Remote Dma Transfer - HP 7957A Hardware Manual

generates an interrupt, if enabled by the
microprocessor.
The DDC does not send ESDI commands to the
HDA or receive ESDI status from the HDA. It
only controls the transfer of read/write data. All
ESDI commands are sent by the microprocessor.
The following is a list of the primary commands
that the DDC executes:
• Read Data (reads data from HDA and transfers
it to the buffers via local DMA).
• Write Data (reads data from the buffers via lo-
cal DMA and transfers it to HDA).
• Remote DMA transfer (between the buffers and
HP-IB Interface IC).
• Error Correction.
• Format
For a read data operation, the microprocessor sets
up the DDC for the number of sectors to be read,
the starting sector number, the starting buffer ad-
dress where the data will be stored, and the size (in
bytes) of each local DMA burst transfer. Once the
microprocessor initiates the operation, the DDC
searches for the beginning sector. Once it is found,
the DDC begins reading in the serial data from the
HDA. The DDC computes the ECC bytes as the
data is read in, and compares it to the ECC bytes
stored on the disc. If the DDC detects an error,
the operation is aborted. After the data is deserial-
ized, it is transferred to the DOC's internal 32-byte
buffer. When the buffer is filled to the local
DMA burst size, the DDC requests a local DMA
transfer by asserting LREQ (Local Transfer
Request).
The DDC begins the local DMA burst transfer af-
ter the DMA Bus Arbiter acknowledges the local
transfer by asserting LACK (Local Transfer
Acknowledge). The DDC then proceeds to write
the data out to the buffers. Each transfer to the
buffers is 16 bits wide and takes 4 DCLK cycles
(333 ns). The DDC performs the following steps
for each 16 bit word transferred.
• Outputs buffer address onto DMA data bus.
Theory Of Operation
7957/7958
• Outputs data onto DMA data bus and generates
NDMAWR to strobe data into buffers.
• Increments buffer address counter by 2 for next
transfer.
This process of filling and emptying the buffer is
repeated until the entire transfer is complete. For
a write data operation, the microprocessor sets up
the DDC for the number of sectors to be written,
the starting sector number, the starting buffer ad-
dress where the data will be transferred from, and
the size (in bytes) of each local DMA burst trans-
fer. Once the microprocessor initiates the opera-
tion, the DDC performs local burst transfers to fill
the internal 32-byte buffer. As in a read data
operation, the DDC must wait for an acknowledge
from the DMA Bus Arbiter before it can begin a
local burst transfer. Each transfer from the buffer
is 16 bits wide and takes 4 DCLK cycles (333 ns).
The DDC performs the following steps for each
16-bit word transferred:
• Outputs buffer address onto DMA data bus.
• Generates NDMARD to read data from buffers.
The DDC strobes the data in on the rising or
trailing edge of NDMARD.
• Increments buffer address counter by 2 for next
transfer.
The DDC then searches for the beginning sector.
Once it is found, the DDC begins serializing the
data in the buffer, and transferring it to the HDA.
The DDC generates the ECC bytes as it transfers
the data to the HDA and appends them to the end
of each sector. When the buffer has been depleted
by the local burst size, the DDC initiates another
local burst transfer by asserting LREQ. This
process of filling and emptying the buffer is
repeated until the entire transfer is complete.
3-34. REMOTE DMA TRANSFER
For a remote DMA transfer operation, the micro-
processor sets up the DDC for the number of bytes
to be transferred, the starting buffer address where
the data will be written to or read from, and the
direction of the transfer. Once the microprocessor
initiates the operation, the DDC asserts the remote
3-15