clock.
The transfer rate of the DMA controller is 44MB/sec at 25 MHz
with parity off. Assuming a continuous transfer rate of 5MB/sec on
the SCSI bus, 12% of the local bus bandwidth is used by transfers
from the SCSI bus.
Note
Four bus clocks with parity off, and
Five bus clocks with parity on
The actual SCSI bus transfer rate is fixed, no matter
what the speed of the microprocessor.
MVME177 Functional Description
4
4-19
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